drag computation

drag computation

Post by andrew.slia.. » Sun, 17 Mar 2013 01:02:06


Hi Thomas,

If we increase the oar length of a Smoothie (or any less-efficient blade) in order to get a similar feeling for the rower, the blade efficiency will not change appreciably.  So, while the rower will feel the same amount of work is being put in - there is more being wasted in the water due to slip.  This in turn leads to a lower boat speed.

Andrew

 
 
 

drag computation

Post by andrew.slia.. » Sun, 17 Mar 2013 01:05:17

To Carl's post - I think the first half of what you said is an expansion of my post immediately below.  (I should have read that first, before responding to Thomas!)

As for the remainder of your post regarding blade depth, it is something that I began to look into after a discussion with Walter Martindale last year - but as of now I have only scratched the surface (haha).

I think that blade depth analysis certainly does warrant further study, and I plan to look at this over the next while.  Stay tuned, and I will report my findings as they come up.

Andrew
Show trimmed content

 
 
 

drag computation

Post by andrew.slia.. » Sun, 17 Mar 2013 01:29:01

...I'm having a hell of a time trying to figure out how to post replies to specific posts on here.  Apologies for my disorganized-looking comments.

Andrew

 
 
 

drag computation

Post by John Greenl » Sun, 17 Mar 2013 10:22:32

Quote:

> To Carl's post - I think the first half of what you said is an expansion of my post immediately below.  (I should have read that first, before responding to Thomas!)
> As for the remainder of your post regarding blade depth, it is something that I began to look into after a discussion with Walter Martindale last year - but as of now I have only scratched the surface (haha).
> I think that blade depth analysis certainly does warrant further study, and I plan to look at this over the next while.  Stay tuned, and I will report my findings as they come up.
> Andrew

Andrew, your work is beautiful-  thanks for letting us in on it!  

I have a very speculative thought about a possible effect of blade depth.  When I watch video of Mahe Drysdale, it strikes me how deep he sends his blades right after the catch, and then they come back up quite abruptly about midway in the drive-  so they make a u-shaped vertical trajectory in the water for the first half or so of the drive.  This always reminds me of the technique used to get high efficiency with an Inuit kayak paddle.  If you have seen these paddles, they look ridiculously narrow- the blades are quite long, but often only 3 inches wide.   I made one for my son a while back, and tried it out.  If you just stick it in and pull it straight through the water like a canoe paddle it slips horribly- about like a giant coffee stirrer.   So, what you do is to hold it out from the boat with the shaft almost horizontal and slice it  downward into the water as you begin the stroke, and then move it in a nearly v-shaped vertical path, taking it in deep and then reversing quickly and bringing it back up to the surface at the end of the stroke.  The combination of the vertical motion and the pull in the propulsive direction causes the blade to have the proper positive angle of attack so that it acts like a very high aspect-ratio  (very efficient) foil, generating lots of lift.  It of course stalls in the middle for an instant as it reverses direction from down to up, but it is quite amazing- when you do it that way, it holds onto the water wonderfully,  you can actually put enough pressure on it to feel it flex.  It is highly efficient, which is why modern sea kayakers like it for long-distance work.  You have to try it to believe what a huge increase it makes in the propulsive force you can apply, and how little it slips.  You can actually feel it twang as it suddenly unloads and un-flexes as it comes out of the water.

Is it possible that what Mahe is doing in the vertical dimension has some similar effect-  particularly in your second regime, when the flow wants to separate off the back side of the oar?  I wonder if adding a vertical component to the velocity, so the water flows partly vertically over the blade at a low angle of attack, might delay or inhibit that separation?

Cheers,

John G

 
 
 

drag computation

Post by John Greenl » Sun, 17 Mar 2013 10:27:22

  The puddle is a monument to the work
 which did _not_ move the boat.

Wonderful!  Exactly true and as always elegantly explained!

