Popular rowing fallacies
Carl Douglas
P35 has 2 sentences on blade depth:
"Sit at the catch with the blades buried - just the blade and not the
shaft. See where your handle height is and then try to maintain it
through the drive".
Whence this belief that none of the shaft should be in the water?
Whence its corollary - that the top of the blade should run at surface
level & no deeper? Where are the written analytical or experimental
justifications for these popular mantras?
A blade should not wash out, as it then moves through the water rather
than staying with it, stirs it to froth, doesn't move the boat so well &
thus - in the words of Sellar & Yeatman - is a Bad Thing. Indeed, the
final paragraph on that same page correctly identifies the need for
tight, dark puddles. Which is blindingly obvious, but in conflict with
what went before
The physical processes around an immersed oar with a variable depth of
immersion have no reason to flip abruptly as the top of teh blade
submerges. So how & by whom was it decided that, suddenly, it all
changes immediately the blade reaches its flotation depth? What
physical transformation introduces a discontinuity or reversal at that
point by introducing a new & contradictory set of rules?
Yes, the oar shaft, being circular in section, is not best shaped for
boat propulsion - it slips too easily through the water (hence that
flatter bit at the end - the blade). But that means its drag will also
be rather low. Are we so dumb that we must invent fictitious rules
defining precise go/no-go boundaries?
Some simple facts:
1. The performance of the blade certainly improves progressively as it
goes from non-immersion through partial immersion (washing out) to
so-called full immersion (top edge level with water surface). But it
continues to improve as you go yet deeper. There is no sudden cut-off.
2. Immersing part of the shaft in the water is, at the worst, a very
slight source of drag but, when the boat is being rowed hard, at
mid-stroke the blade "turns" in the water not about its centre but
around a point a short way up the shaft from the blade. And the concept
of a point of rotation is anyway meaningless near catch & finish.
3. So a fair (if variable) chunk of the shaft can be immersed before it
can cause _any_ parasitic drag in the water. Until then it actually
contributes, marginally, to overall propulsive efficiency around
mid-stroke, where most energy is otherwise lost.
4. If you do go a tad deeper, to where water is slightly back-watering
over the shaft, that water is still moving but slowly relative to the
immersed shaft so, drag losses being proportional to the cube of
relative velocity between water & shaft, any net losses will remain
trivial for some continued increase in depth.
5. Meanwhile, the deeper you row the blade, the less chance of air
being entrained behind it. That entrained air, by forming a cavity
behind the loaded blade, drastically cuts the essential _tensile_
connection between blade & solid water (it's the back, not the face, of
the blade which transmits the propulsive forces into the water). Thus,
up to a surprisingly great depth, the deeper the blade goes the greater
its efficiency becomes
Since going deeper that the popular prescription continues to increase
blade efficiency, & since it only slowly, if at all, incurs any penalty
from shaft back-watering, a fairly strong case can be made for rowing
deeper than conventional wisdom suggests. And, FWIW, study of blade
depths of the more successful rowers & crews tends to support that case.
Despite the above, which is simple to follow, our sport widely maintains
a collective faith in the spurious concept of a divinely-ordained blade
depth limit.
Apologies if, inadvertently in the above, I just broke any treasured icons.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
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Charles Carroll
And you have no comment to make on the cover of this marvelous rag!
Look at the way this younglady is carrying her shell. I took one look at
that cover, photographed it, and sent the photo to Mike Sullivan.
That cover is the sort of thing that would have really thrown me for a loop
had I seen it in my novice days. I probably would have left a post on rsr
asking if this was the way to carry a 1x.
As for Tom Bohrer, the author you quote, I like him a lot. Over the years
his pieces in the Rowing News have helped me immensely. In fact his article
a few months ago on overextending at the catch changed my rowing. But does
this mean he is infallilble? By his own admission, Tom is not the greatest
sculler in the world, although he is still ten times better and faster than
I am.
Just take everything Socratically. Nourish your curiosity. Raise questions.
Learn as much as you can. And be forgiving of the mistakes others make.
"What is the first business of the philosopher? To cast away conceit; for
it is impossible for a man to begin learning what he thinks he knows."
Warmest regards,
Charles
Ps. Our friend sent me photos of his new boat. Talk about "understated!" It
is just magnificent!
Carl Douglas
You know how Sully & I prefer boats to be carried. But none of us
should fire off every time with every loose screw that rattles around
our empty skulls (or sculls). So I let such things pass ;) Besides,
carrying boats that way helps keep the trade in business.
I would never decry Tom Bohrer's expertise. Nor the value of his
columns. Remember: they who get nothing wrong also get nothing right.
I try hard to get many things right, & I may make more errors than most
in the process.
Anyhow, this ain't about perfection, just about highlighting popular
fallacies - for the dubious benefit of those few in the northern
hemisphere who lack the sense or time to be off on their summer hols.
And why does this particularly irrational fallacy hold such sway within
rowing? Why do we just love to make up daft, untested rules? I tried
calling Socrates, but his secretary tells me he hasn't been back in
since he got to work on hemlock, & I don't think it had anything to do
with woodwork....
Glad you like those photos, taken here just before the boat went into
its box for its trip to Seattle. But when will he get to scull his new
beauty?
While on this boaty digression, those with access to World Rowing
on-line magazine might like to view the article on P32 of:
http://www.worldrowingmagazine.com/worldrowingmagazine/200907/
It's an article on Frans Goebel, with a couple of shots of that still
very fine sculler in his rather beautiful boat.
Steven M-M
Actually I thought the more egregious assertion was: "Blades Out
Square--There's no debate here. You are either coming out fully square
or your are not." Now, while on the surface that is a correct
statement, I would certainly debate the effectiveness of waiting to
feather until the blades are completely off the water. This makes for
a needlessly mechanical releases and often upsets the balance & run of
the boat. (I do think square-blade-rowing is an essential drill, but
like all good drills, it exaggerates to teach a lesson.)
Steven M-M
Walter Martindale
Re: Carl's OP. At the 1999 FISA coaches conference, Canberra, for
some long-forgotten reason we were viewing the ITA LM2X, at the time a
highly regarded crew. A coach from another nation stood and asked
something along the lines of "Aren't their blades deeper than we
usually coach?" and the ITA coach replied along the lines of "Yes,
because it's better for speed," or something very close to that. For
me it was a "lightbulb" moment. I recalled working with beginners,
whose "deep" blade I was trying to get more shallow to match the "just
buried" blade, and thought, Hang on - that deep blade is making them
turn, and the shallow blade is slipping - what if I stop worrying
about the deep blade, and get the shallow blade in the water a bit
deeper, maybe the blade-water connection will be more efficient and
they'll go straight, faster than with the blades skimming with the top
edge at the surface... So - now I look for about 40 cm of shaft under
water (or perhaps more) with a sculling blade at moderate pressure,
and I think about how the really fast guys go in pretty deep...
WRT the "out square" stuff. Nolte's work from the 80s (described at
the 1992 Barrie Ontario Canadian coaches conference, if I recall
correctly - it was one of those conferences that stuck in my mind
because of the "discussion" between Nolte and Spracklen) and you can
replicate it with frame-by-frame video of elites)) shows that the
bottom edge of the blade is clear of the water while on the square,
and the handle continues towards the bow while the athlete then
feathers the blade, changes direction, and commences the recovery. To
confirm this, get good side-on video of an elite crew/single at race
pace. Advance the video frame-by-frame, and record the position of
the end of the handle each frame - relative to the boat - and also
record events such as
when the bottom edge of the blade touches;
when the blade is fully buried;
when the top edge of the blade is exposed;
when the bottom edge of the blade is clear;
when the blade is squared up;
when the blade is feathered.
You can also record how many frames it takes for each of these events
(burying, releasing, squaring, feathering)...
You'll most likely find that a lot of coaching doesn't match what
elite performers do - who do you listen to - a coach who's telling you
stuff that's been "the wisdom" forever whether or not it is supported
by evidence, or the people who are making boats go fast, sometimes in
spite of their coaching?
The old fashioned way to do this would be to take a piece of overhead
projector film, overlay it on the screen, trace the gunwale and a
couple of reference points such as riggers, and then put a "dot" where
the end of the handle is. Then advance the film, re-index the film
with the boat, and "dot" (digitise) the handle, repeat until the
handle is back where it started. Put a "tick" on the line of dots
indicating all the above events... I've done this, and it (once
again) changed the way I coach.
I no longer have access to Rowing News, but would suggest that as
reported in this thread, Tom Bohrer is wrong about blade depth, and
right about "out square"..
Cheers,
Walter
Carl Douglas
Walter -
I'd also beg to differ on the blades-out-square aspect (aren't some folk
just plain cussed?). But I have valid reasons (other than natural
cussedness, of course).
Let's first consider whether or not the blade is still loaded as it
rises through & from the water, & whether we want VFM from every inch of
the stroke, or are content with Brownie points for style regardless of
possible loss of content.
If the blade leaves the water square, if not to backwater it must, as
you confirm, be moving astern. Due to its rotation about the pin, the
blade's face-on velocity (angular velocity) WRT the boat is maybe only
~75% of what the boat's speed would otherwise suggest (which makes
finishes a lot easier) but is still substantial. For the sculler, that
reduction in velocity is more meaningful at the handle than for the
sweep rower since the sculler's hands can pass the ribcage diagonally,
whereas the sweep finish brings the hands abruptly to the body - just a
general observation. Anyway, as you say a square finish demands
continuing & useless hand movement to the bow after the blade has
extracted. And it means you have a poor hydrodynamic relationship
between blade & water which tends to prematurely expose the back of the
loaded blade & thus to disconnect it.
So what's wrong with a finish which feathers (partly), before & while it
extracts? I think that you, Walter, accept as I do that as you approach
the finish the blade is once again acting as a foil, working largely by
the generation of lift, due to its action generating a flow from root to
tip that curves along & around its convex back face. And I'm sure that
you'll agree that, while you are extracting the blade, there must also
be a vertical component to that otherwise horizontal flow?
Now if you feather to some degree then, instead of having to keep the
hands moving to the bow, they can move more nearly vertically during the
extraction - just as a part-feathered blade held in the finish position
tends to rise through the water. But also you will now be able to keep
more area of the back of the blade covered during that extraction &, by
the downforce at the hands, to generate useful (propulsive) lift from
this vertical action. And, of course, your blade will be more nearly
feathered as it emerges & more nearly (or actually) stationary as it
clears the water. So, no continuing hand motion into the bow after
extraction, allowing a slightly longer useful draw, & a more effective
last part to your stroke.
Just as we agree that rowing deeper has performance benefits, so I see
real benefit in being prepared to analyse blade fluid dynamics in truly
3D terms (or should I say 4D, it being such a time-dependent action?)
which consider also the fluid dynamics of the vertical motions (catch &
finish, & even through the rest of the stroke) as well as those in the 2
lateral directions. It helps to indicate how hard it may be for some to
see the need for such multi-dimensional thinking if we remember that
many rowers think only in terms of a linear, near 1D, action - which is
why we get that nonsense about needing a hole in the water so we can
extract the blade, & an emphasis on the work done in the midstroke.
Some might think the fluid dynamic significance of these vertical
actions to be of scant importance, but I doubt any rower, let alone the
complete rower, can really afford to spurn minor gains.
I'd welcome your thoughts.
James.
I am mostly convinced by the arguments, in this case put forward by
Carl. To use some layman's terms, it does make sense that the water is
'more solid' further down and that connecting to it will prevent washy
puddles (I don't exclusively mean washing out though - I have seen
many a novice tickling the surface of the water!).
However it raises questions to me:
1) Perhaps this is a complete lack of understanding of hydrodynamics
on my part - when the boat is being driven along it is also lifted out
of the water, and as it decelerates it drops back down again. Whilst
lifted it goes faster partly because the wetted surface area is lower
(main part being that it is the drive phase of course). When the drive
is over the boat drops down as it slows. Surely, if the blades have
been deeper in the water, the shafts will drag the boat downwards as
you extract them? I accept the point that the blades themselves do not
add significant additional drage, but their effect on the hull..? Is
it simply that the additional speed from the depth outweighs the drag?
2) I am always aware that the main discipline discussed here is
sculling and that for one person to change their scull depths can be
done as it is symmetrical. With sweep however this is harder to do (I
know this has been discussed before) but I don't see how to teach
people to achieve a magical unknown optimum depth with people on
either side trying different things. It surely is not going to work?
As far as I can understand it- with sweep at least- we *should* teach
people to row with blades just submerged, but as they gain more
experience that should be progressed into moving the blade deeper.
Particularly with crews used to rowing with each other - possibly
moving the oft seen tape markers to deeper position progressively to
ensure the crew can change together.
I don't receive rowing news so I am not sure of to whom this article
is aimed. It seems to me that for rowing purists, top end athletes
etc, what you are proposing Carl is about right (with the question 1)
still standing) but if you are talking about how to teach people from
beginner to intermediate level, I just don't think it works to take
that purist approach from day one...
