What is the best blade depth?

[Charles Carroll]

Dear all,

I want to thank Walter Martindale who in another thread wrote:

“Classic coaching has the blade depth with the top edge just under the
surface, with the ‘pocket’ developing at the back of the oar to ease
extraction. Sliasas et al., have done modelling on this and published it...
I asked Andrew S. (a rower and coach) if he'd considered checking the
modelling software for blade fluid dynamics with different depths, and I
cited conversations I'd had with Valery Kleshnev about blade depth (1/2 a
bladewidth of water above the blade is best) - he ran the models that
afternoon and came back to me with ‘wow, it's more efficient to be deeper - but all the coaching manuals still say ‘keep the blade at the surface’.”

For years I have been trying to immerse my blades a little deeper. But I
have never been clear about how deep, that is, until I read Walter’s
paragraph.

So yet again, Walter, you have improved my sculling immeasurably. And for
this I am immeasurably grateful.

Warmest regards,

Charles

[Tinus]

The models (allowing free surface effects and adjustment of blade depth)
and/or the computer code have never been published and it is unclear how
much accurate the model is or how possible variations between model and
reality may arise.

I'd go with Kleshnev's experimental data and go for 1/2 blade depth.

[Walter Martindale]

Um... That's what Charles says that he did... 1/2 blade width (or
about 10 cm) of water above top edge of blade... Approximately, of
course, because it's difficult to assess just how deep your blades
are..
I'm sure that some day the models might get published, but .. maybe
only after an Olympiad? There may be some contractual requirements
for time to pass before publication of what might be proprietary info
(but I don't know that - I'm speculating).
W

[Tinus]

On 02/01/2012 11:26 AM, Walter Martindale wrote:
> I'm sure that some day the models might get published, but .. maybe
> only after an Olympiad? There may be some contractual requirements
> for time to pass before publication of what might be proprietary info
> (but I don't know that - I'm speculating).
> W

Aha of course. And in the meantime you are a decoy bringing out the idea
that maybe we should put the blades deeper than 1/2 blade depth.

[Walter Martindale]

Who knows? I don't think I'm being a decoy, but consider - if
everyone thinks 'top edge just below the surface' is right, and nobody
actually does any research, who knows what's the best?
Modelling (nowadays) might provide some pretty good estimates of what
will happen - I believe that once Andrew gets settled in his new place
he's going to continue looking, but - what's stopping people from
trying to see what works best? Should this be done at the
intermediate level with people who might be thinking Rio (it might be
a bit late for major changes for London)? Should people think - "Oh,
I thought the looming would be a problem" and not research it, or
should they think "how much positive slip do I get at what depth, and
how much looming do I get - then - where's the crossover between
gaining advantage from being a little deeper and having too much added
drag from the looming?"
The research is fairly easy to describe, but a nasty one to do..
Anyway - gotta run.

[Roger]

On Jan 31, 12:45 pm, "Charles Carroll" <charles_carr...@comcast.net>
wrote:

This is something I've not really looked at much and really don't
understand why deeper would be better. I, along with many others was
taught that blades should be just below the surface and that blades
were designed with enough buoyancy to allow them to sit there during
the drive, deeper was inefficient. I guess my question is "how do
blade designers design their blades for use, at the surface or below,
surely they are the ones that know how their equipment will work or am
I just another marketing victim?

Roger

[Walter Martindale]

Fiddler on the roof... What was that song? Oh, yes, "Traditions"....

In the realm of "still figuring out why a little deeper is more
effective" -
If you define "positive slip" as the blade going through the water in
the direction of the finish line early in the stroke and "negative
slip" as the blade going through the water in the direction of the
start line, it may be that, because there's a little more mass above
the blade if it's a little deeper, the water in the concave surface of
the blade doesn't move out of the way (i.e., no pocket formed behind
the blade until very late in the stroke) as much if it's right at the
surface, so the blade slip aft less during the drive (later phases of
the stroke) which results in the boat moving more toward the finish
line during the drive. If it's only a centimetre, it adds up to 2.4 m
at the end of a race.

Does the little bit more propulsion outweigh the little bit more
'looming' from having more shaft in the water?

It was an interesting presentation by Andrew at our national coaches
conference last weekend, but there was some information he was not at
liberty to provide - property of the people with whom he's doing the
research (again, that probably explains why some of it isn't
published, but there's also the consideration that it's all very new -
he only ran the modeling software on the deeper blade about 3 weeks
ago, and has since changed residence by about 500 km... Not the best
environment to write up a research paper, and THOSE take more than a
year from submission to the print version...
Cheers,
W

[Carl]

You can't & won't be putting your blade to any given or arbitrary fixed
depth. Blade depth inevitably follows a continuous curve. It should
vary through the stroke &, obviously, it cannot change stepwise. The
question is: how should its depth should vary through the stroke? It
has to go in from zero immersion and, equally, must leave at zero
immersion, so what should it do in between?

We rowers always want hard & fast answers, but the fluid dynamics of the
oar are never that simple, & nor are the rowers. Anyway, so many rowers
seem incapable of controlling their blade depths. Too often you see
blades dig in at the catch, bob up in the middle, dip down again before,
finally, washing out. Depth control is not that easy.

That said, in an athletic pursuit there's not reason why doing it right
should be easy. So don't expect the best rowing to be just about
sticking it & pulling hard?

We had a discussion here (maybe a couple of years back?) on blade depth.
There I described the sort of stroke depth profile one might best use
with a conventional sweep-type oar (by which I mean the kind we use in
racing shells for sculling & rowing).