--John G

 
 
 

drag computation

Post by thomas.k.car.. » Sun, 17 Mar 2013 15:45:43

Hi Carl, thanks for the detailed reply! I am glad to say I can follow almost all of your explanation so many thanks for providing it, however I am still stuck on the same issue I see which is the point below relating to the blade feeling "heavier"

Quote:

> 3. It feels "heavier" - the obvious consequence of reduced slip causing

> it to take longer to complete the stroke, & your impatience encouraging

> you to pull harder, thus _making_ it harder, instead of pulling normally

> & allowing the stroke to finish naturally in a slightly longer time

Now I do understand this point, so if you row deeper "correctly" then you finish in a longer time and equally if you have a more eddicient blade then you also finish in a longer time than you would with a less efficient blade, but how does that then relate to the boat speed? Most importantly what is the outcome of the rowers ability to accelerate the boat thought the stroke?

In my mind (and this is where I struggle) with this added efficiency you will end up rating lower, so a potential for less acceleration through the stroke and some of the information from biorow suggests that good acceleration is key (which makes sense, more acceleration = higher velocity)

http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews11.pdf

 
 
 

drag computation

Post by Carl » Sun, 17 Mar 2013 21:25:34


Quote:

>> To Carl's post - I think the first half of what you said is an expansion of my post immediately below.  (I should have read that first, before responding to Thomas!)
>> As for the remainder of your post regarding blade depth, it is something that I began to look into after a discussion with Walter Martindale last year - but as of now I have only scratched the surface (haha).
>> I think that blade depth analysis certainly does warrant further study, and I plan to look at this over the next while.  Stay tuned, and I will report my findings as they come up.
>> Andrew

> Andrew, your work is beautiful-  thanks for letting us in on it!

> I have a very speculative thought about a possible effect of blade depth.  When I watch video of Mahe Drysdale, it strikes me how deep he sends his blades right after the catch, and then they come back up quite abruptly about midway in the drive-  so they make a u-shaped vertical trajectory in the water for the first half or so of the drive.  This always reminds me of the technique used to get high efficiency with an Inuit kayak paddle.  If you have seen these paddles, they look ridiculously narrow- the blades are quite long, but often only 3 inches wide.   I made one for my son a while back, and tried it out.  If you just stick it in and pull it straight through the water like a canoe paddle it slips horribly- about like a giant coffee stirrer.   So, what you do is to hold it out from the boat with the shaft almost horizontal and slice it  downward into the water as you begin the stroke, and then move it in a nearly v-shaped vertical path, taking it in deep and then reversing qui

ckly and bringing it back up to the surface at the end of the stroke.  The combination of the vertical motion and the pull in the propulsive direction causes the blade to have the proper positive angle of attack so that it acts like a very high aspect-ratio  (very efficient) foil, generating lots of lift.  It of course stalls in the middle for an instant as it reverses direction from down to up, but it is quite amazing- when you do it that way, it holds onto the water wonderfully,  you can actually put enough pressure on it to feel it flex.  It is highly efficient, which is why modern sea kayakers like it for long-distance work.  You have to try it to believe what a huge increase it makes in the propulsive force you can apply, and how little it slips.  You can actually feel it twang as it suddenly unloads and un-flexes as it comes out of the water.
Quote:

> Is it possible that what Mahe is doing in the vertical dimension has some similar effect-  particularly in your second regime, when the flow wants to separate off the back side of the oar?  I wonder if adding a vertical component to the velocity, so the water flows partly vertically over the blade at a low angle of attack, might delay or inhibit that separation?

> Cheers,

> John G

John -
This is exactly how I have described, here on RSR on various occasions
in the past, the action of a traditional straight & narrow whaler oar.
Thank you.

The whaler style of oar-blade has a sort-of elongated diamond
cross-section when cut through its major axis.  It is ideally used with
a lugging stroke, in which the rower drives it diagonally down through
the water in the first part of the stroke, then hoists it diagonally
back up to the finish.  The somewhat foil-shaped cross-section allows
the blade, at the resultant velocities & angles of attack, to sustain
lift in the down & the up phases of that stroke.

'Normal' flat-water rowers imagine this to be a quaint way to row but
they should try first before judging!   This is in reality a far more
useful stroke for propelling a rather slow vessel than a 'conventional'
2D sweeping action with the same implement - which would then operate
largely stalled & with air entrained for most of the stroke.