If I am wrong on the drag I would be very interested to know - it is
the one thing that has really prevented me from rushing out and
teaching it since I first heard the argument!
James
Walter Martindale
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
I'm not theorizing - I'm reporting observations of elite scullers..
Think of "follow through" - when you accelerate the handles to move
the boat past the blades, when the load comes off, you have to follow
through in the direction of movement... Almost like "waving goodbye"
to the puddle. If you try to change directions without this follow
through, you get jerky movements, or unload your blades before you
take them out - frankly I'd rather people were pushing right up to
extraction...
It's what happens. Digitize some elite performers sculling the way
they do, and see what they do... (frame-by-frame - I did this 10 years
ago, - and more recently - ...)
W
Zbigniew A.
> As for Tom Bohrer, the author you quote, I like him a lot. Over the years
> his pieces in the Rowing News have helped me immensely. In fact his
> article a few months ago on overextending at the catch changed my rowing.
Hey Charles,
Can you please tell us which issue it was? I went through a contents of back
issues and can't find anything relevant.
I also may have some questions regarding your post "Feet and Foot Stretcher"
from 13th of July. If you don't mind.
--
Yours Virtually, Zbigniew A.
to e-mail me directly best use zibi(at)netcom,no
Carl Douglas
> I have read this ...debate... on blade depth on RSR several times, and
> I am mostly convinced by the arguments, in this case put forward by
> Carl. To use some layman's terms, it does make sense that the water is
> 'more solid' further down and that connecting to it will prevent washy
> puddles (I don't exclusively mean washing out though - I have seen
> many a novice tickling the surface of the water!).
>
> However it raises questions to me:
>
> 1) Perhaps this is a complete lack of understanding of hydrodynamics
> on my part - when the boat is being driven along it is also lifted out
> of the water, and as it decelerates it drops back down again. Whilst
> lifted it goes faster partly because the wetted surface area is lower
> (main part being that it is the drive phase of course). When the drive
> is over the boat drops down as it slows. Surely, if the blades have
> been deeper in the water, the shafts will drag the boat downwards as
> you extract them? I accept the point that the blades themselves do not
> add significant additional drage, but their effect on the hull..? Is
> it simply that the additional speed from the depth outweighs the drag?
>
Hi James -
I fear that we're into another of those popular fallacies. What you see
as the boat lifting is more a case of the bow lifting because the load
on the blades, acting below the centres of drag & mass, generates a
couple which pitches the boat up at the bow but down at the stern.
Nor would it be a good idea to invest energy ino raising the boat, since
that immediately reduces the energy available to overcome fluid drag. I
know we like to use morally uplifting exhortations about "lifting the
boat", but while this has great psychological value I think its physical
credentials are very questionable.
Now to the consequences of oar shaft immersion: If we suppose that you
did want actual uplift then, slight though it might be, the act of
immersing a bit more shaft will reduce the boat's displacement - on the
Archimedean basis that the mass of water displaced must always balance
the mass of boat, blades & crew, & because there'll be some very slight
uplift from the vertical component of any drag as you lower the shafts
(& blades) into the water. But I'd already noted that there's very
little drag on a short outboard length of a circular section oarshaft &
since at the finish we're not proposing any rapid vertical motion (& the
water at the finish is actually flowing partly _along_ the shaft, just
as it's flowing almost entirely along the blade) you'll get very little
downthrust either.
So, in short, the significant gain in propulsive efficiency from rowing
the deeper blade comes at no significant price - it is all profit. The
deeper blade turns less of your effort into froth & slip, leaving more
for simple propulsive effect. You go faster because you're throwing
less energy away at the end of the blade.
> 2) I am always aware that the main discipline discussed here is
> sculling and that for one person to change their scull depths can be
> done as it is symmetrical. With sweep however this is harder to do (I
> know this has been discussed before) but I don't see how to teach
> people to achieve a magical unknown optimum depth with people on
> either side trying different things. It surely is not going to work?
Good question. Equally, with low-standard crews there's often little
evidence that they row at any set depth, let alone all the same depth.
This is where I'm more with Tom Bohrer: it's a good (or better) idea to
show your rowers the height(s) at which you want them to draw their
hands, then the blades will eventually follow.
The novice has a constellation of coordination problems, plus a
pernicious fear that's too often coached into them that, if they get it
wrong at the finish, they'll catch a crab. Too rarely are they told
that crabs result only when water is allowed to overtake the blade,
either because there's no work on it near the finish, or because the
rower is so keen (through worry) to extract it before the possible crab
that they actually try to push back against the irresistible flow.
First of all, the possibility of crabs should really not be drawn to the
novice rower's attention as that begets fear. Secondly they should be
told that they can & should pull really hard at the finish & the blade
will come out naturally provided they keep the load on the handle.
Once you get that clear, & demonstrate its reality to them e.g. with
single strokes to backstops, they'll be happier stick the blade in & to
row to a solid depth. Until doubts are dead & buried, traces of their
damage remain below the surface of otherwise quite competent strokes.
The other depth problem is that new rowers may try to fix the angle at
which the upper arm projects from the shoulder joint - they tense up
because they're in a boat & this can be one consequence - instead of
learning early to pull with loose arms so the hands can travel in a
straighter line.
All such issues need easing & teasing out of the individuals concerned,
who should not be put under extra pressures until those particular
problems have been resolved.
>
> As far as I can understand it- with sweep at least- we *should* teach
> people to row with blades just submerged, but as they gain more
> experience that should be progressed into moving the blade deeper.
> Particularly with crews used to rowing with each other - possibly
> moving the oft seen tape markers to deeper position progressively to
> ensure the crew can change together.
>
> I don't receive rowing news so I am not sure of to whom this article
> is aimed. It seems to me that for rowing purists, top end athletes
> etc, what you are proposing Carl is about right (with the question 1)
> still standing) but if you are talking about how to teach people from
> beginner to intermediate level, I just don't think it works to take
> that purist approach from day one...
>
> If I am wrong on the drag I would be very interested to know - it is
> the one thing that has really prevented me from rushing out and
> teaching it since I first heard the argument!
>
> James
>
>
If you can get rowers thinking in terms of hand heights (& arm
looseness) rather than bladed depths, until you see they're fully
responsive to that approach, then you'll be able to take them
progressively to being able to row at whatever depth you (& they) want.
Then you can even start to profile the paths their hands take, so as
to control depth variations through the stroke.
We're all fumbling in the gloom towards better ways to do things - hence
these discussions between those of us who really do care. Meanwhile the
shouty types will keep on shouting what their own coaches shouted at
them (& so ad infinitum!).
Steven M-M
Walter,
I don't have access to high quality frame-by-frame video, but take a
look at: http://www.invernessrowingclub.co.uk/strokecycles.html There
aren't many good shots of blades but the sculling sequence of Xeno &
Neykova and rowing of the Aussie 2-, Rom 2-, USA 8+, & Ger 8+ give, to
my eye, support for the observation that top scullers and rowers begin
the feather while the bottom 1/2-1/3 of the blade is still covered.
Xeno coaches square out, then feather, but at least in this sequence,
it appears to me he has a more "rounded" release. Do you see "square
out, then feather" in these sequences?
Steven M-M
James.
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Thank you! A great answer which has caused me to think of things I had
not considered before- the concept (pardon the pun) of immersing a
blade reducing displacement hadn't occurred to me.
When I was asking the question I had in mind a session when my crew
were testing a boat out prior to borrowing it. The crew were "looming"
in a way that was without question slowing it down. To cut an
otherwise long story short, it turns out that the blades were not at
the right pitch for the gates and it was causing the forced error. In
my mind I had the association: blades too deep = slower. Despite
having read here before that depth could generate speed. What I
suppose I should have thought through was that the pitching was
causing the depth AND an awkward angle which caused the extraction
issues and lowering of speed.
The issue of crabs amuses me - I don't ever dwell on them when
teaching beginners, but I did once have an athlete so prone to them,
also to falling off his seat and a host of weird and wonderful reasons
for catching a crab, that we invested some time teaching him how to
recover from them in case he did so in a race. Oddly I don't think he
crabbed again after that lesson...
I do tend to focus on talking about bring hands in to the right place
rather than the depth of the blade, more by accident that design.
Having said that, visual learners like to have something to look at -
their hands at the finish is not a good option! I wonder then whether
I should start the year with the boat rigged a little high for the
crew (which according to Nolte helps keep shoulder position correct)
and then through the year gradually drop the heights so that hands
stay in the same place and the blades get drawn a little deeper?
Perhaps this is a constant way in which a sweep crew could be changed
together?
Does that make any sense?!
James
mruscoe
> James. wrote:
>> [snip]
>> As far as I can understand it- with sweep at least- we *should* teach
>> people to row with blades just submerged, but as they gain more
>> experience that should be progressed into moving the blade deeper.
>> Particularly with crews used to rowing with each other - possibly
>> moving the oft seen tape markers to deeper position progressively to
>> ensure the crew can change together.
>>
>>
>
> If you can get rowers thinking in terms of hand heights (& arm
> looseness) rather than bladed depths, until you see they're fully
> responsive to that approach, then you'll be able to take them
> progressively to being able to row at whatever depth you (& they) want.
> Then you can even start to profile the paths their hands take, so as to
> control depth variations through the stroke.
>
> We're all fumbling in the gloom towards better ways to do things - hence
> these discussions between those of us who really do care. Meanwhile the
> shouty types will keep on shouting what their own coaches shouted at
> them (& so ad infinitum!).
>
> Cheers -
> Carl
>
height of the rig, people will often still put the blade in to the same
depth, even if that puts their hands up at shoulder height, maybe
because that is how they like the connection to feel.
Inadequate depth for sweep rowing does seem to be the norm in club
crews, but I'm not sure whether that is just from coaching, or whether
over-gearing has big influence, and when crews aren't rowing that well
together it's physically punishing for anyone who is getting effective
depth.
Carl Douglas
Rowers do know that rowing shallow & washing out is a real cop-out - a
way of blatantly cheating on your crew-mates. But only to that extent
is it accepted that more depth is better than less.
Once a rower is rowing the blade top flush with the surface, no one can
say they're washing out. It then gets a lot harder for coach to argue
for going still deeper, while a forest of spurious arguments has been
thrown up to defend rowers from having to go deeper - which is probably
why "level with the surface" is the accepted coaching objective. Even
so, you see plenty of washing out with crews going slowly as a result.
As you say, rowing deeper feels a lot harder. That's not because it
makes you pull any harder (only you can decide that) but because it is
more efficient. That means the blade slips less, moves the water less &
the stroke takes a bit longer to complete. There's nothing you can do
to change this - pulling harder makes no difference. But, to add insult
to pain, you then get accused of being late on the extraction!
Then come the creeping, bogus arguments that a bit of blade slip is
desirable as it's "a kind of gearing". No, it isn't. Slip is a
profound source of lost efficiency, just as wheel spin is a way to
generate heat, waste energy & reduce propulsive efficiency & not another
way to gear down a train or car.
Were there an objective way of measuring each rower's individual
contribution to the moving of the boat, it would soon become clear that
there is a continuing correlation between greater blade depth &
increased boat moving effectiveness, regardless of rower power. Yet
because rowing has wrapped itself in an orthodoxy based on unscientific
myths, it continues more obsessed with style than "how it really works".
That's why film footage so rarely shows the blades: we don't as a
sport understand (or much care) what goes on at that end of the stick,
we kid ourselves it's all sorted, & we have instead persuaded ourselves
that it's what the body does that is all important. So we see no need
to study the interaction between blade & water.
Walter Martindale
Hi Steven,
Well, I've had a look at a few of these and yes, the blades do seem to
still be a little in the water when they start to feather. It also
looks like some of their handles are continuing towards the bow during
the feathering, but it's not the best video.
They're definitely square until most of the way out. Are they cutting
off propulsion by stopping the push and feathering before completely
out? I realise that they're champions, but could they have been
faster? I've seen video of some pretty good athletes completely clear
of the water before feathering, too...
When coaching, I ask people what's the most important part of the
stroke, and while they're thinking, I say "All of it."
Rgds,
Walter
Steve
I am wondering as I read through this thread, what is the impact of
blade design on the most effective depth. Macon blades are quite
narrow, and all of the blade is relatively close to the surface.
Hatchets go deeper , even when the blade is carried near the surface.
In fact, the top of the blade is curved forward, to counter the effect
of loss of "traction" with the blade close to the surface. I use
dreher, apex r's, first generation without the more recent
modification. Tried the new smoothie big blades, found the load too
painful at the catch, probably because at the time I had the habit of
really hammering with the legs soon at the catch.
My guess is theat at lot of this is individual experimentatioin and
finding out what depth, and what power applicaion is best for each
person's innate gearing, strength, as well as rigging. Just my humble
opinion from a frequent lurker.
Steve Giddings
> ...
>
> read more �- Hide quoted text -
>
> - Show quoted text -
Charles Carroll
> back
> issues and can't find anything relevant.