Let's first remove a couple of old myths. Both are appealing. Both are
irrational:
1. That getting a bit of the shaft in the water (looming) is
automatically harmful.
No it isn't. What matters is a) whether there is any significant drag
on the immersed shaft & b) whether that drag is greater than the
performance benefit from the deeper blade working more efficiently.
Doubtless you could go too deep - all good things must come to an end -
but it has long been clear that a deeper blade, especially in the
mid-stroke, especially if being pulled by a gorilla, works more
efficiently, while the drag of the first bit of the shaft is rather slight.

2. That the depth at which a blade floats is also the right depth at
which to row it.
No it isn't. It is certainly handy, when not actually rowing, if the
blade sits near the surface but that's no reason to row it at that
depth. It's a tool whose resting position has relatively little to do
with how you should use it.

The depth to bury an oar requires a clear understanding of how the blade
functions at the different parts of the stroke. This a highly complex &
very little studied topic. Hence the widespread application of the KISS
approach, leading to general but misguided disapproval of the deeper stroke.

Since the blade functions first as a foil, then as a crude baffle &
finally as a foil once more, there are many ways to use it & many
different processes to consider:

Take the mid-stroke, which by popular misapprehension is thought to be
the best place to do your work:
Here the blade is stalled, dragging face-first through the small pocket
of immediately surrounding water which it is progressively decoupling,
stirring up & accelerating away from the larger body water. Everything
you can do to reduce that decoupling tendency improves propulsive
efficiency there. And the only way to do that is to go deeper, so that
the pocket surrounding the blade extends above as well as below & around
the blade, making it bigger, & so that air cannot get sucked down from
the surface into the low-pressure region behind the blade which, if not
aerated, provides its main connection to the water.

At the catch the blade, as you make the catch, initially slices both
down & outwards into the oncoming water & if its path relative to the
water gives it an appropriate speed of entry & angle of attack (in the
net direction of that entry), then the compound curvature of the back of
the blade will induce very efficient lift.

Once entry is accomplished, the vertical element of the blade's motion
is initially less important (but not unimportant). At first, if the
blade is covered by a little bit, water flows fast enough along it for
its surface level not to fall below the top edge (under the suction
generated by lift) before moving beyond the blade's root.

As the blade swings towards mid-stroke, the velocity of flow along it
falls but the load you impose on the blade maximises, so the blade need
to go deeper. And by mid-stroke, as already discussed, depth should be
greatest. This matters most for the big pullers since they generate the
highest lift forces & lowest pressures behind the blade (since for some
odd reason we all use pretty much the same blade sizes - funny, that!),
so they should go deeper still.

Since at mid-stroke the blade has stalled & is slipping sternwards, not
only is the shaft unlikely to cause adverse drag but, for a short
distance, will even supplement the useful area of the blade, thus
reducing slip. This won't amount to much, but nor will it hold the boat
back.

And as you approach the finish the flows re-establish along the blade &,
accordingly, the amount of water needed above the top of the blade can
progressively reduce.

Around the finish the blade should _not_ emerge square. Of course, that
is regarded by many as pure heresy, so I'll sit tight and wait to
respond to challenges. Enough for now!

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
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

[Ross Horn]

"Of course, that is regarded by many as pure heresy, so I'll sit tight
and wait to respond to challenges."

I kind of feel like a contestant on QI about to be hit with a huge
blast of wisdom, but hey ho I'll bite.

Surely, extracting the blade square is the best option in terms of
power application and boat balance? A slightly feathered blade would
reduce the surface area of the blade face to apply power and would
scoop water and unbalance the boat at the finish?

The only way I could see it working was if you intended to hold your
blade in the water, feather it 50% (or whatever works best) whilst
still in the water and extract the blade whilst moving sternwards on
the recovery, using the angle of the blade to guide it out of the
water. This would just create drag on the blade during the recovery
and would probably tire out your inside arm if you tried this at race
pace.

Come at me Carl!

[Walter Martindale]

I too, don't understand why you think the blade shouldn't be square on
the way out.
Usually I agree with your contentions about bladework but not this.
Cheers,
Walter

[Carl]

Many thanks for rising to my bait, both Ross & Walter. This is a
philosophical discussion, not a contest, BTW, so I hope not to "come at
you" :) . Let's jointly consider the finish of the stroke & see where
that gets us?

First, 2 basics:
1. You have a limited range of blade movement, especially in sweep
rowing, before the hands must cease moving towards the bow, then stop,
then start backtowards the stern.
2. At that end of the stroke you should (for good hydrodynamic reasons)
wish to maximise the amount of useful work you can do.

If you can bring the bow-wards motion of the hands to a halt while
_still_ doing useful work, then you are likely to increase the
propulsive value of that last part of the stroke. It also means you
have a longer effective stroke & maybe one that's more efficient &
effective - as should become clear during the discussion

Consider first the square-blade finish:
Unless you wish to backwater during the extraction, the hands must
continue to move towards the bow at much the same speed right up to the
instant at which the blade clears the water. Since nothing stops
instantly, the hands must continue a little further after extraction
before coming to a halt. Any part of the stroke length conducted in mid
air is a wasted action in propulsive terms.

Further, as the blade rises, square, through the water its surface area
in contact with the water steadily decreases while the tendency for
complete aeration of its convex back increases - unless you also unload
it while partly immersed.

So, all of the period of the square extraction is a waste of stroke
length. Throughout & beyond this final set of processes the hands must
continue moving towards the bow while the blade breaks the "essential
tensile connection" with the water that ought to cover its back. Thus
it contributes little or nothing to boat propulsion, or it backwaters,
or it remains loaded only to only to throw water uselessly sternwards.