Traditional techniques evolved to make the most efficient use of
traditional oars & paddles - those who didn't learn the perils of simple
orthodoxy caught fewer fish & brought up fewer offspring - & of course
the designer & users of 'Wing-type' kayak paddles caught onto this.

Thus the action with a wing kayak paddle is tailored to sustain lift at
all possible stages of the stroke, with a largely sideways main part
after the entry & the paddle often leaving the water at a point
physically ahead of where it entered.  This contrasts markedly with how
racing kayaks used once to be paddled, with a lot of straight draw on an
increasingly stalled blade.

Marathon canoe paddlers have long adopted the cranked or angled flat
paddle - the blade angled forward to the line of the shaft.  They make a
forceful plunging entry at maximum reach but do not generally take the
stroke past the hip.  The plunging entry, assisted by the forward
angling of the blade, takes a relative gentle resulting angle of attack
(resolving the vertical component applied by the paddler with the
forward motion supplied by the boat) which generates efficient lift all
the way in at the low surface loadings the paddler can impose on a
relatively large blade area, & the paddler extracts before the
sternwards next part of the action becomes too far stalled.  That stall
process, due to the transient nature of any paddle or oar stroke, is
unlike the fully-developed stall that one sees in a steady-state test in
a laboratory water channel.

On that last point: I well remember one very decent former UK oar-maker
spending funds to have water-channel tests 'validate' his blade design -
for advertising purposes.  Those tests held the blade at a typical
barely-immersed depth, perpendicular to a steady flow.  They ignored the
seriously unsteady-state nature of the water flows in a real rowing
stroke, in which flows are continually changing & evolving & always far
from equilibrium at every part of the stroke. They apparently failed to
explore depth of immersion effects.  So what did those results really
mean?  Diddly-squat, in all probability.

You remark on Mahe Drysdale's technique & we both agree he goes deep.  I
think that is good, but as with all personally-evolved techniques it may
contain good & less good aspects, the result being - together with many
other aspects of Mahe's strength, physical capacity & attitude - the sum
of those components.  While we should take lessons from such a master
(and from similarities with the actions of many other fine scullers) we
should not necessarily imitate everything, even of we could, but should
also try to dissect those actions in order to avoid enshrining the less
helpful aspects of their techniques.  Difficult task!

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


 
 
 

drag computation

Post by Carl » Sun, 17 Mar 2013 22:44:32


Quote:
> Hi Carl, thanks for the detailed reply! I am glad to say I can follow almost all of your explanation so many thanks for providing it, however I am still stuck on the same issue I see which is the point below relating to the blade feeling "heavier"

>> 3. It feels "heavier" - the obvious consequence of reduced slip causing

>> it to take longer to complete the stroke, & your impatience encouraging

>> you to pull harder, thus _making_ it harder, instead of pulling normally

>> & allowing the stroke to finish naturally in a slightly longer time

> Now I do understand this point, so if you row deeper "correctly" then you finish in a longer time and equally if you have a more eddicient blade then you also finish in a longer time than you would with a less efficient blade, but how does that then relate to the boat speed? Most importantly what is the outcome of the rowers ability to accelerate the boat thought the stroke?

> In my mind (and this is where I struggle) with this added efficiency you will end up rating lower, so a potential for less acceleration through the stroke and some of the information from biorow suggests that good acceleration is key (which makes sense, more acceleration = higher velocity)

> http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews11.pdf

Let's dissect this a bit?

What matters most is that the work you do is not wasted.

We have come to regard rating as equalling speed, but that is a serious
over-simplification.  And we have come to see rating as related to how
quickly we can "get the blade through", which is also a fallacy.

If a power stroke is a bit more efficient is it likely to take slightly
longer to complete, for the same arc.  That's inevitable.  But let's
kill the fallacy about rating.  Rating is determined not by how quickly
you get from catch to finish but by how quickly & _smoothly_ your
recovery takes you from finish to catch.