Zbigniew,
Delighted to. It is The Rowing News, Volume 16 Number 7, September 2009.
Also I just sent you a private email.
Cordially,
Charles
Carl Douglas
> I am wondering as I read through this thread, what is the impact of
> blade design on the most effective depth. Macon blades are quite
> narrow, and all of the blade is relatively close to the surface.
> Hatchets go deeper , even when the blade is carried near the surface.
> In fact, the top of the blade is curved forward, to counter the effect
> of loss of "traction" with the blade close to the surface. I use
> dreher, apex r's, first generation without the more recent
> modification. Tried the new smoothie big blades, found the load too
> painful at the catch, probably because at the time I had the habit of
> really hammering with the legs soon at the catch.
>
> My guess is theat at lot of this is individual experimentatioin and
> finding out what depth, and what power applicaion is best for each
> person's innate gearing, strength, as well as rigging. Just my humble
> opinion from a frequent lurker.
>
> Steve Giddings
>
Superimpose a Macon blade onto a Hatchet & you'll find the differences
are not quite as you describe. The hatchet shape is created by
stretching the outer part of the upper edge up towards the waterline &
the inner part of the lower edge downwards. (On the surface of the
earliest Hatchets you could actually see how the Macon shape was
extended into the Hatchet in just this way). Thus the Macon blade runs
diagonally through the Hatchet, with its upper edge almost everywhere
running _deeper_ than that of the Hatchet.
This greater submergence of most of the top edge on Macons may well
explain why the originally-claimed astounding difference in performance
with Hatchets was not seen in practice.
Which brings me to your reference to that curved top lip. If blades
worked by pushing water, as is so commonly but wrongly believed, then a
lip might help. In reality, however, the blade locks into the water not
by pressure on its concave face but by tension within the water covering
its convex back. As I noted in an earlier posting, shallower immersion
(e.g. top just below water surface, or higher) allows the fall in
pressure in the water behind the blade (due to that tension) to
precipitate a rapid fall in water level behind the blade. This
sufficiently reduces the immersed area & increases aeration of the back
of the blade that the tensile connection is first weakened & then
disrupted - breaking the essential blade/water connection. A similar
phenomenon is well understood in sailing, where entrainment of air down
the low-pressure face of a rudder, or even a keel or dagger-board, can
cause sudden loss of control.
Which returns us to the original argument that, contrary to Tom Bohrer's
advice in the latest Rowing News, for the best results you should row
the blade significantly _deeper_ than the "flush with the surface"
target of conventional wisdom. And you should do this regardless of
your innate gearing, strength or rigging, since the objective (if you
wish to move fast) should always be to row an efficient stroke, not to
row a less efficient stroke because it's easier that way.
Sure, a more efficient stroke takes longer to pull through as it slips
less but, by slipping less, it throws away less of the energy you invest
in each stroke. Throwing invested energy away is no way to get the best
out of your rowing, or anything else & is certainly not a cunning type
of gearing. As pulling harder on a deeper, thus more efficient, stroke
won't reduce the time it takes to complete, you may also need to adapt
your stroke/recovery rhythm, & maybe even your stroke length at first,
to the new realities of the deeper stroke.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Pete
> I have read this ...debate... on blade depth on RSR several times, and
> I am mostly convinced by the arguments, in this case put forward by
> Carl. To use some layman's terms, it does make sense that the water is
> 'more solid' further down and that connecting to it will prevent washy
> puddles (I don't exclusively mean washing out though - I have seen
> many a novice tickling the surface of the water!).
>
> However it raises questions to me:
>
> 1) Perhaps this is a complete lack of understanding of hydrodynamics
> on my part - when the boat is being driven along it is also lifted out
> of the water, and as it decelerates it drops back down again. Whilst
> lifted it goes faster partly because the wetted surface area is lower
> (main part being that it is the drive phase of course). When the drive
> is over the boat drops down as it slows. Surely, if the blades have
> been deeper in the water, the shafts will drag the boat downwards as
> you extract them? I accept the point that the blades themselves do not
> add significant additional drage, but their effect on the hull..? Is
> it simply that the additional speed from the depth outweighs the drag?
There's not much boat lifting effect (the bow lifts, the stern doesn't
dip quite as much), for one thing. Then - well, if you were dragging
the boat down, you'd be doing it by pushing down with your hands on
the sculls to extract them. Think about how the finish feels - you
don't even notice that you're tapping down, normally. Certainly
there's not any significant force; and since there is already the
significant force of your body weight pushing the boat down, whatever
little extra your hands provide will be not worth worrying about.
> 2) I am always aware that the main discipline discussed here is
> sculling and that for one person to change their scull depths can be
> done as it is symmetrical. With sweep however this is harder to do (I
> know this has been discussed before) but I don't see how to teach
> people to achieve a magical unknown optimum depth with people on
> either side trying different things. It surely is not going to work?
On that theory, why not just stay out of sweep until everyone can row
the same way on both sides..? All that will happen is that the balance
will be slightly upset (and it will be a small effect) until both
sides compensate to sit the boat again, or to row better. So, why not
simply coach the less effective people to row better instead of asking
the people who are already rowing better to do the wrong thing?
Pete
James.
Pete,
I think perhaps there are two conversations or sets of uses going on
at once and one assumed statement:- Carl is talking about the ideal to
which we all aspire. He knows from what I have said before that I deal
with complete beginners - there is no need to consider those of
different standards - there are none, they are all completely new to a
boat never mind the fine art of establishing the optimum blade depth.
My first question was for the assumption of that ideal - however I am
a cox by origin (though competent with a blade) so sometimes I do miss
how things feel.
The latter part was to do with those beginners. I defy anyone to tell
a group of people with a blade in their hands for the first time to go
out and put the blade in the water to an unspecified depth with no
visual references or experiences against which to compare. No matter
how hard they try they will not get it right or consistent across the
crew. They need a visual reference to compare to until they have
learned the basics and the feel, then they can be coached into
changing what they now perceive as the correct feel/depth.
I think I am to take it from "then why not stay out of sweep..." that
you mean to teach everyone to scull first? Oh for that little luxury!!
I moved clubs at the start of last academic year to try and assist a
club with 1 pair/double and one 4+. Trying to send people out sculling
to get the finer points right in a scull was unlikely in the
extreme...! [If anyone has any sculls in their clubs they don't need I
would be VERY grateful, we may be able to find some small funds!]
Apologies for assuming everyone would know something of what I am
involved in!
James
Walter Martindale
>
(snip)
> Sure, a more efficient stroke takes longer to pull through as it slips
> less but, by slipping less, it throws away less of the energy you invest
> in each stroke. Throwing invested energy away is no way to get the best
> out of your rowing, or anything else & is certainly not a cunning type
> of gearing. As pulling harder on a deeper, thus more efficient, stroke
> won't reduce the time it takes to complete, you may also need to adapt
> your stroke/recovery rhythm, & maybe even your stroke length at first,
> to the new realities of the deeper stroke.
>
> Cheers -
> Carl
And - just for the sake of really flogging the horse - the less the
blade slips, the more the boat moves, yes?
8-)
Walter
Pete
> The latter part was to do with those beginners. I defy anyone to tell
> a group of people with a blade in their hands for the first time to go
> out and put the blade in the water to an unspecified depth with no
> visual references or experiences against which to compare. No matter
> how hard they try they will not get it right or consistent across the
> crew. They need a visual reference to compare to until they have
> learned the basics and the feel, then they can be coached into
> changing what they now perceive as the correct feel/depth.
>
> I think I am to take it from "then why not stay out of sweep..." that
> you mean to teach everyone to scull first? Oh for that little luxury!!
> I moved clubs at the start of last academic year to try and assist a
> club with 1 pair/double and one 4+. Trying to send people out sculling
> to get the finer points right in a scull was unlikely in the
> extreme...! [If anyone has any sculls in their clubs they don't need I
> would be VERY grateful, we may be able to find some small funds!]
No, the intention was that you are never, however you do things, going
to coach a boat where everyone does the same thing on both sides,
whether that's the right or the wrong thing. And even if you did, when
you tell them to change something then they will not do it all
together and your boat will do different things on different sides.
So, why not simply coach them to do the right thing? I assume you
aren't coaching your beginners to row the whole outing watching their
blades from the catch to the finish, so they can't see where their
blades are until near the finish most of the time anyway. The blade
height will surely change through the stroke, so they'll have no idea
whether they really have their blades in the position you coach except
near the finish. Fine, you can worry about how telling them to row at
some loosely specified depth may make them inconsistent and the boat
unbalanced - but why? They are inconsistent anyway, the boat is
unbalanced already, your coaching will be pretty similar whether
you're coaching blades at the surface or a bit lower: '3, too deep at
the catch.. better... 4, deeper, ..'. If you're really set on the
idea of a visual reference (which they cannot see for most of the
stroke except by looking round and rowing with a different body
position to normal...), put red tape a little way up the oarshaft and
tell them to cover up to the tape.
Pete
Steve
> URLs: �www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)- Hide quoted text -
>
> - Show quoted text -
So, to my thinking, the question becomes, how much deeper is enough,
and does the increased efficiency outweigh the negative impact of the
greater time and/or energy required for the vertical movements at the
entry and the extraction, which do not effect forward motion, and also
may impact the timing of the catch and the finish? Is this blade
specific? Is it predictable? Is it dependent on the amount of force
applied to the blade?
Steve Giddings
Carl Douglas
There can be no hard & fast answers, but first I should say that I think
you will spend no significant extra time or effort in entry or
extraction. We are not talking about immediate deep burial, just a
well-buried catch followed by progressive deepening through the
mid-stroke, and then a gradual reduction in depth (but still properly
buried) towards the finish. Let me explain further:
The tendency for the water to recede from & expose the convex back of
the blade at any depth of immersion is amplified by several factors:
1. Load - the greater the load, the deeper you need to be to ensure the
blade is completely embedded within the water & not aerating. At the
catch, & shortly thereafter, the load is not very high. We think it is.
We are told it should be. It would be good if it was. But the
reality is that the load takes time to build. However, in the
mid-stroke the load is high, which is where depth becomes most important.
2. Flow direction - if the flow past the blade is relatively swift & is
parallel with its back surface, then the low-pressure region is being
continually re-filled with new, smooth, previously undisturbed water.
The flow is along the blade near the catch & finish, which thus reduces
the tendency for the water level behind it to drop & the blade to
aerate. In the mid-stroke the flow is from face to back, under the
blade, around its ends &, but only if it is "properly" buried, over its
top edge. But if the blade is near the surface, this water going behind
the blade is very frothy & can't in any case adequately replenish the
cavity growing there.
3. Duration. The longer the blade spends with water coming face-on,
the more likely that an air-filled or aerated cavity will form behind it
- which describes the mid-stroke to late mid-stroke situation & is a
special case of 2 above.
The deeper you row, the more efficient the blade becomes, but clearly as
you go deeper the process must be subject to a law of diminishing
returns since at no depth will you get 100% propulsive efficiency. So,
while it is easy to explain & justify the need to row deeper when most
of us row evidently too shallow, & while one can further expand on where
in the stroke to row deepest, it is not possible (without detailed
experiment on a variety of stroke force profiles, & a knowledge of each
rower's stroke force profile) to give precise rules on exactly how deep
to go & in what progression through the stroke.
We are here setting out the hydrodynamic reasons why rowing deeper than
flush with the surface must enhance stroke efficiency. Indeed, the
simple fact that if you row deeper it feels harder & takes longer is a
strong indicator that it must be more efficient. Now it is up to
individuals & coaches, if they wish to, to run with this ball & see
where it takes them. It's a journey of exploration, not a predetermined
bus ride. Rowers so often want precise rules spelled out to them (even
though they will probably break or adapt these!) before doing something,
but this is a case where the initial benefit is very clear & the point
where new sources of lost efficiency may start to take the shine off the
process have yet to be established & may differ for each person.
As an aside: had you considered how the traditional lugging oar works -
you know, the whaler or longboat oar which has a long straight blade
which is an elongated diamond shape in its cross-section? With this oar
you don't stick it in at the catch & pull horizontally through, 'cos
that is very inefficient, with lots of slip & turbulence. Instead a
dipping stroke is used, the blade being driven & cutting downwards,
edge-first, at a steepish angle & then being being drawn back to the
surface again at an opposite, steepish angle. Used thus, this kind of
oar is really rather efficient. It acts as a hydrofoil or under-water
wing on both paths - due to its diamond or lozenge cross-sectional
shape, which is a reasonably efficient foil in either direction. Its
vertically angled motion through the water generates lots of lift in the
boat's forward direction & relatively little drag, & it lends itself
well to use in a relatively slow & heavy craft. Only as the boat speed
increases towards that of rowing shells, giving insufficient time for
performing this largely vertical action, do we need to use the long
stroke & the conventional scull-shaped rowing oars. But that doesn't
mean we should completely flatten out our stroke action, nor that we
should row right at the very surface.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
MagnusBurbanks
being told that the net pitch on my blades should be 6 degrees what
with me being a novice, and then as I got better it should be reduced.