Now consider a finish taken with the blade part-feathered. I won't
propose an actual amount of rotation, but suppose that it varies during
extraction from fully square & fully buried (as you approach full
distance) to 1/3 rotated (i.e. still way off fully feathered) as it
leaves the water. What does this do & how does it work? I'll take this
in 2 steps:

1. You approach the finish, then partly feather & stop pulling. Unless
you try to hold it immersed, the blade will now move quickly (depending
on how much & how swiftly you feather) up through the water. In doing
so it will apply very little force to the hands & will not backwater.
So you were able to pull pretty much full length with a completely
buried blade, & that has to be more efficient. And then you let the
flow of the water fly the blade up & out, incurring a small penalty but
giving a clean self-extraction.

2. Now instead, as you approach the finish & begin to reduce the load,
start to part-feather & to pull your hands slightly downwards. Instead
of the blade flying _itself_ upwards WRT the water surface & the boat
(as it moves diagonally up while also moving forwards through the water)
you are using it as a wing, or as a dolphin's flipper or tail. The
upwards force you are applying to the now-angled blade will generate a
continuing forward thrust for as long as water is flowing around it
(i.e. it will foil, generating a propulsive lift reaction, until it
leaves the water), yet your hands need not move even a millimetre more
towards the bow. Nor will water be flicked out or astern. Nor will the
blade backwater.

By method 1 you get to pull the fullest possible length of the stroke &
then make the water's flow safely extract the part-feathered blade. By
method 2 you draw the maximum possible length with a fully buried blade,
& then get a bonus of propulsion with zero horizontal hand movement by
foiling the part-feathered blade, under vertical load, out of the water.

You again get a very clean extraction, plus the benefit of being already
part-way to full feather as you clear the surface.

To a fish or an aquatic mammal this would make perfect sense. It is by
angling their flattened fins to the direction of motion & moving them up
& down that they propel themselves, sometimes at great speed. Such
effects are not unfamiliar to kayak & canoe paddlers. sailors & flyers
understand them. They only seem alien to us rowers, who are (quite
wrongly) taught that our blades simply push water & are left blissfully
unaware of the various foiling (lift-generating) processes which
contribute so much to many parts of our stroke.

If that is insufficient as an explanation, please get straight back.

[Carl]

It deeply puzzles me that rowers & coaches keep waiting to be told what
to do by some uber-person before daring to try anything different. Why?
I think it's a case of, "Always keep a-hold of nurse / For fear of
finding something worse", which is a strangely timid attitude for those
who spend so many hours flogging themselves silly in the hope of winning
this or that race.

We have in our hands all the equipment we could need if we wanted to
assess changes in technique (& equipment) for their potential to make us
move boats faster & steer them better (& hence again move boats faster),
except apparently for the one vital element - an enquiring mind.

So, with the simple exception of training even harder, we collectively
fall for doing whatever our forebears did, confident it can't make us
slower by unconcerned by certainty that it won't make us any faster.

Just occasionally we fall for some change - e.g. in blade shape -
because "surely" a blade of bigger area which keeps all of its top edge
at the surface "must" be better, & everyone switches. Yet where is the
evidence that this change changed anything. Or we convince ourselves
that because some champion used a different bit, or even colour, of kit,
then we too can be champions if we invest in similar stuff. And when no
change results, we are sufficiently invested in that piece of folly that
we carry on regardless, since to revert would make us feel foolish.

I claim no credit, but for too many years I've been advocating a deeper
mid-stroke phase, & pointing out that many successful scullers have
independently found it works for them. Yet when I discuss stroke depth
with rowers & coaches they respond with the same arguments about rowing
level with the surface being "designed into the blade so it must be
right", & that any amount of looming must bring your instant downfall.
But they have never put it to the test!

The snag, if & when they do try a deeper stroke, will be one of
perception. Their deeper stroke will be more efficient, so it will take
longer to complete. Then they'll interpret this as being "too heavily
geared", so they'll argue they need a bit of slip. Utter tosh! Let it
take longer - that small increase in stroke duration is the consequence
of reduced slip, & slip is all wasted energy.

Yet rowers somehow kid themselves that slip is "like gearing" &
therefore "a good thing in the right place & used in the right way".
No! It darned well isn't "a good thing". Gearing is highly efficent;
slip is 100% inefficient. Only rowers see skid marks (i.e. big puddles)
as signs of strong & efficient propulsion, whereas anyone involved in
car racing knows that skidding is to be avoided & improved adhesion is
required if you wish to maximise straight-line performance.

The simple test of a more efficient stroke (one that wastes less energy)
is that it _does_ feel harder, while any inefficiency in your stroke is
bound to shorten its duration & thus feel "lighter". You will, of
course, have to learn to row with that slightly slower, longer duration
stroke. It might mean you have to make a slicker recovery (_not_ by
rushing off backstops, please!) to sustain the same rate. It might also
mean you actually marginally reduce how hard you pull, so as not to
over-exert yourself against the enhanced resistance resulting from
reduced slip. But it will also mean you move the boat faster.

I though going faster, not conforming to the crowd, is what it's all about?

[Carl]

On 23/02/2012 22:17, Carl wrote:
> On 22/02/2012 23:15, Ross Horn wrote:

<snipped>

>
> If that is insufficient as an explanation, please get straight back.
>
> Cheers -
> Carl
>

Is that the stunned silence born of sheer incredulity, or did I actually
succeed, for once, in explaining the very hard to explain?

