Hull acceleration
boatie
coaching convention
http://www.maxrigging.com/could-i-have-been-mistaken-for-30-years-about-how-to-make-a-rowing-shell-fast
Quote "For all my years of coaching I went by the mantra that the hull
of a boat moves the fastest at the finish, after the drive is over and
the blade is just being removed.
Good chance (darn good chance) that this is not so. That I was
mistaken for 30 years is a real ego knock.
Why do I think I was wrong?
I spent about an hour discussing a very interesting project with the
creator—Jim Mitchell of PowerView.
In essence his combination of GPS, video, accelerometer graphs out
that the fastest part of the stroke may well be the catch. (I always
thought it was at the release.)" End Quote
What does this group think?
Walter Martindale
> Just reading Mike Davenport's Max Rigging site and his visit to the US
>
> Quote "For all my years of coaching I went by the mantra that the hull
> of a boat moves the fastest at the finish, after the drive is over and
> the blade is just being removed.
>
> Good chance (darn good chance) that this is not so. That I was
> mistaken for 30 years is a real ego knock.
>
> Why do I think I was wrong?
>
> I spent about an hour discussing a very interesting project with the
> creator—Jim Mitchell of PowerView.
>
> In essence his combination of GPS, video, accelerometer graphs out
> that the fastest part of the stroke may well be the catch. (I always
> thought it was at the release.)" End Quote
>
> What does this group think?
The C of G of the boat-crew-blades "movement system" moves fastest at
or near the end of the drive phase, but the hull in a sliding seat
boat moves fastest during the recovery.
That's not really all that new...
Haven't seen velocity curves of a sliding rigger boat, but that's
kinda irrelevant because they're not allowed in competition.
Walter
Charles Carroll
Do you know of anyone who has used one of these PowerView kits?
Also, did you attend the US Coaching convention? Perhaps I have gotten
things wrong, but I seem to remember the last time we discussed "hull speed"
someone's suggesting that there may be a connection between the point at
which the hull reaches peak velocity during the stroke and the technique the
oarsman is using.
Also, I seem to remember Xeno's insisting that one must think of the catch
as the end of the recovery, not the beginning of the drive. For those who
share Xeno's view, if the hull speed is fastest at the catch, wouldn't that
still make it technically fastest very late in the recovery?
Cordially,
Charles
Carl Douglas
> Just reading Mike Davenport's Max Rigging site and his visit to the US
> coaching convention
> http://www.maxrigging.com/could-i-have-been-mistaken-for-30-years-about-how-to-make-a-rowing-shell-fast
>
>
> Quote "For all my years of coaching I went by the mantra that the hull
> of a boat moves the fastest at the finish, after the drive is over and
> the blade is just being removed.
>
> Good chance (darn good chance) that this is not so. That I was
> mistaken for 30 years is a real ego knock.
>
> Why do I think I was wrong?
>
> I spent about an hour discussing a very interesting project with the
>
> In essence his combination of GPS, video, accelerometer graphs out
> that the fastest part of the stroke may well be the catch. (I always
> thought it was at the release.)" End Quote
>
> What does this group think?
Deeply improbable, IMHO, since:
1. a relatively small part of the total work of the stroke cycle has
been done during the recovery
2. the recovery is normally of longer duration than the power stroke
3. energy is being sucked away by fluid drag during the recovery just as
during the stroke
4. there must therefore be the least total energy in the moving system
at the instant of the catch & about the most around the point of extraction
5. at catch, as at finish, both body & boat share the same velocity
since there is no relative movement between them
Thus if the kinetic energy of the system has to be least at the catch &
greatest near the finish, it cannot be that the boat velocity is highest
at the catch
As a comment on your headline, I would note that this has nothing to do
with hull acceleration.
Any comments?
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)
Tinus
Carl Douglas
> Maybe if one substitutes 'before catch' for 'at the catch'.
But isn't everything in the recovery just a bit before the catch,
including the finish ;)
Charles Carroll
Tinus,
Ah! That is precisely the question.
Walter's graph shows that peak velocity of the boat occurs roughly at the
midpoint between the finish and the catch.
Now redefine the catch.
Propose, for the sake of argument, that the catch is part of the finish and
actually starts midway through the recovery. Voila! Hull speed is fastest at
the catch.
But if we embrace Carl's argument that the rowing stroke is an absolute
continuity of motion, then we can't really redefine the catch as such, can
we?