I can't remember asking for a reason or if one was given, but in
retrospect and in the light of present discussion it seems like good
advice.
Yours, Magnus
Carl Douglas
Those are excellent points, Pete.
I would add that in a crew the effect of unequal blade depths on balance
tends to be less than in a single or pair, there being a fair chance of
a deep one cancelling a shallow one. The real pig is when all of one
side rows shallow, or finishes low, & most of us have been in one of
those in the past.
The novice rower has many problems to overcome &, sadly, they get
inflicted not just on that person but on the rest of the crew. So the
task of coaching novices, which can be the most rewarding of all
coaching tasks, does need the patience of Job on both sides of the bank,
plus a few wise words to increase relaxation, some simple drills to
establish better confidence & reduce the human tendency to over-correct,
& lots of little bits of individual encouragement.
I'd prefer not to put tape markers on the shaft as they give the false
notion that you must go straight down to that mark & stay there. Better
to accept a degree of instability & irregularity & progressively to
nudge each rower towards your own objective, thus wearing away their
overall instability & giving the whole crew a steadily improving
experience. No one rower's changes can have much effect on the boat,
which makes it that harder for them to learn those changes. So it is
best if everyone can be helped to make their own most important
improvements, which will not be the same as the next person's, which is
why instruction should be individual & low key whenever possible - &
limited to only 1 topic per person per outing. In that way the crew
will feel itself improving together, as the different bumps are ironed
out simultaneously, & each member will have the feeling that it had a
lot to do with changes they themselves made.
Obviously that kind of subtlety is less showy than megaphone
dictatorship, but worth a try.
Charles Carroll
The less the blade slips, the more efficient the stroke!
The less the blade slips, the less energy you throw away!
The less the blade slips, the longer the pull through!
The less the blade slips, the more the boat moves!
Does this remind you of Bourne's famous passage?
"The oarsman should have a clear conception of what is required of him . . .
He should understand, and be quite sure that he does understand, that he is
not so much required to set water in motion and shovel it along past the
side of the boat, as to stick the blade of his oar as firmly as he can into
a given spot in the water and to lift the boat as far and as quickly as
possible past that spot. His attention must be concentrated on moving the
boat, not stirring up water."
I never tire of that passage, although sometimes I find myself wishing that
Bourne had not tried so hard to be poetic. If only Bourne had refrained from
using the word "lift!" Maybe then he would have written the perfect
description of moving a boat with an oar.
Cordially,
Charles
Ps And that's really flogging a dead horse, no?
Charles Carroll
I didn't think of this until after I had sent the first reply, but I have
what may be a really good example of how a more efficient stroke with a
longer pull through results in moving a shell faster through the water.
My friend, David Lay, just purchased an ancient Van Dusen. For the last
month he has been making some minor modifications to it. But the one thing
he hasn't been able to do is change the spread.
For several years David has been sculling with a 158 cm spread, but the Van
Dusen came with a 161 cm spread. The Van Dusen has flat plates that are
attached to the riggers and into which the oarlocks are bolted. To change
the spread you have to unscrew the plates and moved them in or out.
Unfortunately the screws holding the plates to the riggers are frozen. David
has not found a way to loosen them.
For this reason David has adapted to a 161 cm spread and lengthened his oars
accordingly. In conventional thinking isn't this new gearing suited to a
bigger, younger man? But it is what David had to work with, so he had no
choice but to accommodate himself to it.
We were discussing the results yesterday morning. He had just come back from
a row around Angel Island. Was he ever surprised! For the first half, while
the water was flat, he had rowed his best time ever. A significant personal
best!
David attributes this to the slightly larger spread and longer oars, which
force him to have a longer pull through. The longer the pull through, the
longer the blades are in the water, the more you move the boat
To accommodate the longer pull through David said he had to lower his rate.
But this obviously didn't slow him down. When he goes around the island his
rate has been normally 28 to 30. Yesterday morning his rate was 24 to 26.
Interesting?
Cordially,
Charles
Walter Martindale
wrote:
Hi Charles,
I have a copy of Bourne, somewhere, but it's been quite a while since
I opened it.
Horse well killed.
W
Walter Martindale
wrote:
Sounds like dipping the rigger plates into an industrial ultrasound
cleaner with a good detergent, after soaking for a few days in a
penetrating solvent might be called for.
Or talk to Carl about new riggers.
However, I believe that the speed depends to a great extent on the arc
length described by the blade, and the blade movement during the first
part of the stroke while it is travelling end-on through the water.
(boat speed depends on a lot of other factors, too - just as elite
level target pistol shooting requires that the shooter be knee-deep in
brass spent during deliberate practice, elite rowing boat speed
depends on having a heck of a lot of water flow past the fin - also in
deliberate practice...) Yes, moving the boat farther per stroke will
(well, should) make it faster. At some stage, if racing, the stroke
rate needs to go up so that there can be more time spent in the water
propelling the boat, and less time out of the water recovering. If
that increased time in the water is done slowly, you get the "oh my
gosh that's heavy" drive and the "flaming slides" recovery. By having
a long arc with a short overall blade, you can get lots of good blade
action in the water, a fast, dynamic drive, and a controlled
recovery. All the bilge about whether the bottom edge is free of the
water before feathering or not is probably an irrelevant distraction,
but I don't have the research to back that up.
Charles Carroll
I have a friend who says that years ago he found a way of taking care of
frozen screws. He thought it up when he was the "go to guy" at the Rad Lab
(Lawrence Hall of Radiation at UC Berkeley). Actually he was hunting heavy
elements at the time, but he was also very good with his hands and blessed
with loads of imagination and common sense.
He says to get one of those small, hand held torches sold in Gourmet Stores,
the ones used to caramelize sugar on top of a Cr�me Br�l�e. Their flames are
tiny and can be adjusted to be white hot and as narrow as a sewing needle,
and are very good at oxidizing rust and breaking bonds. He claims it worked
for him every time.
Of course dipping the rigger plates into an industrial ultrasound cleaner
with a good detergent, and soaking for a few days in a penetrating solvent
also works. But it is messy and takes longer.
Cordially,
Charles
Carl Douglas
Means of separation for corroded mild steel include immersing in
Phosphoric acid solution (some swear by Coca Cola, which tells you why
it's best avoided!).
Tinus
the water most easily is by applying horizontal force and not
mentioning the blade depth too much or relaxing the vertical motion.
It seams like the blade is already positioned very easily by itself.
If no force is applied it will float in the water just below the
surface.
If forces are applied the change of position of the blade in the water
is partly determined by the direction of the force on the blade (or
alternatively the angle of the blade) and the direction of the force
on the handle (or alternatively the height of the handle influencing
this direction). During the stroke there is some balance between these
forces resulting in the blade traveling in horizontal direction
relative to the water surface and little vertical motion.
If one desires a rower to move the blade deeper trough the water in
the middle of the stroke, then one can instruct the rower to do so.
But, is it also good idea to just alter the angle of the pin a degree
towards the bow side and have the rower maintain the notion of
applying the force in the direction of the stretched arms instead of
trying to actively move the arms up or down.
Carl Douglas
May I gently question your remark about moving the blade through the
water easily, Tinus, although I am sure I partly misinterpret what you
say? The objective in rowing should be to minimise the face-first blade
movement through the water, since that's what generates your losses.
And moving it easily confirms that losses are being made.
Oar makers may or may not mean their blades to float at what is
popularly thought to be the "right" level, but none of them writes to
RSR so we can't say. I would not blame them for working to get that
result, since you'll sell most by supplying what the market thinks it wants.
However, it takes little effort to row a little deeper for part of the
stroke. We do hear arguments that better rowers need less pitch on
their blades, which might be a skewed or accidental reflection of your
point that less pitch will help achieve a deeper stroke.
How the blade depth varies through the stroke can be strongly influenced
by small variations or irregularities in blade shape. I recall an
"interesting" outing when a 2x I was coaching went out with a set of new
blades which had been made by the husband of one member. (I'll give the
short version here). Her partner stopped sculling after a few strokes,
saying she found one blade's depth impossible to control. There was a
bit of an argument on the water. They came back in. They swapped
blades. They came back in, having agreed that that 1 blade was
unrowable. Yet at first sight the 4 blades were identical. Only on
closer inspection could we see that the offending blade had a small
local difference in shape, a warp, from the other 3.
I don't think that changing depths through the stroke is an exercise in
actively moving hands up & down. That sounds like a set of separate
motions which is not how the body or a well-made machine really works.
What we should seek is that the hand path develops into the well-learned
smooth but vertically (slightly) curved line that will achieve the
desired outcome. Thousands of kilometres of good practice is all that
should need....
Tinus
> water easily, Tinus, although I am sure I partly misinterpret what you
> say? The objective in rowing should be to minimise the face-first blade
> movement through the water, since that's what generates your losses.
> And moving it easily confirms that losses are being made.
I meant moving easily as in easy coordination. For a rower it is easy
if the arms are used passively in the early phase of the drive and let
the tension in the arms be the force to move the handles. In that
case, if the arms work as if they are pieces of strings, the force on
the handles is parallel to the arms. It is difficult to apply and
control forces in perpendicular direction. So, it would be a good
thing if blade depth would be controlled "automatically" by geometry
of the blade.
> We do hear arguments that better rowers need less pitch on
> their blades, which might be a skewed or accidental reflection of your
> point that less pitch will help achieve a deeper stroke.
> How the blade depth varies through the stroke can be strongly influenced
> by small variations or irregularities in blade shape.
I wondered if this idea of moving the blade moving deeper trough the
water is an issue of changing a rowers technique or an issue of
changing rigging and blade shapes. It seems to me (assuming the forcec
on the handles to be "simple") as if the changes need to be made by
adjusting the boat+oars instead of adjusting the rower (besides a need
for the rower to have better skills in balance because moving the
blades at the surface allows for better control of balance by using
the blades as if they are training wheels). It would also be nice if
there would be an oar designed to reduce the drag of the oar shaft in
the water.
Carl Douglas
>> May I gently question your remark about moving the blade through the
>> water easily, Tinus, although I am sure I partly misinterpret what you
>> say? The objective in rowing should be to minimise the face-first blade
>> movement through the water, since that's what generates your losses.
>> And moving it easily confirms that losses are being made.
>
> I meant moving easily as in easy coordination. For a rower it is easy
> if the arms are used passively in the early phase of the drive and let
> the tension in the arms be the force to move the handles. In that
> case, if the arms work as if they are pieces of strings, the force on
> the handles is parallel to the arms. It is difficult to apply and
> control forces in perpendicular direction. So, it would be a good
> thing if blade depth would be controlled "automatically" by geometry
> of the blade.
I think it would be wrong to trust in string theory here ;)
I measured some forces before responding:
1. Downforce on the hand of a relaxed arm = 1.05kgf
2. Downforce at handle to balance a sculling oar in air= 0.68kgf
And, not measured:
3. Downforce to slightly cover blade in water, say = -0.2 kgf
(negative, so it takes positive uplift)
So to amply cover the static blade I'd have to provide a positive uplift
of ~1.2kgf, while to keep it on the feather needs ~0.4kgf.
That's the statics done. but how about the dynamic effects? Is your
arm pulling level from the shoulder of is the hand lower than that, in
which case what vertical force component is generated thereby to bury
the blade & how does it vary through the stroke? What's the dynamic
uplift generated by the blade's pitch in its varying hydrodynamic
situations?
And how, then, about the varying, but significant, effects of head &
tail winds on the oar during recovery?
I'd suggest to you that we have evolved to be inherently very good at
ensuring that our hands, regardless of the vertical & lateral forces
imposed on them, follow whatever path we choose. And we're even better
at this after a bit of practice.
>
>> We do hear arguments that better rowers need less pitch on
>> their blades, which might be a skewed or accidental reflection of your
>> point that less pitch will help achieve a deeper stroke.
>
>> How the blade depth varies through the stroke can be strongly influenced
>> by small variations or irregularities in blade shape.
>
> I wondered if this idea of moving the blade moving deeper trough the
> water is an issue of changing a rowers technique or an issue of
> changing rigging and blade shapes. It seems to me (assuming the forcec
> on the handles to be "simple") as if the changes need to be made by
> adjusting the boat+oars instead of adjusting the rower (besides a need
> for the rower to have better skills in balance because moving the
> blades at the surface allows for better control of balance by using
> the blades as if they are training wheels). It would also be nice if
> there would be an oar designed to reduce the drag of the oar shaft in
> the water.
>
So, as indicated above, I think we're well able to make the blade take
whatever path we choose within the water.
Rowers are so good at imagining that they, poor dears, have such limited
control over what they do. To me that's deeply embarrassing -
especially when we see the splendid bodily skills & coordination
routinely demanded & displayed in so many other sports & games. From
the way rowers talk, many would long ago have determined that
ice-dancing was impossible & that no one could, with a flattened stick,
ever hope to accurately hit a ball travelling at IRO 200km/hr. In
reality, the rowing action is so incredibly simple, if subtle, & we get
to repeat it so often, that learning it should present no problems. And
the balance problems we encounter are also trivial compared with those
for gymnasts or K1 paddlers.