If you try to get a quicker power phase to the stroke you will either
finish short or row shallow.  Well, we've all done that to keep up with
the other guys in the crew after a hard outing - guilty as charged!
Unfortunately so many coaches are shouting at their crews to get the
rate up in the water, & that simply can't be done.  If you pull 10%
harder without affecting the efficiency or length of the stroke you
won't go even 3% faster.  If your power phase takes 0.8 sec before you
increased the loading, now it will take 0.77 sec.  So you've shaved all
of 0.03 sec off the stroke time.  If you were rating say 30spm, that
means every stroke that was taking 2 sec will now take 1.97 sec.  And,
if you do nothing else, that will only raise your rate from 30 to 30.46 spm.

What often happens is that rowers shorten up when told to "raise the
rate in the water". What should happen is that you accept the stroke
will take the same time and work to make a smoother, swifter recovery.
I hope you get my point?

How do we make the faster recovery needed to raise the rate?  Well, it
ain't done by sudden gestures, so forget about throwing the hands away
faster.  Ideally we need to keep the same controlled action as at lower
rates, free from all sudden changes of speed (which simply bucket the
boat & waste energy), & simply make a small speed increase in every part
of that action.  In reality, all many crews need to do is to cut out
their tendency to hang about before the catch.  Why get forward & then
stop?  It only checks the boat.  You've come forward to take the catch
so get on with it - take the catch on the fly, not from stasis & the
boat will check less, your catch will be slicker, sooner & harder.

Most crews can easily save 1/10sec in this way and many can save a lot
more.  So just by not waiting for the catch a rate of 30 spm can rise
effortlessly to 31.6 spm & many could do better.  But your recovery is
still the longest part of the stroke during which you do the least
actual work.  It's not there so you can 'recover' your energy &
composure.  It's there only because the funny cyclic nature of the
rowing action demands we waste our time moving back to the catch for
another bite of the cherry.  So if you have 1.2 sec to make the recovery
& can reduce that time to 1 sec - which isn't very hard to do provided
you don't rush at it, just move more slickly - then your rate will rise
effortlessly from 30 to 33.3 spm.  My word, how did that come about?

Next, this stuff about 'accelerating the boat'.  Sorry to be boring but
the object of rowing is _not_ to accelerate the boat.  That's just
another of those lovely motivating-but-meaningless coaching mantras.  We
wouldn't want it accelerating away from us, would we, even if that were
a possibility?

Inevitably the boat surges & checks under us, but that's because it's
very much the junior partner in the exercise.  The biggest lump by far
is AKA 'the crew'.  So the boat is like a somewhat weighty pair of
skates.  I doubt the skater wants to accelerate his or her skates.  They
just to complete the course in the least possible time without parting
from those skates.  It's the same in rowing.  The boat is attached to
our feet & goes wherever we go, but because we keep lengthening &
shortening the gap between our thorax & the stretcher, the boat speed is
constantly varying while our own CofG travels at a pretty constant speed.

What we do in rowing is try to deliver as much useful power as we
possible in the least disruptive way - not increasing the surge & check
on the boat, not bouncing it, & definitely not accelerating it.  In
fact, during the first part of the power stroke every shell actually
decelerates.  Then in the last part of the stroke it speeds up until it
reaches the speed of the crew.  And during recovery, if we're skilled
enough, we decelerate our own passage over the water (_not_ up the
slide) by moving astern WRT the boat through pulling on the stretcher &,
thereby, sustaining the boat's run through the water despite the fluid
drag which is trying to slow it down.

That's a whole bundle of worry - sorry about that, but it probably needs
saying in view of the wall of technical disinformation which normally
surrounds every rowing outing.  Rowing is a simple thing too often made
complicated.