[Walter Martindale]

On Mar 4, 3:35 pm, Carl <s...@sss.jjj> wrote:
> On 23/02/2012 22:17, Carl wrote:
>
> > On 22/02/2012 23:15, Ross Horn wrote:
> <snipped>
>
> > If that is insufficient as an explanation, please get straight back.
>
> > Cheers -
> > Carl
>
> Is that the stunned silence born of sheer incredulity, or did I actually
> succeed, for once, in explaining the very hard to explain?
>
> Cheers -
> Carl
>
> --

Nah..
Spent last week behind a steering wheel with minimal access to the
net. Contemplating the business of feathering partly in the water -
having a lot of trouble with it. Not sure I care enough to get into a
great long bun-fight about it while we're waiting for the water to
thaw with no opportunity to go and try stuff.
W

[Carl]

Who said there'd be a fight over it? Not me.

This is just the kind of challenge for a guy like you, with the
rock-hard cold stuff keeping you away from real water & something
definitely not rock-hard between the ears, to nag & worry into shape
while sipping whisky & listening to the distant howling of wolves.

Like anyone, I find it tricky to think processes through in 4-D (time as
the 4th dimension) but worth doing. OK, that's not what we are
encouraged to do when rowing but, since every little counts in race
winning, it could repay the effort - provided it doesn't irreparably
damage that grey matter.

We have plenty of time, & it could be fun.

[fastyacht]

My sense of the old rules is that it comes from focusing on the
handle-end of the oar, and "rowing the table". The thought is that too
much vertical motion is inefficient or gets the hands too high to be
efficiently used.

And yet the reality is that the hand height is adjustable! You shouldn't
be forcing your blades to be high just to get the handle height optimal.

The reality of the blade depth is very simple and easily learned through
feedback: you can feel the difference between a high air-sucking blade
and a properly immersed one.

How deep is deep enough? Simple! Deep enough that you don't have any
ventilation. I doubt a rower can induce cavitation but yo would feel
that, too.

It may work out to be 1/2 blade deep. A simple analysis will get you in
the ballpark. Find the loads at the handles, figure out the load on the
blade, from that use classical analysis both for stalled and for
circulating and come up with a -deltaP value. From that, you know that
you will know how deep--if your -deltaP/gravity is greater than the
pressure head of the immersion, then you are sucking air :-)

[Carl]

> Tinus wrote:
>> On 01/31/2012 06:45 PM, Charles Carroll wrote:
>>> Dear all,
>>>
>>> I want to thank Walter Martindale who in another thread wrote:
>>> “Classic coaching has the blade depth with the top edge just under
>>> the surface, with the ‘pocket’ developing at the back of the oar to
>>> ease extraction. Sliasas et al., have done modelling on this and
>>> published it... I asked Andrew S. (a rower and coach) if he'd
>>> considered checking the modelling software for blade fluid dynamics
>>> with different depths, and I cited conversations I'd had with
>>> Valery Kleshnev about blade depth (1/2 a bladewidth of water above
>>> the blade is best) - he ran the models that afternoon and came back
>>> to me with ‘wow, it's more efficient to be deeper - but all the
>>> coaching manuals still say ‘keep the blade at the surface’.”
>>>

Proving what I have for so long said, & confirming that ours is a
terribly backward sport in which we put the most faith in the most
antiquated, least tested & least scientific notions.

>>> For years I have been trying to immerse my blades a little deeper.
>>> But I have never been clear about how deep, that is, until I read
>>> Walter’s paragraph.
>>>
>>> So yet again, Walter, you have improved my sculling immeasurably.
>>> And for this I am immeasurably grateful.
>>>
>>> Warmest regards,
>>>
>>> Charles
>>
>> The models (allowing free surface effects and adjustment of blade
>> depth) and/or the computer code have never been published and it is
>> unclear how much accurate the model is or how possible variations
>> between model and reality may arise.
>>
>> I'd go with Kleshnev's experimental data and go for 1/2 blade depth.

Presumably by that you mean that much completely below the surface? How
deep must depend on more than such simple rules, since there are
different phases to the stroke imposing different loads & flow regimes.
The essence should be that at no stage do you allow aeration of any
part of the back of the blade.

On 03/05/2012 18:44, fastyacht wrote:
> My sense of the old rules is that it comes from focusing on the
> handle-end of the oar, and "rowing the table". The thought is that too
> much vertical motion is inefficient or gets the hands too high to be
> efficiently used.
>
> And yet the reality is that the hand height is adjustable! You shouldn't
> be forcing your blades to be high just to get the handle height optimal.
>
> The reality of the blade depth is very simple and easily learned through
> feedback: you can feel the difference between a high air-sucking blade
> and a properly immersed one.
>
> How deep is deep enough? Simple! Deep enough that you don't have any
> ventilation. I doubt a rower can induce cavitation but yo would feel
> that, too.
>
> It may work out to be 1/2 blade deep. A simple analysis will get you in
> the ballpark. Find the loads at the handles, figure out the load on the
> blade, from that use classical analysis both for stalled and for
> circulating and come up with a -deltaP value. From that, you know that
> you will know how deep--if your -deltaP/gravity is greater than the
> pressure head of the immersion, then you are sucking air :-)
>

>

Bill -
It's very unlikely that a useful rower can have an efficient stroke with
the blade only part-immersed. You're bound to get significant
ventilation of the back (convex) face & as a result generate a lot of
flows around the blade edges, not to mention doing unproductive work in
moving the water surfaces vertically.