I was recently sent some images by Jim Dwyer which, among other things,
neatly confirm, as I've said already, that the shaft does not backwater
during the stroke - even when substantially immersed. That notion which
infests our sport - that "looming" causes backwatering which, suddenly &
mysteriously, imposes deleterious amounts of drag - is, in short, total
hogwash. So perhaps we should not try to solve a non-existent problem
in oarshaft design, but rather we might look at flattening out the loom
so it presents a larger face area to the water during the stroke.
Any comments on the analyses I sent you in my reply, Jim?
In fact, we could even go back to longer, thinner blades ;) That way
we'd get deeper immersion of that upper edge & perhaps a higher total
drag during the mid-stroke stall phase. As I've noted on RSR at other
times, I have a deliberately unmatched pair of sculls: one blade has had
a strip about 25mm deep by 150mm long cut from its upper edge, from the
tip towards the root, so it's quite a lot smaller than its partner but
has its upper edge (over that distance) markedly deeper. Despite the
big area difference, you can use (& many have) those blades without
noticing even the slightest difference in feel or performance between
them. The conclusion has to be that any loss in performance due to the
reduced area is neatly counteracted by an improvement due to the reduced
scope for air entrainment.
As I've said before, we should open our blinkered eyes, make more
experiments, use the evidence everywhere before us, actually study what
we warble on about, take more simple measurements (as above) & be deeply
sceptical of all those popular but unproven coaching mantras & the
authoritative pronouncements of bar-room experts.
Some might call that iconoclasm. Such folk are appalled when I suggest
that rowing ignorantly misinforms its participants about the mechanics
of the stroke. They cling for safety to the bogus "certainties" rammed
down their throats as they learned the sport. Meanwhile, those with
open minds go & do more digging, study, thinking & experiment. Thus it
cost me 10 minutes to supposedly "wreck" a pair of sculls, but I learned
so much & they still work just as well as ever. Even if they had
actually been wrecked, wouldn't the lesson learned have been worth the cost?
Tinus
> tail winds on the oar during recovery?
I am not talking about the recovery but the start of the drive when
arms act to channel the force from the trunk to the handles. Of course
in other parts the handle position is more directly determined by the
vertical forces actively applied by the arms. Just imagine the catch
and the release. Still there are varying effects which do effect the
drive. But, these effects can make rowing feel very awkward. I am used
to row on a river with a lot of current and this doesn't really make
the stroke feel better.
> I'd suggest to you that we have evolved to be inherently very good at
> ensuring that our hands, regardless of the vertical & lateral forces
> imposed on them, follow whatever path we choose. And we're even better
> at this after a bit of practice.
The rower having a certain idea about the path the hands should follow
does not oppose the idea that during the first part of the drive the
arms are used like pieces of string. The way the rower feels the
motion could be a composite of both the rower feeling to move the
hands along a certain path and the rower feeling a balance in the
vertical forces with little effort needed from angular movement/forces
of the arms.
You posted an example yourself in which the rowing didn't feel nice
because a blade wasn't shaped well. If rowers would only feel and
control the path of the handle/hands and not the vertical forces then
this experience with a wrongly shaped blade wouldn't have been felt as
a problem. This example shows that rowers must be feeling the amount
of effort needed to move the blade horizontally or a certain path.
Rowers are very good at feeling the force from the handle. I suggest
that it is best if this effort is as low as possible. On an erg it can
be seen very well. In the first part of the stroke, forces tangent to
the arms are zero and the position of the handle is determined by the
tension in the arms (at least for those who don't flex the arms at the
start). Only in the recovery and the last part of the stroke do rowers
actively control the path of the handle. Also, the harder a rower
pulls the higher the hands are moved.
So, if we want to change the path along which we want a rower to move
the blade (and consequently the path of the handle) should we alter
the rigging and the oar such that this path becomes a path of least
resistance? As in no angular forces needed from the arms but only
tension. You are right that the rower must also reconsider the path
along which the handles move if the blade is to be moved along a
different path.
> In fact, we could even go back to longer, thinner blades ;) That way
> we'd get deeper immersion of that upper edge & perhaps a higher total
> drag during the mid-stroke stall phase.
A longer blade has a fulcrum further away from the blade tip which
increases the amount of slip. That wouldn't be a problem if the blade
wasn't rotating. But since an oar has to rotate in order to apply
force the optimum length of the blade can't be increased much. My idea
about this rowing style in which the blade is immersed more deeply is
that it moves the fulcrum away from the tip which will decrease the
positive effect of the increased drag of the blade. The deeper
immersion should be realised without increasing the length of the
blade. (It is also debatable whether the better rowers which immerse
the blades more deeply are really rowing more efficiently or just need
the extra drag because they are so strong).
Your idea about cutting away the upper edge of the blade is
interesting. If it would be done without reducing the blade area, by
adding the area back at the lower edge, then we might have a super
blade. It does require more vertical space though and the torque/twist
along the axis parallel to the oar shaft would increase. Do you really
believe(/know) these kinds of experiments are not performed?
Tinus
> be seen very well. In the first part of the stroke, forces tangent to
> the arms are zero and the position of the handle is determined by the
> tension in the arms (at least for those who don't flex the arms at the
> start).
There is no edit function here isn't there? tangent should have been
perpendicular.
Carl Douglas
>> And how, then, about the varying, but significant, effects of head &
>> tail winds on the oar during recovery?
>
> I am not talking about the recovery but the start of the drive when
> arms act to channel the force from the trunk to the handles.
I used the reference to the recovery only to illustrate how well we can
cope with all forces that rowing imposes on our arms.
Of course
> in other parts the handle position is more directly determined by the
> vertical forces actively applied by the arms. Just imagine the catch
> and the release. Still there are varying effects which do effect the
> drive. But, these effects can make rowing feel very awkward. I am used
> to row on a river with a lot of current and this doesn't really make
> the stroke feel better.
>
>> I'd suggest to you that we have evolved to be inherently very good at
>> ensuring that our hands, regardless of the vertical & lateral forces
>> imposed on them, follow whatever path we choose. And we're even better
>> at this after a bit of practice.
>
> The rower having a certain idea about the path the hands should follow
> does not oppose the idea that during the first part of the drive the
> arms are used like pieces of string. The way the rower feels the
> motion could be a composite of both the rower feeling to move the
> hands along a certain path and the rower feeling a balance in the
> vertical forces with little effort needed from angular movement/forces
> of the arms.
I am, as you'll have seen, slightly unsympathetic to the view that
rowing is already so perfect that all we do has to feel right to be
right. Rowing is a compromise, an activity for which we were not
designed & for which our equipment is probably imperfectly designed. So
we must be prepared for forces we'd prefer not to have to handle, &
learn to handle them well.
>
> You posted an example yourself in which the rowing didn't feel nice
> because a blade wasn't shaped well.
It was not that it didn't feel right but that a quite subtle error in
shape made that 1 blade respond so differently from its companion,
generating a significantly different profile of vertical forces, that it
made it impossible for its user to row as well as required or to
contribute to the smooth balance of the 2x. You can drive a car with a
flat tyre, but not as fast or as safely as with all tyres working properly.
If rowers would only feel and
> control the path of the handle/hands and not the vertical forces then
> this experience with a wrongly shaped blade wouldn't have been felt as
> a problem. This example shows that rowers must be feeling the amount
> of effort needed to move the blade horizontally or a certain path.
It was not the feeling that mattered; it was the actual imbalance of
vertical forces out at the end of the blades. Except by leaning over,
there was no way to correct that, but leaning as required prevent the
user from working fully or well. So you are drawing inappropriate
conclusions from my example.
> Rowers are very good at feeling the force from the handle. I suggest
> that it is best if this effort is as low as possible. On an erg it can
> be seen very well. In the first part of the stroke, forces tangent to
> the arms are zero and the position of the handle is determined by the
> tension in the arms (at least for those who don't flex the arms at the
> start). Only in the recovery and the last part of the stroke do rowers
> actively control the path of the handle. Also, the harder a rower
> pulls the higher the hands are moved.
>
> So, if we want to change the path along which we want a rower to move
> the blade (and consequently the path of the handle) should we alter
> the rigging and the oar such that this path becomes a path of least
> resistance? As in no angular forces needed from the arms but only
> tension. You are right that the rower must also reconsider the path
> along which the handles move if the blade is to be moved along a
> different path.
As I say, it would be nice if the blades could be forced to follow a
set, hopefully optimal, path. To do that would require additional
machinery which would not go well in this sport. So, if you want to get
the best from yourself you may have to learn to profile stroke depth,
regardless of the (I think minor) vertical forces that requires. I've
indicated how very slight those forces will be, which leaves no grounds
not to apply them. But you are free to row as you wish, even if that
limits your potential performance.
>
>> In fact, we could even go back to longer, thinner blades ;) That way
>> we'd get deeper immersion of that upper edge & perhaps a higher total
>> drag during the mid-stroke stall phase.
>
> A longer blade has a fulcrum further away from the blade tip which
> increases the amount of slip. That wouldn't be a problem if the blade
> wasn't rotating. But since an oar has to rotate in order to apply
> force the optimum length of the blade can't be increased much. My idea
> about this rowing style in which the blade is immersed more deeply is
> that it moves the fulcrum away from the tip which will decrease the
> positive effect of the increased drag of the blade. The deeper
> immersion should be realised without increasing the length of the
> blade. (It is also debatable whether the better rowers which immerse
> the blades more deeply are really rowing more efficiently or just need
> the extra drag because they are so strong).
I didn't say you should, just that you might. You seem not to have
understood that the supposed fulcrum is already inboard of the inner end
of the blade, as I did say. And that fully explains why the shaft of
the oar does not backwater, even if buried for some distance. I can
tell you this, but you are still free to ignore what I tell you, or to
disbelieve not just me but the evidence of most of the solid strokes
that are ever rowed. And by doing so you can generate unsound notions
which try to put the centre of rotation in the water out at the tip
during the mid-stroke stall phase. That would, of course, result in
nearly all of the blade backwatering.......
>
> Your idea about cutting away the upper edge of the blade is
> interesting. If it would be done without reducing the blade area, by
> adding the area back at the lower edge, then we might have a super
> blade. It does require more vertical space though and the torque/twist
> along the axis parallel to the oar shaft would increase. Do you really
> believe(/know) these kinds of experiments are not performed?
If you cut from the top edge & add the same amount to the bottom, how is
that in any way different from rowing the original blade deeper buy that
same amount?
The top part of the blade has so little tension behind it that it makes
no meaningful contribution to the torque generated.
And, no, I have never found anyone who has made, or tried to make, even
so simple experiment as that which I have described. I have met only
with incredulity that I would even think it OK to row with such
seemingly mismatched blades - which tells us plenty about how little is
understood about the way a blade interacts with the water & with its
free surface.
Donal
perhaps there could be an alternate way of looking at this. Washing
out of being locked deep are aspects of the overall gearing giving a
load on the individual. Providing the sculler or crew is consistant in
what happens with the blade throughout the stroke and the gearing is
appropriate is there necessarily any real difference in how this is
achieved? Im not talking about chucking the finish away having been
locked up early in the stroke but some people get away very well with
an element of froth to their stroke...
Donal
Carl Douglas
It is also debatable whether the better rowers which immerse
> the blades more deeply are really rowing more efficiently or just need
> the extra drag because they are so strong).
>
I missed this one first time around - sorry.
We are here discussing the mid-stroke stall zone, so it is fair to talk
in terms of drag & slip
Extra drag is what we all need. Extra drag is what reduces slip, for
all of us. Sure, the stronger guy will get more slip when using the
same size of blade. And I agree that by going deeper will reduce that
slip. But all of us will reduce slip by going deeper. If you are
racing someone stronger than you are, then you have less power & less to
throw away, so you have more need of reducing slip than he does.
Remember that drag, like lift & slip, is something which develops in
direct response to whatever load you apply to the blade & how you use
that blade (including how deep you row it. It is only there in response
to the loading you apply, not something that the water imposes on the
blade regardless of how you pull, & the water never imposes a loading on
the blade except in a direct & exactly equal reaction to the load you
apply to the blade.
Carl Douglas
If you are suggesting, Donal, that washing out is in some way equivalent
to gearing - i.e. more washing out = easier gearing - then you could not
be more wrong. Washing out is like wheel spin, which generates smoke,
noise & heat but does _not_ produce motion.
Donal
> Donal wrote:
> > washing out.....
>
> > perhaps there could be an alternate way of looking at this. Washing
> > out of being locked deep are aspects of the overall gearing giving a
> > load on the individual. Providing the sculler or crew is consistant in
> > what happens with the blade throughout the stroke and the gearing is
> > appropriate is there necessarily any real difference in how this is
> > achieved? Im not talking about chucking the finish away having been
> > locked up early in the stroke but some people get away very well with
> > an element of froth to their stroke...