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


 
 
 

drag computation

Post by Carl » Sun, 17 Mar 2013 22:54:00


Quote:

>> Hi Carl, thanks for the detailed reply! I am glad to say I can follow
>> almost all of your explanation so many thanks for providing it,
>> however I am still stuck on the same issue I see which is the point
>> below relating to the blade feeling "heavier"

>>> 3. It feels "heavier" - the obvious consequence of reduced slip causing

>>> it to take longer to complete the stroke, & your impatience encouraging

>>> you to pull harder, thus _making_ it harder, instead of pulling normally

>>> & allowing the stroke to finish naturally in a slightly longer time

>> Now I do understand this point, so if you row deeper "correctly" then
>> you finish in a longer time and equally if you have a more eddicient
>> blade then you also finish in a longer time than you would with a less
>> efficient blade, but how does that then relate to the boat speed? Most
>> importantly what is the outcome of the rowers ability to accelerate
>> the boat thought the stroke?

>> In my mind (and this is where I struggle) with this added efficiency
>> you will end up rating lower, so a potential for less acceleration
>> through the stroke and some of the information from biorow suggests
>> that good acceleration is key (which makes sense, more acceleration =
>> higher velocity)

>> http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews11.pdf

> Let's dissect this a bit?

> What matters most is that the work you do is not wasted.

> We have come to regard rating as equalling speed, but that is a serious
> over-simplification.  And we have come to see rating as related to how
> quickly we can "get the blade through", which is also a fallacy.

> If a power stroke is a bit more efficient is it likely to take slightly
> longer to complete, for the same arc.  That's inevitable.  But let's
> kill the fallacy about rating.  Rating is determined not by how quickly
> you get from catch to finish but by how quickly & _smoothly_ your
> recovery takes you from finish to catch.

> If you try to get a quicker power phase to the stroke you will either
> finish short or row shallow.  Well, we've all done that to keep up with
> the other guys in the crew after a hard outing - guilty as charged!
> Unfortunately so many coaches are shouting at their crews to get the
> rate up in the water, & that simply can't be done.  If you pull 10%
> harder without affecting the efficiency or length of the stroke you
> won't go even 3% faster.  If your power phase takes 0.8 sec before you
> increased the loading, now it will take 0.77 sec.  So you've shaved all
> of 0.03 sec off the stroke time.  If you were rating say 30spm, that
> means every stroke that was taking 2 sec will now take 1.97 sec.  And,
> if you do nothing else, that will only raise your rate from 30 to 30.46
> spm.

> What often happens is that rowers shorten up when told to "raise the
> rate in the water". What should happen is that you accept the stroke
> will take the same time and work to make a smoother, swifter recovery. I
> hope you get my point?

> How do we make the faster recovery needed to raise the rate?  Well, it
> ain't done by sudden gestures, so forget about throwing the hands away
> faster.  Ideally we need to keep the same controlled action as at lower
> rates, free from all sudden changes of speed (which simply bucket the
> boat & waste energy), & simply make a small speed increase in every part
> of that action.  In reality, all many crews need to do is to cut out
> their tendency to hang about before the catch.  Why get forward & then
> stop?  It only checks the boat.  You've come forward to take the catch
> so get on with it - take the catch on the fly, not from stasis & the
> boat will check less, your catch will be slicker, sooner & harder.

> Most crews can easily save 1/10sec in this way and many can save a lot
> more.  So just by not waiting for the catch a rate of 30 spm can rise
> effortlessly to 31.6 spm & many could do better.  But your recovery is
> still the longest part of the stroke during which you do the least
> actual work.  It's not there so you can 'recover' your energy &
> composure.  It's there only because the funny cyclic nature of the
> rowing action demands we waste our time moving back to the catch for
> another bite of the cherry.  So if you have 1.2 sec to make the recovery
> & can reduce that time to 1 sec - which isn't very hard to do provided
> you don't rush at it, just move more slickly - then your rate will rise
> effortlessly from 30 to 33.3 spm.  My word, how did that come about?

> Next, this stuff about 'accelerating the boat'.  Sorry to be boring but
> the object of rowing is _not_ to accelerate the boat.  That's just
> another of those lovely motivating-but-meaningless coaching mantras.  We
> wouldn't want it accelerating away from us, would we, even if that were
> a possibility?