I mean how can we say that the catch comes at some point in the rowing
stroke if the rowing stroke is not made up of points?
Does this mean that the catch doesn't exist? Or does it mean that the catch
is merely arbitrary?
Carl would bestow on the rowing stoke an attribute more traditionally
reserved for the divine, namely, he conceives of it as an entity without
beginning or end.
But then a perfect stroke would be divine, wouldn't it?
But the catch in all this is that if the catch were divine, then it would
exist outside the province of human reason. It would exist in a mode beyond
word, or logos, or measurement. How do we speak about that which cannot be
put into words? How many times must we be told, "Whereof one cannot speak,
thereof one must not speak?"
Oh dear! How does one begin even to discuss the catch? And if the catch is
beyond discussion, does that mean the finish is beyond discussion?
And if the finish is beyond discussion, does that mean that the rowing
stroke itself is beyond discussion?
Can we actually discuss anything about rowing? Or is Fairbairn right? We can't
really discuss rowing; we "can only illustrate in a boat what we think
rowing is."
Well, these are my silly thoughts this morning.
As always, cordially,
Charles
David Jillings
with a clean finish, the boat will keep accelerating for a little while
after the blades leave the water. Any problems at the finish and that
last bit of acceleration is lost and the boat starts slowing down
immediately. That's why a constant acceleration from catch to finish
and a clean finish are more important than a "big" finish.
David.
Carl Douglas
> I have always taken the view that if you are sculling (or rowing) well
> with a clean finish, the boat will keep accelerating for a little while
> after the blades leave the water. Any problems at the finish and that
> last bit of acceleration is lost and the boat starts slowing down
> immediately. That's why a constant acceleration from catch to finish
> and a clean finish are more important than a "big" finish.
>
> David.
But what point is there in accelerating the boat after extraction?
Everything done after the blades cease pulling ought to be about
achieving the most economical distribution of boat speed through the
recovery, while also preparing you to move into & through the catch, etc.
Rowing is a process of intermittent power application. During the
stroke you do work, some of which increases the kinetic energy of the
whole system & some of which is absorbed in overcoming the continuous
force of fluid drag. During the recovery you try to release you body's
stored kinetic energy in that way which best sustains the boat at a more
uniform run than other possible ways of making your recovery, & you do
this by pulling your feet towards you (thus doing work at the expense of
your body's kinetic energy store) in a well-programmed manner.
Since power consumption through fluid drag is approximately proportional
to the cube of boat velocity, the mathematical argument seems clear -
you should seek to manage you bodily movements in the a way that reduces
both peaks & troughs in boat velocity. For every surge in boat speed
you pay disproportionately in fluid drag while for every matching period
at below average boat speed you get a much smaller reduction in drag
losses. So to engineer a velocity surge after extraction, exhilarating
as it might be, is going to cost you dearly. So why would we want to do
anything to accelerate the boat which, at the finish, is already moving
much faster than its mean velocity?
Charles Carroll
> with a clean finish, the boat will keep accelerating for a little while
> after the blades leave the water.
David,
Assuming I am reading Walter Martindale's graph correctly:
The boat reaches peak velocity roughly 0.468 seconds after the finish.
The sculler, on the other hand, reaches peak velocity 0.273 seconds before
the finish.
So sculler and boat reach peak velocity at different times during the stroke
(i.e. 0.741 seconds apart). But this is hardly surprising inasmuch as the
sculler has to move faster than the boat the travel down the slide rails
during the drive.
Now consider the velocity for the Total System (i.e. sculler plus boat). The
velocity for the Total System peaks at about 0.156 seconds before the
finish. So the velocity for the Total System keeps increasing most of the
way through the recovery until very shortly before the finish.
To my thinking this makes the recovery extremely important. I should think
that we would want to do as little as possible to slow down the ever
increasing velocity of the Total System during the recovery?
Cordially,
Charles
Charles Carroll
> through the recovery until very shortly before the finish.
>
> To my thinking this makes the recovery extremely important. I should think
> that we would want to do as little as possible to slow down the ever
> increasing velocity of the Total System during the recovery?
How embarrassing! This couldn't be more wrong!
It is most of the way through the drive until very shortly before the finish
that the velocity for the Total System keeps increasing. Arrrrrgh!
My apologies,
Charles
Tinus
To get the foot stretcher towards the rower in order to make the next
stroke.