From hydrodynamic first principles no one would choose a propulsion
method which deliberately allows the very unstable air/water interface
which we call the surface (held flat only by gravity & therefore so
easily deformed) to have any continuing influence. So your blade top
edge should always be sufficiently buried that the surrounding pressure
field it generates is unable to disrupt the water surface.

The snag for a rower in trying to judge subjectively when their blade is
working well is that the more efficient the action the 'heavier' their
stroke will feel. This counter-intuitive result arises, as you know but
others may not, because what we loosely call 'blade slip' is both the
manifestation of a less efficient stroke & the means by which we too
often judge a 'good & quick' stroke. No one likes it when the blade
takes longer to complete the stroke, but a more efficient stroke must
always slip less, so it will take a little longer to complete the same
stroke arc at the same boat speed. Pulling harder won't change that.

I often use the analogy of wheel spin to convey this message. The car
gets away faster for the same power output when the wheels do not spin,
since spin, exactly like blade slip, takes energy away from driving the
car by diverting it into producing heat, noise, smoke & grinding rubber
off the tread.

Unfortunately, rowers are very sensitive, however inept we may be as
rowers, to stroke duration. We feel the boat goes better & the crew
rows better if we don't row any deeper than the other guys since this
allows us to catch & extract together. Perversely, that confirms my
argument, as does the seemingly helpful (for us) effect of rowing a bit
less deep than the rest of the crew when our inferior fitness makes it
all feel 'too heavy'.

[fastyacht]

Perhaps a revolution in rowing would be to correlate stroke rate to
speed, by measuring both. With some planning and statistics and skillful
precise coxwaining, it might be possible to train an improved
stroke--one which is maximally "heavy" and therefore efficient with
minimum slip.

The beauty of this approach is that it is a "no cheating!" approach. Any
effort to keep up by washing out early or catching late will show up in
lackluster times...

These are merely the possible impractical musings of an engineer rather
than a professional oarsman...
-Bill

[davie...@gmail.com]

Reducing recovery time to improve the stroke ratio (drive time:recovery time) is an established method of increasing power & speed. Top scullers can row 2k at close to 1:1 ratio. ISTR Fiona Milne winning world champs LW1x at 1:<1. Not being a very powerful athlete, it was a sensible approach, but one that requires good technique.

[Tinus]

On 05/11/2012 02:06 AM, fastyacht wrote:
> Perhaps a revolution in rowing would be to correlate stroke rate to

> speed, by measuring both...

After many years of evolution we've ended up with strokes which differ a
lot among boat types. Doesn't this mean the correlation is already present?

[Carl]

Probably not.

You can only optimise where there is freedom to allow all variants to be
tested. Orthodoxies obstruct optimisation. The very reasonable need to
get all members of a crew doing the same thing discourages coaches from
venturing into more difficult or less intuitive techniques.

We have inbuilt preferences for what "feels most natural", & sensory
feedback tells is "if it feels harder it must be wrong", stopping us
from experimenting to see whether, even so, that change improves overall
performance. The myth that where the blade floats is the level at which
we should pull it keeps shifting techniques back towards that norm.

And your easy assumption of the existence of a valid correlation is
another part of the barriers to exploring what really does work best.
Any coach who gets moderate material going faster by adopting a
noticeably different technique is certain to be told that this crew
would be faster still if they reverted to "standard" technique. So is
every member of that crew going to hear the same siren song from those
outside the crew. Thus evolutionary optimisation is stopped & reversed.
Coaches soon learn that, to keep their jobs, they need their crews to
"look right" & to hell with what others see as airy-fairy physics, since
there is no absolute test of whether their crews are performing at,
below or above their actual potential.

Sad, isn't it?

Cheers -
Carl

Only when we become truly dispassionate about techniques, are prepared
to challenge preconceived notions & apply the cold, hard lessons of
fluid dynamics are we likely to get a real improvement in performance.
Great scullers do this by dint of fortune or determination, but are
often roundly criticised by armchair and active rowers for, e.g., "going
too deep".

[donal...@gmail.com]

Now that Croker have strain guages(?) in blades surely there will be
some possibilities for objective data with
use of known athletes cross referenced against speed coach data and
some visual clues as to what is happening
to the blade? Though whether athletes will want their personal data
published is of course another matter....

Donal

[Carl]

Strain gauges will tell you the bending load on the oars (i.e. how hard
you're pulling) but will not tell you how much of that effort was useful
& moved the boat & the remainder which only stirred water & did no boat
moving. You can have the same load on a blade but, depending on how you
use it, you will dissipate more or less of the work you do into useless
water-stirring.

Cheers -
Carl

[donal...@gmail.com]

Appreciated but isnt that where the speedcoach and the eye and
knowledge of the athletes come in to complement the process?
http://www.crokeroars.com/images/aronditepresentation.pdf

Donal


>
> Strain gauges will tell you the bending load on the oars (i.e. how hard
> you're pulling) but will not tell you how much of that effort was useful
> & moved the boat & the remainder which only stirred water & did no boat
> moving.  You can have the same load on a blade but, depending on how you
> use it, you will dissipate more or less of the work you do into useless
> water-stirring.
>
> 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

> Email: c...@carldouglas.co.uk  Tel: +44(0)1932-570946  Fax: -563682
> URLs:  www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)

[Carl]

On 15/05/2012 00:25, donal...@gmail.com wrote:
> Appreciated but isnt that where the speedcoach and the eye and
> knowledge of the athletes come in to complement the process?
> http://www.crokeroars.com/images/aronditepresentation.pdf
>
> Donal
>
>
>>
>> Strain gauges will tell you the bending load on the oars (i.e. how hard
>> you're pulling) but will not tell you how much of that effort was useful

>> & moved the boat& the remainder which only stirred water& did no boat

>> moving. You can have the same load on a blade but, depending on how you
>> use it, you will dissipate more or less of the work you do into useless
>> water-stirring.
>>
>> Cheers -
>> Carl
>>

Donal -

The Croker strain-gauged oar is a great device, no question. It enables
you to measure the amount of oar bend, & hence the combination of
applied stress & transient strain, throughout the stroke.