>
> > Donal
>
> If you are suggesting, Donal, that washing out is in some way equivalent
> to gearing - i.e. more washing out = easier gearing - then you could not
> be more wrong. Washing out is like wheel spin, which generates smoke,
> noise & heat but does _not_ produce motion.
>
> Cheers -
> Carl
>
> --
> Carl Douglas Racing Shells -
> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
> Find: http://tinyurl.com/2tqujf
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Carl - no Im not suggesting that the froth produces motion simply that
providing the stroke is consistant and the blade partially locked that
there is still a level of gearing with the part of the blade enjoying
"lock" beneath the froth - that if the rower is consistant in how he
applies his work can he nevertheless be efficient despite the apparent
visual flaw.
Donal
Zbigniew A.
> If you are suggesting, Donal, that washing out is in some way equivalent
> to gearing - i.e. more washing out = easier gearing - then you could not
> be more wrong. Washing out is like wheel spin, which generates smoke,
> noise & heat but does _not_ produce motion.
Still have a question: If what you say is 100% true then how come that jet
aeroplanes move at all? They have no way to "hook" themselves in air, all
they do is throw lots of air backward, and throw real fast. If drag was so
important for them, all aeroplanes should have propellers as big as wind
mills have.
I am not trying to say that you are wrong, just trying to understand...
--
Yours Virtually, Zibi
Alasdhair Johnston
> aeroplanes move at all? They have no way to "hook" themselves in air, all
> they do is throw lots of air backward, and throw real fast. If drag was so
> important for them, all aeroplanes should have propellers as big as wind
> mills have.
> I am not trying to say that you are wrong, just trying to understand...
Aircraft do indeed propel themselves that way (and they stay up by
effectively throwing a lot of air downwards) That this isn't the most
efficient way of getting around is demonstrated by the fact that up to 47%
of a Boeing 777's take-off weight is the fuel needed to complete its
journey - even the porkiest sculler won't match that ;-)
Carl Douglas
Not a problem, Zibi.
All aircraft are propelled by increasing the rearwards momentum of
imaginary tubes of air. If you ignore the type of engine, whether gas
turbine, reciprocating, Wankel, electric or even a wound-up rubber band,
it is by accelerating that the mass of air flowing in that imaginary
tube that they generate their propulsive force.
It is this momentum change - the mass of air flowing per unit of time
multiplied by its change in velocity - that determines the propulsive
force. However the energy given away, not to be recovered, to achieve
this thrust is the change in kinetic energy of that same air mass.
Momentum is directly proportional to velocity, but KE is proportional to
velocity squared.
If you achieve your required thrust by a 10% increase in velocity of a
large mass flow of air you increase that air mass's KE by a factor of
1.21, so energy absorbed = mass flow. Now suppose for the identical
thrust you double the velocity of a smaller mass flow of air? For the
same thrust you will need only 1.1/2, = 55% as much air flow, but you
will give that reduced flow four times as much total KE.
If you have to give a large flow just a 21% energy boost per unit mass
of air, but a 300% energy boost is required if you reduce that flow to
55%, the energy ratio is
(21 x 1.0) : (300 x .55) = 1 : 7.9
It costs you almost 8x as much energy for the same thrust if you
accelerate only 55% as much air.
Early jet (gas turbine) engines were pretty inefficient - they had high
speed exhaust jets & used a lot of fuel for what they did. But the
relatively high power they could generate made them interesting for
fighter planes while their relatively good altitude performance was a
further attraction.
Modern aircraft gas turbine engines are not usually pure jets, as the
first ones were, but are turbofans. These have huge fans as the first
stages of their intake compressors, & a high proportion of the flow
through this first stage never enters the rest of the engine. Instead
it "by-passes" & flows around the much smaller-diameter outer casing of
the engine core (which remains a pure jet engine). The engine is now
acting as driver of the big fan, & the bypassed air only rejoins the jet
exhaust at the rear, where it also lessens the screaming noise otherwise
generated by the acute shearing of a high-speed jet exhaust slicing into
the outside air. But the most important fact is that, now, a much
larger tube or mass-flow of air is being accelerated to generate the
same thrust, needing proportionately much less acceleration &
disproportionately much less energy.
So, in a sense, aircraft do now have "propellors as big as wind mills" -
see: http://www.grc.nasa.gov/WWW/K-12/airplane/aturbf.html
And another variant on gas turbine propulsion is the turbo-prop, in
which the jet engine drives a much larger diameter propellor. In some
modern transports we see the same principle in use, with the propellors
starting to look a bit more like the first stages of the turbofan engine.
HTH?
Tinus
> > increases the amount of slip. That wouldn't be a problem if the blade
> > wasn't rotating. But since an oar has to rotate in order to apply
> > force the optimum length of the blade can't be increased much. My idea
> > about this rowing style in which the blade is immersed more deeply is
> > that it moves the fulcrum away from the tip which will decrease the
> > positive effect of the increased drag of the blade. The deeper
> > immersion should be realised without increasing the length of the
> > blade. (It is also debatable whether the better rowers which immerse
> > the blades more deeply are really rowing more efficiently or just need
> > the extra drag because they are so strong).
>
> I didn't say you should, just that you might. You seem not to have
> understood that the supposed fulcrum is already inboard of the inner end
> of the blade, as I did say. And that fully explains why the shaft of
> the oar does not backwater, even if buried for some distance. I can
> tell you this, but you are still free to ignore what I tell you, or to
> disbelieve not just me but the evidence of most of the solid strokes
> that are ever rowed. And by doing so you can generate unsound notions
> which try to put the centre of rotation in the water out at the tip
> during the mid-stroke stall phase. That would, of course, result in
> nearly all of the blade backwatering.......
I don't see the back watering of the shaft as a problem but instead
the increased slip of the blade tip. I remember a research in which it
was found that the fulcrum is more towards the outside and slip at the
tip was reduced when the blade was shorter in size. I believe I have
the particular research on paper (I'll search for it) as I can't find
it quickly on the internet. It was pretty old so I agree if it's
findings might already be countered but there is something to say for
caution with simply changing style to have the blade move more deep in
the water just because it increases drag.
> Extra drag is what we all need. Extra drag is what reduces slip, for
> all of us. Sure, the stronger guy will get more slip when using the
> same size of blade. And I agree that by going deeper will reduce that
> slip. But all of us will reduce slip by going deeper. If you are
> racing someone stronger than you are, then you have less power & less to
> throw away, so you have more need of reducing slip than he does.
The same again here. I don't want to say that moving the blades more
deeply is not increasing drag. I just wonder if it will improve speed
for everyone and what the practical implications could be. The idea
about changing the rower style indirectly by or together with rigging
or blade shapes is just an example of such implications. And what are
the moderators of this theory. what situations could increase or
decrease its effect? I was just posing the idea that this rowing style
needs more then different rowing but also different types of oars or
rigging and maybe also other parts of the style should be changed to
get the best out of this technique. I am not opposed to the idea but
was just trying to get some of it's implications clear. The idea that
it increases drag is for me too much theory. It is still questionable
if this increased drag will also result in more speed.
The idea is, as well, that the better rowers do not necessarily show
effective rowing in general but just the most effective way for very
high speed. It could be that the technique turns out to be positive
for those rowers because they need the increased drag for more force
and not for improved efficiency. More force could also be achieved by
changing the ratio inboard/outboard or pulling faster but it might
have reached an optimum for them already. The optimum being determined
because it either decreases amount handle displacement or increases
stroke length. When both of those changes are detrimental the rower
will need to increase force by increasing the drag of the blade. I
believe this is very well comparable with ergometer dragfactor being
related to the rower's physiological preferable mode of movement. An
optimum amount of drag could be calculated for a certain power output
(I wonder why Atkinson finds an optimum for inboard/outboard ratio but
not for blade area). At some point increasing drag won't be positive,
yes it's more effective reducing slip but the rower won't have the
force to move the oar at the required speed for a certain pace and
stroke length. This is a good reason for rowers to work on decreasing
recovery time.
> I am, as you'll have seen, slightly unsympathetic to the view that
> rowing is already so perfect that all we do has to feel right to be
> right.
Maybe I haven't showed it very well but I am in favour of this view as
well. I just wonder if speed automatically increases as the drag of
the blades is being optimised. This change could have effects on other
parts of the stroke as well and there may also be physiological
limits. For instance more vertical displacement of the hands is
needed. Possibly a higher position of the hands in the middle of the
stroke or a lower position at catch and release is physiologically
less favourable. If one is going to increase the vertical movement of
the blade one could just as well try a bigger blade. For a given blade
moving it more deeply might increase it's drag but does this really
mean as well that for a given blade it's drag increases as the area is
decreased from the upper side. Even if this would be true to some
extend than the amount in which one should wish to decrease the blade
area at the top should have some close optimum to the current
situation as the decreased area might increase drag due to increase
bade depth but it would decrease lift forces.
Carl Douglas
Hi Donal -
One question for you:
The making of froth requires the expenditure of energy, & energy spent
on making froth can't be spent on moving the boat. So how can the
making froth be counted as a legitimate or contributory part of a more
efficient stroke?
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Carl Douglas
> Zbigniew A. wrote:
>> Carl Douglas wrote:
> If you achieve your required thrust by a 10% increase in velocity of a
> large mass flow of air you increase that air mass's KE by a factor of
> 1.21, so energy absorbed = mass flow. Now suppose for the identical
> thrust you double the velocity of a smaller mass flow of air? For the
> same thrust you will need only 1.1/2, = 55% as much air flow, but you
> will give that reduced flow four times as much total KE.
Whoops!
Something got edited out in error, & this should have read:
If you achieve your required thrust by a 10% increase in velocity of a
large mass flow of air you increase that air mass's KE by a factor of >
1.21, so energy absorbed = mass flow x (Vout^2 - Vin^2)/2g where Vout &
Vin are outgoing & incoming velocities & g is the acceleration due to
gravity. Now suppose for the identical thrust you double the velocity
of a smaller mass flow of air? For the same thrust you will need only
1.1/2, = 55% as much air flow, but you will give that reduced flow four
times as much total KE.
Cheers -
Christopher Kerr
>
> Still have a question: If what you say is 100% true then how come
that jet
> aeroplanes move at all? They have no way to "hook" themselves in
air, all
> they do is throw lots of air backward, and throw real fast. If drag
was so
> important for them, all aeroplanes should have propellers as big as
wind
> mills have.
> I am not trying to say that you are wrong, just trying to
understand...
>
I'm not an aeronautical engineer, but the way I understand it is that
aircraft engines are not only optimised for efficiency (which would
lead them to be much larger than they are, as you suggest) but also
to minimise noise production and weight and to ensure safety in the
event of an engine failure, birdstrike etc. The nacelle around the
engine blocks out some of the noise (although you might not believe
it) and also acts to contain flying turbine parts if they become
detached from the shaft for whatever reason.
All of these engine components must be built from real materials,
which have finite strength etc, and this limits the size of the
engine. Indeed, if you compare aircraft designs from the 1960s and
today, the fuselage shape doesn't seem (to the casual observer) to
have changed much, but the engines have got continually bigger as
advances in materials technology allow them to be made larger and
still remain safe.
Donal
In itself it wont make for a more efficient stroke as you have
demonstrated it creates energy to create it which is diffused...though
there has also been talk about different levels of drag depending on
the surface(blade or loom) in contact with the water. Its been stated
that the drag is small...but could small froth be less dissipation of
energy than the drag by not having the blade buried so deep?
Personally I think I've always tended to row deeper at the catch than
at the finish rather than at a constant depth which may have its own
set of problems!
Donal
Carl Douglas
Donal -
In the mid-stroke, drag (on the blades) is a good & necessary thing.
It's what resists the force you apply, providing the reaction that moves
the boat. Drag at that stage is all you have & it exactly matches to
force you apply at the blade. This, I suspect, may be the source of the
confusion.
It's hard to grasp that the drag on the blade can only be equal to the
applied load, yet we talk about increasing drag. Clearly that can't be
right! Well, yes & no. Let me try to clarify:
What we're really talking about is not the _amount_ of drag there is (it
matches the force you apply) but how much unnecessary movement of the
blade through the water occurs under that load. You've heard car makers
burble on about drag coefficients? They don't bother to explain that
the drag a car creates at any given speed is proportional to that drag
coefficient multiplied by the frontal area. Thus a car with a small
frontal area but high drag coefficient may generate the same total drag
force as one with more frontal area but lower drag coefficient. With
with oarblades we are in similar territory - we have a given frontal
area & shape, but would like to _increase_ the drag _coefficient_ of
that area.
Why do we want to increase the drag coefficient?
The enemy of efficient rowing is blade slip in the mid-stroke phase -
the blade moves face-first through the water. This slip is all a dead
loss. The force on the blade x the distance slipped equals that the
work you did which did not & could not move the boat. It is the prime
source of loss & wastage from the total work you did in the stroke.