I believe we are discussing too much semantics here. We all agree that
the velocity of rower and boat oscillate around the velocity of the
entire system (centre of mass) which oscillates by itself as well.
It just depends on the specific curves (which can change a lot) and
definitions whether or not one should say that the maximum boat
velocity is just before catch. But we all have somehow about the same
picture of the case (I believe).
Carl Douglas
>> But what point is there in accelerating the boat after extraction?
>
> To get the foot stretcher towards the rower in order to make the next
> stroke.
But the boat has no need to accelerate for that to happen.
When you pull the boat towards you on the recovery you do this as part
of a system which is decelerating due to drag on that boat. The result
is that the boat still decelerates, but by less than you do.
>
> I believe we are discussing too much semantics here. We all agree that
> the velocity of rower and boat oscillate around the velocity of the
> entire system (centre of mass) which oscillates by itself as well.
>
> It just depends on the specific curves (which can change a lot) and
> definitions whether or not one should say that the maximum boat
> velocity is just before catch. But we all have somehow about the same
> picture of the case (I believe).
Getting this clear is key to understanding rowing dynamics. Rowing
abounds with stuff about needing to accelerate the boat, to let it run &
to "get the weight out of the bow". This current discussion has exposed
similarly fundamental misunderstandings of what really can, should or
does happen.
If A says maximum boat velocity is at the catch & B says it is at the
finish, A misunderstands the inevitable consequences of energy
dissipation during the recovery & B has the better chance of minimising
fluid drag therein.
To describe the stroke in ways divorced from reality, or claim to know
just when, after extraction, the maximum boat speed is attained also
makes the unfounded implicit assumption that every rower has identical
stroke dynamics. You need only step into a pair or 2x with a new
partner to realise how very different are the stroke & recovery dynamics
of every rower. Similarly, you have only to see the large differences
between individuals in form of the curves of variation of speed &
acceleration of boat & sculler
Back to your last paragraph:
The only sense in which the boat may even be imagined to accelerate as
you continue your recovery is WRT your own body. But that's just one of
those confusing illusions which leads people to think they're sliding up
& down the boat (as on an ergometer) when the reality is more closely
that the real boat is sliding back & forth under them.
Charles Carroll
> with a clean finish, the boat will keep accelerating for a little while
> after the blades leave the water.
David & al.,
Allow me again to apologize for the extremely poor writing in my earlier
post. I'll try again and hope I do a little better this time.
Walter Martindale collected data on a female sculler sculling at 36 spm. He
then published this data with comments in a paper titled "Control of
Momentum in 2007: View from the boat's perspective." Along with his paper
Walter published a graph. Assuming I am reading the graph correctly, what
follows are some of the data that Walter collected regarding the boat's
velocity during the rowing stroke.
The boat reaches peak velocity during the recovery, roughly 0.468 seconds
after the finish.
The sculler, on the other hand, reaches peak velocity during the drive,
about 0.273 seconds before the finish.
So sculler and boat reach respective peak velocities at different times
during the stroke (i.e. 0.741 seconds apart). But this is hardly surprising
inasmuch as the sculler has to move faster than the boat to travel down the
slide rails during the drive; and in the same way the sculler must move
slower than the boat to bring the stern towards him during the recovery.
Now consider the velocity for the Total System. Walter defines the Total
System as the sculler plus the boat. The velocity for the Total System peaks
towards the end of the drive at about 0.156 seconds before the finish. So
Walter's data shows that the velocity for the Total System keeps increasing
most of the way through the drive until shortly before the finish.
But Mike Davenport has recently offered a challenge to Walter's data. He
says that after talking to Jim Mitchell of PowerView he is rethinking the
idea "that the hull of the boat moves the fastest at the finish."
Jim Mitchell has an instrument, a "combination of GPS, video, accelerometer"
that collects data on a crew rowing. This instrument apparently suggests
that "the fastest part of the stroke may well be the catch."
A number of questions instantly leap to mind. First, of course, is everyone's
definition of the catch the same? Since both Walter and Jim are graphing
data, they are graphing points. Does everyone agree that the catch happens
at the same point?
Then, is everyone looking a hull speed? (I assume that what Walter
Martindale calls "boat speed" is what Jim Mitchell calls "hull speed).
Another way of saying this might be, is everyone making a distinction, as
Walter did, between the separate velocities of the sculler, the boat, and
the Total System?