I'm very interested by what the graphs on sheet 11 indicate:
1. the bits they label earlier as negative strokes, which I'm guessing
are where oar shaft curvature rebounds past straight as extracted when
still loaded (i.e. still flexed & suddenly unloaded), or back-waters
when extracted with the infamous attempted vertical tap-down.
2. the orange curve 11, which shows the blade depth _reducing_ in
mid-stroke - where the load is greatest but the blade efficiency is
lowest. Is that actual blade depth, or a predictive measurement that
has been affected by rigger, oar & boat flexure?
3. the big variations in blade square through stroke & recovery (the
green line).

It is certainly a great empirical tool. If used as intelligently as its
price requires, it should assist a good coach. But it still does not
measure the stroke work wasted through technique errors. Yes, you
should combine its use with boat speed monitoring (local &
stroke-averaged), but the one really solid lump of data - how much work
gets thrown away in unproductive water-stirring - stays out of reach, as
still does a broad acceptance of a valid interpretation of the complex
fluid dynamics of the blade/water interaction.

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

[Phil]

> SNIPPED LOTS

"> but the one really solid lump of data - how much work gets thrown away
> in unproductive water-stirring - stays out of reach, as still does a
> broad acceptance of a valid interpretation of the complex fluid dynamics
> of the blade/water interaction.
>
> Cheers -
> Carl
> "

Let's see if the collective brain can come up with a way of measuring
the wasted work then!
I'll start us off by postulating that a strain-gauge equipped oar with
position sensing would give us the 'input' work (force*distance moved in
direction of force).
A boat of known mass equipped with accelerometers could supply the total
output work, by integrating M/a over the same time distance.
The challenge would then be apportioning the difference between the two
values to the various loss systems.

Over to the floor for discussion....

Phil.

[Carl]

In principle that sounds like a good approach, although the mass in
question is not that of boat alone but of boat, crew & blades.

But:
1. accelerations of the boat (& crew) average to zero over the stroke cycle
2. work is done between rower & boat during recovery as well as during
the stroke
3. the loss sources (hydrodynamic drag in all its manifestations) are
not directly measurable - drag under widely cycling velocity & load
distributions is not the same as drag under steady conditions (for a
range of reasons, including that the boat drags along with itself a
substantial fluid mass in the form of its accompanying boundary layer,
which interacts with the varying velocity of the boat & the relatively
non-moving body of water beyond that).

So significant elements remain outside your initial framework & we need
to ponder whether we can find ways to deal adequately with these.

I do really like the idea of a collective approach, so more suggestions,
please.

[Phil]

I'll try to address point (1) - are only the positive accelerations due
to work input during the stroke? Negative numbers due to other systems
slowing the boat down - losses due to hull drag / windage etc.

Let's take one stroke from the catch to the finish (no recovery) on boat
already moving at a steady state.

Mtot = total mass of boat+rower+all.

W1 = total work done on the oar, total input work, measureable.

L1 = Lost work due to inefficiencies in the oar system

W2 = work done in accelerating the boat+rower system, measured by
integrating Mtot/a (for positive 'a') over the stroke time (equivalent
to NET force * distance).

L2 = total work done by 'hull losses' in slowing the boat

So I reckon that (for energy to be conserved, give or take a little bit*):

W1 - L1 = W2 + L2

So it remains to quantify the work done by the various 'hull losses' in
slowing the boat down - lets call this L2, in order to find L1....

If I could stir my mind back to various equations of motion, I reckon
L2 could be found empirically by measuring how the boat slows on the
recovery. Any takers?

Phil.

* - excluding waves, sound, heat, farting etc

[Tinus]

You talk bad about habits in rowing but you don't have much proof that
you are right.

I don't believe that rowing strokes are only optimised by what feels
right. This is because differences in boat size, speed, propulsive
power, acceleration dictate the type of stroke we make. Your idea about
people rowing a certain style because it feels or looks right can not
explain why rowing in an eight feels and looks different from rowing in
a pair.

[Carl]

Phil - no one ever allows adequately for the effects of wind ;)

Unless I've misunderstood you, at steady state shouldn't W2 be zero?

As for L2, what you are proposing amounts, I think, to an analogy with
how ergs compute the constant in their own energy absorption calcs -
from the rate of spin-down of the wheel. But in a rowing shell we do
useful amounts of work during the recovery to decelerate our bodies by
more than the deceleration of the deceleration of the boat - or we
wouldn't move to frontstops (or move frontstops towards us) - which
energy input helps to keep the boat moving against the forces of drag.
And that's like having to push the chain back into the erg, thereby
adding further energy to the flywheel (which we don't do).

Furthermore, boat velocity is quite variable during recovery, with
(usually) a biggish if brief check around the catch. So I think we'd
have some difficulty in defining what is recovery & what is stroke for
the purposes of your proposed analysis.