So we want to reduce the slip. How do we do that? Well, we could use
bigger spoons (same drag coefficient but larger area), & no immediate
reason not to except that it gets unweildy.
Now go back to those 2 cars & just suppose they are identical, except
that one has the windows open. They have equal frontal areas, equal
engine power. But wes know that open windows allow air to rush in & out
of the car & screw up its fancy aerodynamics. And we know that the car
with open windows will nolonger be able to match the top speed of the
one with windows closed. For the same motive force, it goes slower.
I.e. for the same force it slips through the air less. In short, it now
has a higher drag coefficient.
If you had that more draggy car out as the end of one oar instead of its
blade (crazy notion, but I use it to make a point so please suspend
disbelief) & the other car in place of the other blade, the one with the
higher drag coefficient would also slip less through the water. Less
energy would then be wasted on slip. So that blade/car would be more
efficient at propelling the boat.
OK? So how do we increase the oarblade's drag coefficient? That's the
whole point of this discussion!
If the water can separate from the back of the blade (& remember that
blades _don't_ work by pushing water but by the tension created & held
between the covered back of the blade & the water directly behind it),
the drag coefficient falls right down. So you get lots of slip, a
shorter stroke, thus a shorter pulling period - & much more energy
dissipation as movement through the water causes froth, the swirling
puddle & other monuments built of wasted energy. And this lowers the
blade's drag coefficient.
If you now bury deeper, so air can't creep down behind, you retain full
connection with the water behind the blade. Slip is much reduced, so
the energy losses are also reduced. And this causes an increase in drag
coefficient.
Previously we'd been talking of increasing blade drag when, sometimes,
we meant we were increasing the drag coefficient. But the 2 do come
together when the blades go deeper - they slip less due to resulting
higher drag coefficient & take longer through the stroke. Because they
take longer, we may be tempted to pull harder. Then we do feel that the
blade is imposing a load on us which, as you see, is not the case.
Any help in that?
Cheers
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Tinus
> the increased slip of the blade tip. I remember a research in which it
> was found that the fulcrum is more towards the outside and slip at the
> tip was reduced when the blade was shorter in size. I believe I have
> the particular research on paper (I'll search for it) as I can't find
> it quickly on the internet.
It was from a 1957 paper by Gutschow which was in it's turn mentioned
by Volkert Nolte in 'Die effektivitat des Ruderschlages'
"Wahrend der Schlupfbewegung der Blatkante um etwa 65 cm heckwarts
bewegt sich der Hals um etwa 10 cm bugwarts ... Die
dazwischenliegenden Teile der Blattes bewegen sich daher jenach ihrer
lage hechwarts oder bugwarts und bilden entweder fordernden Schub oder
hemmenden Wasserwiderstand. Daraus folgt fur den, der die korperliche
Arbeit gut ausnutzen will, dass der Blattlange gewisse Grenzen gezogen
sind, und zwar gehort zu einem geringen Schlupfe ein entsprechend
kurzeres Blatt ..."
This was in a time when blades were shaped very thin and long but
bugwarts movement is also noted today:
http://www.phys.washington.edu/users/jeff/courses/ken_young_webs/208A/scull.lift.html
Carl Douglas
You will always get a tendency for the blade to leave the water ahead of
its entry point, whatever its immersed length.
During the early phase of the stroke, the blade is angled forwards &
driven both forwards & outwards by the motion of the boat, so it loops
in a curving path both forwards & outwards. The blade "flies" well
through the water & has ample area & hydrodynamic lift in this situation
to sustain its path under heavy loading, so does not move backwards.
During the mid-stroke (stalled) phase the blade moves back, in the
oposite direction to the boat's motion, becasue there it is slipping or
stalling, just as a parchute moves down through the air. This is the
region in which we need to maximise resistance to slip by increasing the
blade's drag coefficient (see my response to Donal). The only way to do
this is to bury it deeper, which prevents aeration of the back of the
blade & ensures better maintenance of tension between the back of the
blade & the water.
And during the last phase of the stroke, the motion of the boat together
with the rotation of the oar about the moving pin again act to move the
blade forward through the water, while the load on teh blade is well
supported by hydrodynamic lift.
In the early & late phases, as long as it is adequately covered, the
blade is more than large enough to sustain the loads applied without the
slightest risk of stall or appearing to slip trough the water becasue
the flow that there passes along its length maintains the necessary
conditions for any load to be countered by hydrodynamic lift, which
disributes that load across larger & continually changing masses of
water. In the midstroke there is nolonger any lengthwise flow along the
blade, which is why it stalls & why we then should take it deeper to
maximise its stalled drag coefficient.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Donal
Thankyou..for me that comes across much better.
Regards
Donal
> Email: c...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Tinus
> This is the region in which we need to maximise resistance to slip by increasing the
> blade's drag coefficient (see my response to Donal). The only way to do
> this is to bury it deeper, which prevents aeration of the back of the
> blade & ensures better maintenance of tension between the back of the
> blade & the water.
But isn't the amount of slip also determined by the geometrical shape
of the blade? In a uniform field the drag might be improved by making
the blade longer or placing it deeper under the watersurface but the
movement of the water around the blade isn't uniform in rowing as the
blade rotates as well. The speed of the blade tip is different from
the speed of the blade base. When the blade length is increased (for
instance by using the shaft as part of the blade) this difference will
increase. At some point slip won't decrease by increasing the length
or the depth of the blade because the difference in speed of blade tip
vs blade base becomes larger. The average slip of the entire blade can
never be zero as the blade does not move uniformly and zero slip for
every part of the blade can not be obtained.
The measurements by Ken Young show a speed of the blade tip of 1 m/s
(30cm slip in 0.3 seconds) during the middle phase of the stroke. The
oarlock moves at about 4 m/s. So the point of rotation is at about 40
cm from the tip (assuming a 200cm outboard). This means that for that
particular situation the slip of the blade at the tip can't be much
decreased by increasing drag alone. That is because the slip at the
base of the blade is already zero (or even negative as the blade is
slightly larger than 40 cm). Without changing the dimension of the
blade decreasing the slip at the tip would mean that the base has to
move backwards and is not contributing in positive work.
Carl Douglas
I would be cautious about basing an argument on 1 source. Ken Young's
work was a big eye-opener for those thoughtful rowers who'd not
previously thought too hard previously about blade mechanics, but his
overhead photo-sequence taken from a bridge should perhaps be taken a
qualitative rather than quantitative data. Walter makes similar points
in discussing his own excellent work derived from data taken from film
sequences.
Please consider that 30cm slip which you detect at the tip, timed over
0.3 seconds, in that film sequence. Was it all at a uniform rate?
Surely not. Like all fluid flow processes, indeed all dynamic
processes, that slip will have built up to a maximum rate & then fallen.
So we cannot average that process over that 0.3 seconds, corresponding
to the middle 1/3 of the stroke - the period during which stall occurs,
with aeration & consequent sudden slip.
Notwithstanding the late & great Ken Young's work, there's lots of
evidence that strong scullers maintain significant sternwards slip of
the immersed part of their oar-shaft, & we are talking about going fast
& how to go faster, I think.
You appear to propose that, if there is a non-uniform flow-field, due to
rotation of the oar among other many factors, we would not want to
increase the drag coefficient of the blade. I would respond that we may
very well want to improve the blade shape once we get it down to
sufficient depth to increase its stalled drag coefficient, but I can see
no reason to suppose that enhancing drag coefficient without changing
blade shape would be disadvantageous. I guess it is all a matter of
degree. Please remember that throughout the stroke the flow regime is
continually changing, & we move from an engagement with the water based
very efficiently upon hydrodynamic lift, through the stall process in
mid-stroke where drag is everything & I can see neither theoretical nor
evidential argument for not increasing depth to enhance drag coefficient
- the turbulent, more-aerated puddle & the highly visible slip are
compelling physical evidence of the losses incurred by the more shallow
blade, not to mention the longer stroke duration that accompanies a
slightly deeper stroke.
So I am drawn to think that you are clutching at straws, but happy to
discuss the issue further.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Tinus
> to the middle 1/3 of the stroke - the period during which stall occurs,
> with aeration & consequent sudden slip.
I don't believe this is a big problem. An average speed of 1 m/s for
the 0.3 seconds in which the blade tip stalls must mean that at least
for some parts of the stroke the speed of the tip is minimally 1 m/s.
> Notwithstanding the late & great Ken Young's work, there's lots of
> evidence that strong scullers maintain significant sternwards slip of
> the immersed part of their oar-shaft, & we are talking about going fast
> & how to go faster, I think.
My argument based on one example wasn't very strong but it did show
some a moderator which is important to take into consideration. The
amount in which the blade length can be increased or the shaft can be
immersed is limited.
> You appear to propose that, if there is a non-uniform flow-field, due to
> rotation of the oar among other many factors, we would not want to
> increase the drag coefficient of the blade.
In the before mentioned example the blade turns about 45 degrees
during the period that the blade stalls (I believe it is not wrong to
assume this is also the case in general.). In the example the blade
tip and the base of the blade moved about 33cm relative to each other
during this period. This means that no matter what the drag is, the
blade tip must either slip or either part of the blade must move
backwards. The longer the blade the larger this effect. So possibly
some rowers may still improve increasing the drag by adjustments which
increase the length of the blade (especially the gifted ones but we
are not all like that) but it is limited and for some it won't help. I
don't say we should not ultimately try to increase the drag
coefficient. I just say that it might have no effect when it would
mean that the blade length increases.
I have been under the impression that for many rowers the point of
rotation is close to the blade. But this is not based on scientific
measurements so there could be some corrections here. However I
believe that even if parts of the blades don't move backwards it would
still be better to decrease the length of the blade in order to
decrease slip. Not that increasing the length of the blade wouldn't
improve the drag and reduce the slip in this case but doing it while
keeping the blade short would be better. For two blades which slips
some average amount the one which is shorter has less spread.
So there should be made some oar with shorter blades which are to be
moved deeper under water. Ideally they should have a shaft which
reduces drag while under water. Funny thing is that they have been
building oar shafts which do the opposite: flattened ends which reduce
resistance while in air and increase drag while under water. I believe
that it should only be logical to see blades becoming shorter while
technique finds ways to increase the dragcoefficients. Making the
blades shorter is an integral part of the increased drag-coefficient
to have an optimal effect. As at the current time the big blades are
optimal size, this same size can't be optimal for future blade shapes.
Changing one part is almost always changing other parts.
Tinus
> compelling physical evidence of the losses incurred by the more shallow
> blade, not to mention the longer stroke duration that accompanies a
> slightly deeper stroke.
These are yet only indicators. The longer duration does not exclude
the possibility that part of the blade moves towards bow while other
parts of the blade move towards stern. This would still allow for less
average slip and the resistance due to the bowards movement could
reduce speed and duration as well. The aerated puddles are evidence of
high losses of energy but if our blade moving under water would behave
inefficiently how would it look like?
Carl Douglas
>> So we cannot average that process over that 0.3 seconds, corresponding
>> to the middle 1/3 of the stroke - the period during which stall occurs,
>> with aeration & consequent sudden slip.
>
> I don't believe this is a big problem. An average speed of 1 m/s for
> the 0.3 seconds in which the blade tip stalls must mean that at least
> for some parts of the stroke the speed of the tip is minimally 1 m/s.
At that stage you already have a rotating flow field developing around
the blade - it can't be otherwise in view of the rotational input to
which you refer & which I don't dispute.
>
>> Notwithstanding the late & great Ken Young's work, there's lots of
>> evidence that strong scullers maintain significant sternwards slip of
>> the immersed part of their oar-shaft, & we are talking about going fast
>> & how to go faster, I think.
>
> My argument based on one example wasn't very strong but it did show
> some a moderator which is important to take into consideration. The
> amount in which the blade length can be increased or the shaft can be
> immersed is limited.
When I mentioned the longer & narrower blade I was not seriously
suggesting that we should go back to such blades (although the lack of
evidence against them is intriguing), but that they might prove no less
efficient because their outboard end was better buried than that of a
hatchet.
How an oar acts at various parts of the stroke is a function of many
variables, but I would agree with you that it is likely that a shorter
blade will be more efficient.
>
>> You appear to propose that, if there is a non-uniform flow-field, due to
>> rotation of the oar among other many factors, we would not want to
>> increase the drag coefficient of the blade.
>
> In the before mentioned example the blade turns about 45 degrees
> during the period that the blade stalls (I believe it is not wrong to
> assume this is also the case in general.). In the example the blade
> tip and the base of the blade moved about 33cm relative to each other
> during this period. This means that no matter what the drag is, the
> blade tip must either slip or either part of the blade must move
> backwards. The longer the blade the larger this effect. So possibly
> some rowers may still improve increasing the drag by adjustments which
> increase the length of the blade (especially the gifted ones but we
> are not all like that) but it is limited and for some it won't help. I
> don't say we should not ultimately try to increase the drag
> coefficient. I just say that it might have no effect when it would
> mean that the blade length increases.