Could it be that what Jim Mitchell's marvelous instrument really shows is
that the Total System reaches its highest velocity just before the catch? If
so, then Jim's data would reaffirm Walter's and this discussion could end.
On the other hand, if Jim's instrument shows that the boat (separate from
the sculler and Total System) actually does reach its highest velocity just
before the catch, then Jim's instrument would be revolutionary. And if so,
then I would want to know how it works. How is it taking its readings? Why
are they so at variance with Walter's?
Walter plainly says that "Boats go fastest during the early part of
recovery."
Jim Mitchell is quoted as saying that "Boats go fastest at the catch."
Who's right?
Cordially,
Charles
Marcorow
seeing a curve of the speed of the boat when I took my 1st class to
become a rowing coach I guess that was in the mid 80's (let not get
too precise).
M
On Dec 7, 1:15 pm, boatie <rebe...@caroe.com> wrote:
> Just reading Mike Davenport's Max Rigging site and his visit to the US
> coaching conventionhttp://www.maxrigging.com/could-i-have-been-mistaken-for-30-years-abo...
Walter Martindale
wrote:
Oh.. Ow. The total system Vx is greatest just before the release
while most of the blade(s) surfaces are still buried and the crew is
(are?) still applying propulsive force. Once the blades are in the
air, DRAG takes over the velocity curve and causes the SYSTEM to slow
(ok, decelerate). The movement system is at its lowest velocity about
the same time as the next catch is taken.
We saw this in my "stuff" in the 2007 article. I've got other curves
(plotted the same way but Excel doesn't let you mark it up - the graph
in the 2007 article was generated using Lotus 123 or Works or
something, back in 99 or 00..) showing similar profiles at different
stroke rates.
If you spend about $35000NZ and buy a set of Peach Innovations force-
gates, and put it into one of your boats, you'll get not only force
(resolved in the direction of travel because that's the only force
measured by their transducer - not the total force), but due to the
accelerometer in the boat (in the same module that senses the NK
impeller) you get acceleration and boat velocity (from the impeller's
magnetic pulses, picked up by the module). I've seen these curves,
but don't have copies - if I did, they'd still be considered
proprietary by Rowing NZ, even though I don't work there any more.
You can do all you want to regulate and smooth out the recovery hull
velocity, but it is not possible to get the boat into the "catch"
position without the use of a force. That force is exerted against
the foot-stretchers, (about 50 N or about 5 kg of force per foot (from
Smith, R)) by the - um - feet. This force moves the boat towards "the
finish line" underneath the crew, (or - the boat is then moving faster
than about 95% of the mass of the crew - the only thing in the crew
that's moving at the same velocity as the boat during the recovery is
the FEET - because they're attached to the boat. All other body
segments, and indeed the crew CG moves at a lower velocity in the same
direction as the boat - the crew gives up some of its momentum/kinetic
energy/velocity (choose yer measure) to the hull so that the hull can
move past the crew (carrying with it the oars) to the next catch.
Ain't no other way possible unless you detach the rigger from the boat
and bolt in the seat. That merely results in all of these velocity
curves being changed around - i.e., the sliding rigger will have a
velocity curve similar to that of the fixed rigger boat, while the
fixed-seat boat will have a velocity curve similar to that of the
crew's seat/hip joints in a sliding seat boat.
Gotta go...
W
Carl Douglas
Charles -
Don't be seduced by millisecond data! That it has timing reported down
to 0.001 seconds does not necessarily mean it is that accurate (though
it may be), that is repeatable stroke on stroke to that sort of accuracy
(it won't & can't be, since none of us can manage such repeatability) or
that it in any way represents what another sculler, good or bad, will
achieve.
Next, as I tried to point out earlier & Walter has done similarly:
The kinetic energy content of boat + crew _has_ to be at its lowest at
the catch. A large amount of energy has been sucked out of teh system
by fluid drag, & only the rather smaller amount work done by the sculler
pulling on his/her feet to bring the boat towards them during the
recovery has been put back into the kitty. If the energy content is at
its lowest, & both sculler & boat are at equal speed (which at a 1st
approximation they must be at the catch), then both boat & system
velocity are also at their lowest. QED.