This irregular velocity profile further complicates the business of
establishing a reliable basis for drag loss calculation, due to the
roughly cube-law relationship, even at a constant speed, of these losses
to velocity. And the substantial surrounding, but diffuse, mass of the
boundary layer, whose decelerations & accelerations will lag behind
those of the boat, adds further complication.

Still, it was never meant to be an easily solved problem. It might be
easier to optimise the stroke depth, force & arc profile problem in a
lab by applying a single stroke from a passing "boat" to a large but
defined volume of water while performing 3D velocity change measurements
throughout that volume to determine the energy gained by the totality of
that water. Still very tricky, as local kinetic energy rapidly converts
into very low-grade heat which might prove impossible to measure.

So it might be better to use the existing tools of hydrodynamics,
including CFD, to consolidate useful insights into what should be better
ways to use an oar, & then work with coaches & rowers who share a
determination to properly apply those insights to rowing in real boats.
Or we construct rowing automata to row test boats with varying
techniques and compare performances vs technique at different work
rates, since we can certainly measure the total power consumption of an
automaton? Anyone got a handy couple of millions?

[Phil]

...all good stuff, but my aim was to outline some 'thought experiments'
which could be easily tried and refined for real to give us some real
results.
Sometimes it's surprising the amount of theory that can be glossed over
by a solid empirical approach.

Phil.

[Carl]

And you were quite right to do so, Phil. Please understand that I have
no wish to hit good ideas on their heads, but I am wary over the
assumptions we can safely make.

[Carl]

For a start, a pair is quite a bit slower than an eight, yet the rigging
differences are not so great. The result is quite a different (heavier,
slower) feeling for the crew - perhaps more like rowing with only 6 men
doing the work in an eight. This should be enough to explain why 2
people rowing a pair does not look like any adjacent 2 members of the
crew of an eight. But it doesn't affect the case I was presenting.

As I said, some great scullers scull so differently from others & from
the presumed 'best technique' that when they win there are folk waiting
to say they'd have gone faster still with a more orthodox technique.
When I see anyone going fast I look first for what they do differently,
to see if there might be a rational technical explanation for why those
differences might help their superior performance. Others like to
suppose they are fast despite those differences, & won't try to work out
what benefits those difference might bring. I think this may be due to
the generally minimal insight & interest, across our sport, into
rowing's inertial- & hydro-dynamic interactions. Easier to believe what
others say than to get inside the science?

Cheers -
Carl

[Tinus]

>
> For a start, a pair is quite a bit slower than an eight, yet the rigging
> differences are not so great. The result is quite a different (heavier,
> slower) feeling for the crew - perhaps more like rowing with only 6 men
> doing the work in an eight. This should be enough to explain why 2
> people rowing a pair does not look like any adjacent 2 members of the
> crew of an eight. But it doesn't affect the case I was presenting.
>

You are only being descriptive here and don't explain why, if people rig
based on feeling, people don't rig in such a way that they get the same
feeling in an eight and a pair (or any other boat type).

Sure I agree that styles/techniques are different and can be improved
upon. However I responded on the idea that boat speed and stroke speed
could be better correlated or revolutionised. These correlations already
exist and sure techniques may be revolutionised but the stroke rate is
limited to physical boundaries which rowers have already found or at
least have approached closely.

[Carl]

Speed ratio for Pair to Eight is ~0.85:1.
Oar length ratio is closer than 0.99:1.
Inverse inboard length ratio is ~0.98:1
Spread ratio is ~0.97:1

So however you juggle with the numbers, there's no way a pair-oar stroke
can feel the same as the stroke taken in an eight. For the same stroke
rate, either the pair shortens up or they allow longer for the stroke &
less time for the recovery.

I think that's a fairly fundamental difference, even before we start
considering the issues of balance & keeping the boat running straight
(most pairs show a degree of asymmetry between the actions of bow &
stroke). And the pair is likely to pitch & bounce more than the eight.
It is a very different beast from the eight - much more like a 2-man
single sculling boat.

[Tinus]

On 05/24/2012 06:48 PM, Carl wrote:
> Speed ratio for Pair to Eight is ~0.85:1.
> Oar length ratio is closer than 0.99:1.
> Inverse inboard length ratio is ~0.98:1
> Spread ratio is ~0.97:1
>
> So however you juggle with the numbers, there's no way a pair-oar stroke
> can feel the same as the stroke taken in an eight.

There is a way. You could jiggle with the last three numbers to make the
feeling more similar but it isn't done because it would make a less
efficient stroke.

So I agree with you, in practice, there's no way a pair-oar stroke can

feel the same as the stroke taken in an eight.

And as a corollary I stated: We've got strokes which differ a lot among
boat types which means that the correlation is already present.

[howard...@gmail.com]

On Tuesday, January 31, 2012 5:45:18 PM UTC, Charles Carroll wrote:
> Dear all,
>
>
>
> I want to thank Walter Martindale who in another thread wrote:
>
>
>
> "Classic coaching has the blade depth with the top edge just under the
>
> surface, with the 'pocket' developing at the back of the oar to ease
>
> extraction. Sliasas et al., have done modelling on this and published it...
>
> I asked Andrew S. (a rower and coach) if he'd considered checking the
>
> modelling software for blade fluid dynamics with different depths, and I
>
> cited conversations I'd had with Valery Kleshnev about blade depth (1/2 a
>
> bladewidth of water above the blade is best) - he ran the models that
>
> afternoon and came back to me with 'wow, it's more efficient to be deeper - but all the coaching manuals still say 'keep the blade at the surface'."
>
>
>

> For years I have been trying to immerse my blades a little deeper. But I
>
> have never been clear about how deep, that is, until I read Walter's
>
> paragraph.
>
>
>
> So yet again, Walter, you have improved my sculling immeasurably. And for
>
> this I am immeasurably grateful.
>
>
>
> Warmest regards,
>
>
>
> Charles

My twopenn'orth on the subject of blade depth available here: http://bit.ly/1m3YODk

Constructive feedback welcome.