As I said above, there are many reasons to believe that a shorter blade
should be better. At the catch & finish stages it seems self-evident
that a shorter but wider blade could have advantages, since the
extremely low aspect ratio of even a typical hatchet blade is wholly
inappropriate to any normal hydrofoil. This is to some degree supported
by the fairly competent performance of an experimental delta-shaped
blade design of some years ago in the UK, called the "Feathor", & the
similar Alden "Deltor" - although this introduces the somewhat different
fluid dynamics of the delta wing into this discussion....
What is against the shorter, wider blade is that you might never get
users to bury it enough to get the best out of it. I wonder?
>
> I have been under the impression that for many rowers the point of
> rotation is close to the blade. But this is not based on scientific
> measurements so there could be some corrections here. However I
> believe that even if parts of the blades don't move backwards it would
> still be better to decrease the length of the blade in order to
> decrease slip. Not that increasing the length of the blade wouldn't
> improve the drag and reduce the slip in this case but doing it while
> keeping the blade short would be better. For two blades which slips
> some average amount the one which is shorter has less spread.
>
> So there should be made some oar with shorter blades which are to be
> moved deeper under water. Ideally they should have a shaft which
> reduces drag while under water. Funny thing is that they have been
> building oar shafts which do the opposite: flattened ends which reduce
> resistance while in air and increase drag while under water. I believe
> that it should only be logical to see blades becoming shorter while
> technique finds ways to increase the dragcoefficients. Making the
> blades shorter is an integral part of the increased drag-coefficient
> to have an optimal effect. As at the current time the big blades are
> optimal size, this same size can't be optimal for future blade shapes.
> Changing one part is almost always changing other parts.
As you can see, there is less that divides us than that which unites us.
However, I really would like to see oar-shafts create less wind
resistance.
On that last point: it is perfectly possible to markedly reduce the
wind resistance of the circular-section oar shaft. It is found in all
good texts on fluid dynamics. It is used elsewhere. And it was
successfully applied by the GBR M8+ in Sydney 2000. It has a
predictable effect in reducing wind drag without in any way changing teh
oar shaft itself. And it is dirt cheap to implement. But rowing is so
remarkably ignorant of fluid dynamics, & is so sure that the only way to
go faster is to pull harder (regardless of blade depth, steering
hydrodynamics or the overall aerodynamics of a rowing shell & its crew)
that the necessary measures were rarely adopted in the years immediately
following 2000 & are never seen now. How sad & how silly!
Walter Martindale
(snip)
>
> On that last point: it is perfectly possible to markedly reduce the
> wind resistance of the circular-section oar shaft. It is found in all
> good texts on fluid dynamics. It is used elsewhere. And it was
> successfully applied by the GBR M8+ in Sydney 2000. It has a
> predictable effect in reducing wind drag without in any way changing teh
> oar shaft itself. And it is dirt cheap to implement. But rowing is so
> remarkably ignorant of fluid dynamics, & is so sure that the only way to
> go faster is to pull harder (regardless of blade depth, steering
> hydrodynamics or the overall aerodynamics of a rowing shell & its crew)
> that the necessary measures were rarely adopted in the years immediately
> following 2000 & are never seen now. How sad & how silly!
>
> Cheers -
> Carl
>
Hi Carl,
Ok... I'll ask... I just watched the M8+ final from Sydney. Sure, the
POMs won - rowing well, deep strokes, well timed catches, synchronised
crew. We've heard before about them using your riggers - I've always
wondered - are they actually going fast enough for a significant
difference, in air resistance, to be made (in the very nearly still
conditions of Sydney that day) by varying the riggers?
However - the blades - they look like "bog standard" Croker racing
oars... I'm not up to reading about fluid dynamics - my little old
brain doesn't like the math any more (it was never really fond of it,
but now it just says nighty-night).
What _can_ you do with a circular section oar shaft to reduce wind
resistance without modifying the shaft? Sorry I'm so thick.
Walter
Carl Douglas
Not thick at all, Walter! You can't see it because the modification is
physically very small in 2 of its 3 dimensions.
When a fluid flows perpendicularly across a cylinder it must split at
the stagnation point to flow equally around both sides. That results
immediately afterwards in a compression of the streamlines (the same
flow, close to the cylinder, has to pass through less space) & a
corresponding increase in velocity as the flow moves towards the widest
part of the cylinder, and a corresponding fall in its internal (static)
pressure.
If we ignore fluid friction, for minimum drag we want there to be a full
pressure recovery & smooth deceleration in the fluid in contact with the
cylinder as the flow continues to its rear side & beyond. In short, we
get the best result if the streamlines look the same both ahead of &
behind the widest point.
At very low flow velocities (actually, at low Reynolds numbers), you
will get that kind of flow - like treacle flowing around a stick, but
note that when you stir a thick fluid you still get a free surface
effect, whereby the surface level drops behind the stick just as it does
behind an oar blade.
As you increase velocity (Re), you reach the point at which the fluid
immediately adjacent to the surface (the boundary layer) has been slowed
enough by friction against the cylinder to lack the kinetic energy to
make it flow along the surface & turn down around the back face of the
cylinder. Instead it is torn off in shreds or vortices, & that
separation process not only prevents the flow from re-closing smoothly
around the cylinder, but also soaks up lots of energy & creates a messy,
turbulent wake. That wake is not a hole as such - it is fluid filled &
full of random energy stolen from the main flow - but it acts very much
as if it were a hole, & prevents the desired pressure recovery behind
teh cylinder. So the drag jumps spectacularly. And this is form drag,
not frictional drag.
As you further increase the velocity, you then get into a regime in
which an oscillation develops. Instead of the flow breaking up messily
behind the cylinder, it breaks away first on one side & then on the
other, shedding large, discrete vortices. You can see this effect
clearly if you drag a cylindrical object, held vertically, at the right
speed through water - those big vortices drop off the back, first one
side, then t'other, those on one side spinning in the opposite direction
to those on the other. This, called a von Karmann vortex street,
imposes real lateral forces, can make struts & wires hum in an air or
water flow & create oscillatory forces causing real structural problems
(you can accurately predict the oscillatory frequency), e.g. for tall
stacks, & flutter or buffeting in other objects. And it associated with
much increased fluid drag (the drag coefficient jumps).
At still higher velocities there is enough energy in the flow to improve
the re-closing of the streamlines, which reduces the drag coefficient
(but not the total drag, since that is proportional to V^2).
There's a way to mitigate this separation-induced loss, & that involves
what's called re-energising the boundary layer. The idea is to disturb
the flow, right near the surface, some way before you reach the thickest
part of the cylinder. At that point there is still a good & increasing
flow velocity close to the surface (the boundary layer is quite thin,
but much slower than the undisturbed stream). If you poke something up
from the surface in the right way, at the right point, you can induce a
temporary flow separation, followed by an immediate re-attachment of a
more energetic flow from the free stream. This more energetic (faster)
flow has enough the energy to carry the boundary layer a considerable
distance on around the back of the cylinder before it, to separates.
The result is that the area of the back of the cylinder over which the
flow has detached, & the scale, volume & energy content of the vortices
shed over that region is also reduced. So there is greater pressure
recovery there & the drag coefficient of the cylinder is reduced.
This can achieved by a variety of different but small-scale (in
proportion to the cylinder) devices. The simplest is a pair of
longitudinal ribs, maybe only 1-1.5mm deep, running along the oarshaft
in the appropriate positions (getting the positions right is most
important). Stacks are sometimes stabilised by winding spiral strakes
around them - they have to be spiral 'cos you don't know where the wind
will come from at any time, so it is a compromise, but it helps break
the tendency to form a vortex street.
In the critical flow regimes you might, by this means look to reduce the
oar-shafts' drag coefficient by a good 50%. Working on the back of an
envelope, so I am open to correction, that could amount to ~ 70 watts of
power saved when going into in a stiff headwind (10m/s), assuming the
oar shafts move forwards at around 8m/s relative to the water for around
50% of the time with an eight moving at a bit under 6m/sec (slow due to
the headwind). If you correct that 70 watts up to 100 watts, since the
propulsive work you achieve is ~70% of the actual work you do, due to
losses at the blade, and you can see that we are talking about a small
but not-insignificant proportion of the crew's total output.
Walter Martindale
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
So... A spiral wire down the shaft? A couple of wires, one on top,
one on bottom, of the shaft when the blade is feathered? Some degrees
off top/bottom?
W.
Carl Douglas
2 wires, or better still a pair of thick but narrow tapes. I'll reply
to your earlier email with better details when I've a moment.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Kit
Hmmm. Not sure we want those Kiwis being any quicker than they already
are !!
Kit
Steven M-M
a visualization: http://www.youtube.com/watch?v=_AJgEa2dbJU
Walter Martindale
>
> Hmmm. Not sure we want those Kiwis being any quicker than they already
> are !!
> Kit
I'm just a high school coach.
W
Walter Martindale
> a visualization:http://www.youtube.com/watch?v=_AJgEa2dbJU
That's pretty cool. Have they done these studies (and recorded and
uploaded them) with the bits of tape or whatever that Carl's talking
about?
W
Tinus
> being entrained behind it. That entrained air, by forming a cavity
> behind the loaded blade, drastically cuts the essential _tensile_
> connection between blade & solid water (it's the back, not the face, of
> the blade which transmits the propulsive forces into the water). Thus,
> up to a surprisingly great depth, the deeper the blade goes the greater
> its efficiency becomes
>
> Since going deeper that the popular prescription continues to increase
> blade efficiency, & since it only slowly, if at all, incurs any penalty
> from shaft back-watering, a fairly strong case can be made for rowing
> deeper than conventional wisdom suggests. And, FWIW, study of blade
> depths of the more successful rowers & crews tends to support that case.
> If the water can separate from the back of the blade (& remember that
> blades _don't_ work by pushing water but by the tension created & held
> between the covered back of the blade & the water directly behind it),
> the drag coefficient falls right down. So you get lots of slip, a
> shorter stroke, thus a shorter pulling period - & much more energy
> dissipation as movement through the water causes froth, the swirling
> puddle & other monuments built of wasted energy. And this lowers the
> blade's drag coefficient.
I appreciate the idea of the influence of the separation of the water
from the back of the blade. However, I have another issue which might
influence the way and how much we want to move the blade below the
water surface. The influence of wave drag is such that the drag
coefficient of the blade is actually higher if the blade is moved
closer to the surface. There must be some ideal depth instead of the
case in which continuously going deeper continuously improves the drag
coefficient. The most advantage is probably made by only slightly
submerging the blade, making the best effect of the layer of water at
the back of the blade and the wave drag.
'the magnitude of the drag coefficients for the surface-piercing
plates was greater than for published data on fully submerged plates'
In order to determine the effect of depth on the drag coefficient I
tried an experiment today but it failed because it was only
qualitative and I had some disturbing factors. Maybe I'll try a better
quantitative experiment next week.
Carl Douglas
It is very difficult, reading only the abstract (& I won't pay >$30 for
the PDF - an outrageous rip-off), to understand what that paper is
saying. They appear to be studying extremely slow drift rates of keels.
Normally keels are attached to boats, which effectively fence them
from the free surface. I'm not sure if & how they are testing plates
which emerge through the surface. The abstract tells us nothing of the
test protocols. And it is worth remembering that we are discussing the
consequences of aeration behind a loaded but relatively shallow
oarblade, not the slow drift of a yacht keel which normally has a far
higher high aspect ratio. When keels do aerate, you sure know all about
it. If you have a full copy that you can send me, Tinus, I'll read it
with interest.
Please also bear in mind that experiments which deal with a steady-state
flow regime do not simulate any part of a rowing stroke
I'll await the results of your experiment. Meanwhile you might care to
repeat one of my own experiments - cut a 25mm strip from the top edge of
one blade of a pair of sculls & then go sculling using the same amount
of shaft coverage on both. Does the now smaller blade in any way affect
how the boat goes or how the blade responds? No one who has used my
butchered pair can tell any difference.
While I am sure that there will be an optimum depth at which to row the
blade, & that this optimum depth must vary continuously through the
stroke, according to boat speed, according to load, according to blade
area, shape, etc., I see & have found no grounds to suggest that the
kind of coverage you can get by rowing what others would consider to be
"deep" (e.g. 30 cm of shaft immersion) takes you past the optimum.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Tinus
> repeat one of my own experiments - cut a 25mm strip from the top edge of
> one blade of a pair of sculls & then go sculling using the same amount
> of shaft coverage on both.
I am missing the funding for this project. I don't even own a pair of
sculls of my own. Maybe I'll try some trics with blades if I ever come
across some old stuff which I dare to abuse.
Walter Martindale
I see & have found no grounds to suggest that the
> kind of coverage you can get by rowing what others would consider to be
> "deep" (e.g. 30 cm of shaft immersion) takes you past the optimum.
>
> Cheers -
> Carl
30 cm? That's practically washing out...
Walter
Carl Douglas
LOL!
BTW, Walter, the answer to your question is: + & - 70 degrees