Finally, as a reprise on my first point:
There have been very many plots published over the last 30 years showing
measured & inferred boat & crew velocities & accelerations. Most
recently we have the excellent work which Magnus has published & which
has been discussed here (but surprisingly little discussion, I'd have
thought...). What these show is how very different are these plots for
any individual & even between consecutive strokes. Yet the overwhelming
consensus confirms the simple logic of my second paragraph - that system
energy & speeds are least at the catch.
Science is straighforward, words are malleable. One report of a swallow
does not make a summer, nor does one report of that swallow flying
backwards upset established science (although it does make us review it.
The first essence of conducting a scientific review is that the
conditions of the observation & the observations themselves should be
repeatable.
For those of a less scientific bent it may sometimes seem that one or a
few guys swimming against the overwhelming tide of observation & science
should be accorded special status. Maybe they should, in which case
time will eventually tell. However, we both know the huge investment
over the years by coal & oil companies in seeking to spread doubt on the
consensus over global warming & impugn the motives of the preponderance
of those researching this topic. We know, too, how such unrelenting
assaults pushes people into corners, & into shunning the nay-sayers, in
ways which then may, if you cherry-pick private emails, look dodgy.
Time & science will continue quietly & inexorably towards an ever fuller
exposition of the facts (which was how the underhand techniques of Big
Tobacco were laid bare). The important difference between commercial &
politically motivated screwing with science & today's entertaining
discussion of an alleged, probably anomalous, high boat speed at the
catch is that there are no material costs if we get the boat speed issue
wrong, but may be ghastly consequences for us all if climate policy gets
diverted by the tactics of Big Energy.
mruscoe
> Science is straighforward, words are malleable. One report of a swallow
> does not make a summer, nor does one report of that swallow flying
> backwards upset established science (although it does make us review it.
> The first essence of conducting a scientific review is that the
> conditions of the observation & the observations themselves should be
> repeatable.
>
I think this is all a fuss about nothing. I think Mike and others have
been exposed to something that has blown apart their strongly-held
preconception, and are suffering from a bit of culture shock, but I
don't get the impression that anyone is making any definite claims,
other than showing people that the world is conclusively not what they
thought it was.
I don't think there's anything new here at all, other than the shiny new
piece of equipment with interesting video capabilities. The data capture
is a limited subset of what has been used by various rowing
biomechanics boffins for years.
Carl Douglas
I'm left wondering which particular preconceptions have been dismembered
by what?
zeke_hoskin
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Carl wrote:
The only sense in which the boat may even be imagined to accelerate as
you continue your recovery is WRT your own body. But that's just one
and he's usually right. But in this case, it is easy to accelerate the
boat
(not the total system) by yanking in your feet good and hard. I think
the point is
that it's overall a poor idea, since doing so increases drag. On the
other hand,
too slow a recovery means too long a time without the oars in the
water, so
at least for the last few strokes of a tight race, I'd conjecture that
it's best to
operate in a region of lowered efficiency in order to be able to
expend maximum
power. And for the first few strokes, drag is less important than
inertia and
the faster the recovery, the better.
//Zeke. It's also a good idea not to get your oars stuck on top of the
surfski
you're trying to pass - you guys who stick to rowing-only races miss
so much.
Tinus
> Tinus wrote:
> >> But what point is there in accelerating the boat after extraction?
>
> > To get the foot stretcher towards the rower in order to make the next
> > stroke.
>
> But the boat has no need to accelerate for that to happen.
But in that special case the stretcher can't accelerate towards the
rower faster than the boat decelerates (times the ratio between mass
of the total system and mass of the rower). What if I want to move
faster?
Carl Douglas
Nothing wrong with special cases, except that they can be highly
unrepresentative, & may be hard or pointless to achieve except under
particular circumstances which might relate to a single stroke.
Thus if you start rowing a stationary boat from sitting at backstops, &
you manage (as I would wish) to engage your blades before your feet hit
the stretcher, then the boat might be moving faster than your CoG at
that catch & might indeed be the fastest thing around. But that unique
situation will not apply to the subsequent strokes.
Similarly, you might wish, when part way through the recovery to
suddenly raise the rate. So you pull hard up the last bit of the slide
to get an earlier catch &, perhaps, that might again bring boat speed
right up at the catch.
However, in your normal rowing stroke in a succession of similar strokes
that cannot happen, due to the simple energy deficit that exists at the
catch & the inevitability that, as at the finish, there will be a close
identity between the velocities of boat & bodily CoG.