Best wishes,

Howard

[Carl]

Why would feedback not be constructive, Howard? We might not agree with
your proposition but on RSR we do try always to be constructive.

As one suggestion:
It'd have been slightly easier, & more helpful to RSR readers &
responders, had your argument been outlined within RSR rather than as a
link.

Now to the core:
I believe that the fundamental flaw in your argument is to be found
within these 2 sentences:
"Rowers need to understand that their centre of rotation is out beyond
the pin, at the tip of the blade. When they understand that, they can
understand how to use the blade efficiently."

Why would we "need" (an emotive, persuasive term one ought not to us in
a rational analysis) to understand what what you there when it is
delivered entirely without supporting evidence? I accept that you may
believe what you claim, but where's the evidence? To me it is no more
than what you presume, in which case it'd be better to start: "If we
were to presume that..... then....."

What I see as the fallacy in your argument is as follows:
Were the tip of the blade really to be the centre of rotation, then
_every_ other part of the blade would be moving towards the bow, i.e.
it'd be back-watering. In that case there'd be no possibility of
delivering useful work, only of wasting energy & bringing the boat to an
early halt.

So your initial statement must fail that test. And it fails to test of
observation too.

The stroke is in any case a far more complex beast than you propose.
But by the middle part of the stroke the blade has already stalled, &
observation (as well as fluid dynamics) shows that in that condition it
is ploughing sternwards through the water. Sure, the tip is moving
faster to the stern than the neck of the blade, but in most cases even
the neck has some sternwards motion (the blade is, after all, a lot
shorter from tip to neck than the shaft is from blade tip to pin).

This case has been made quite a few times here on RSR & not as yet
challenged. And the easily-observed fact, science & an extensive body
of experimental evidence have all shown that a deeper blade slips less &
is less bale to entrain air down from the surface, that the case is made
for going significantly deeper than having the "upper edge level with
the surface".

There being no rules about oar dimensions, & the differences in blade
loadings between champions & the average rower being not so great as to
change blade depth requirements, I don't see how you can make a case for
treating the average rower differently from the champion who, as you
accept, does tend to row deeper than the pundits advise.

To make your case you'd need to provide meaningful test results to show
at what point in depth, & at which points throughout the stroke
trajectory, the energy losses through friction & back-watering on the
blade & shaft exceed the energy savings through reduction of blade slip.
From the work I (being naturally curious about such things) have
actually done, I see no evidence that burying 30cm of the shaft in order
to get the blade buried deeper has other than beneficial consequences
for boat propulsion efficiency. And there may well be a case for going
deeper.

Please note that I'm not saying you need that depth throughout the
stroke. What I am saying is that the most efficient stroke comes from
having a larger (i.e. deeper) vertical component.

But I am, of course, completely open to challenge - on the science, & on
the practical issues too.

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: tinyurl.com/2tqujf
Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

[James HS]

Howard,

I have seen what you have suggested coached often - pop the blade in and it floats with approximately an inch above the water.

However, I have never seen any information from an oar designer state that is the ideal height - in fact, if a blade is dropped in the water it naturally floats in a feathered position - but again that is an unintended side effect of a buoyant blade.

We do a simple experiment - take the rower to the edge of the river with their blade, get them to drop it in where it naturally floats as you suggest and sweep it along. Then position it an inch lower and sweep it along - the second is MUCH harder. (The former having the air cavitation issue).

So if one took your rotational visualisation, the deeper blade would have a much better grip and the cavitating blade would slip.

The drag issue, were it an issue, would be much worse on the face of the blade anywhere inboard of the tip than on the shaft, which is so much smaller than the blade face.

Having been taught the floaty position and the depth sufficient to avoid cavitation position, I have though observation and measurement found that deeper is better with the same input.

I actually don't think that 'elite' scullers are so different - the kit is designed for them and we just benefit from being lesser mortals.

IMHO and all that!


James

[graham]

Interesting that Carl talks of feathering while the blade is still partly submerged.
In my 'Rhecon' style this is required for 2 reasons.
Firstly, the lower part of the blade will push water back as it is feathered, thus actually driving the boat. Make sure to control the feather so that water is driven back, but not lifted. The Macon type blades now commonly used have more spoon below the shaft, and this gives the feather more drive.
Secondly, not lifting squared blades out of the water allows the vertical hand movement at the release to be reduced. By lowering the rigging(mine is 125mm, rather than a common 170mm) the pull during the work is lower, kinder to the lower back, and better for balance.
Have fun.

[A. Dumas]

On 22/06/2017 01:54, graham wrote:
> Interesting that Carl talks of feathering while the blade is still partly submerged.
> In my 'Rhecon' style this is required for 2 reasons.

> Firstly, the lower part of the blade will push water back as it is feathered, thus actually driving the boat. Make sure to control the feather so that water is driven back, but not lifted. The Macon type blades now commonly used have more spoon below the shaft, and this gives the feather more drive..

> Secondly, not lifting squared blades out of the water allows the vertical hand movement at the release to be reduced. By lowering the rigging(mine is 125mm, rather than a common 170mm) the pull during the work is lower, kinder to the lower back, and better for balance.
> Have fun.

Fun is what you should have because speed ain't it.