> Inevitably the boat surges & checks under us, but that's because it's
> very much the junior partner in the exercise.  The biggest lump by far
> is AKA 'the crew'.  So the boat is like a somewhat weighty pair of
> skates.  I doubt the skater wants to accelerate his or her skates.  They
> just to complete the course in the least possible time without parting
> from those skates.  It's the same in rowing.  The boat is attached to
> our feet & goes wherever we go, but because we keep lengthening &
> shortening the gap between our thorax & the stretcher, the boat speed is
> constantly varying while our own CofG travels at a pretty constant speed.

> What we do in rowing is try to deliver as much useful power as we
> possible in the least disruptive way - not increasing the surge & check
> on the boat, not bouncing it, & definitely not accelerating it.  In
> fact, during the first part of the power stroke every shell actually
> decelerates.  Then in the last part of the stroke it speeds up until it
> reaches the speed of the crew.  And during recovery, if we're skilled
> enough, we decelerate our own passage over the water (_not_ up the
> slide) by moving astern WRT the boat through pulling on the stretcher &,
> thereby, sustaining the boat's run through the water despite the fluid
> drag which is trying to slow it down.

> That's a whole bundle of worry - sorry about that, but it probably needs
> saying in view of the wall of technical disinformation which normally
> surrounds every rowing outing.  Rowing is a simple thing too often made
> complicated.

> Cheers -
> Carl

In fact my example of the rate changing effect of "getting it through
faster" was an overestimate.  It should have read:

"If you pull 10% harder without affecting the efficiency or length of
the stroke you won't go even 3% faster.  If your power phase takes 0.8
sec before you increased the loading, now it will take 0.777 sec.  So
you've shaved all of 0.023 sec off the stroke time.  If you were rating
say 30spm, that means every stroke that was taking 2 sec will now take
1.977 sec.  And, if you do nothing else, that will only raise your rate
from 30 to 30.35 spm."

So, even less rate gain for a whole lotta pain.

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


 
 
 

drag computation

Post by John Greenl » Mon, 18 Mar 2013 09:13:17

Quote:

> We have come to regard rating as equalling speed, but that is a serious  
> over-simplification.  And we have come to see rating as related to how  
> quickly we can "get the blade through", which is also a fallacy.

Yes, thanks!  This makes me feel better about not having a stroke rate meter.  I have only a GPS for speed.  So I have always just worked on trying at any given speed to scull at the lowest possible (estimated) stroke rate.  What Carl says is immediately apparent-  at a given speed, the drive has to take a certain (fixed) time, if you keep the same arc length.  The only variability in rate comes with how much time the recovery takes.  If you try to go the same speed at a lower rate, that means a longer recovery time and fewer drives per minute, so you have to do more work during each drive.  I don't know whether coaches would agree with this, but for me it seems to be a nice way to work to develop a stronger, more efficient drive.

cheers,

John G

 
 
 

drag computation

Post by John Greenl » Mon, 18 Mar 2013 09:47:53

Quote:

> John -
> This is exactly how I have described, here on RSR on various occasions
> in the past, the action of a traditional straight & narrow whaler oar.  
> Thank you.
> The whaler style of oar-blade has a sort-of elongated diamond  
> cross-section when cut through its major axis.  It is ideally used with  
> a lugging stroke, in which the rower drives it diagonally down through  
> the water in the first part of the stroke, then hoists it diagonally  
> back up to the finish.  The somewhat foil-shaped cross-section allows  
> the blade, at the resultant velocities & angles of attack, to sustain  
> lift in the down & the up phases of that stroke.

Ah, that's great, Thanks- sorry to make you repeat!  I had thought that the long, narrow-bladed oars must work the same way, but I've never had the pleasure of rowing with one.  

By the way, the pattern I used to make the Inuit paddle blade also has a symmetrical, elongated diamond-shaped cross section at its root, tapering through a progressively thinning and widening oval section to a nearly flat section at the tip- a nice, easily formed, strong shape that has quite good lift.   I love the fact that people who presumably had no access to wide timbers for their paddles came to this elegant shape and way of using it.  I do think, from my experience, that once the shape was in hand, so to speak, the use would have come quickly-  it's very easy to feel how it works, the effect is not at all subtle, and quite delightful.  I expect the feel of the paddle is more direct to the hand than an oar would be, with this happening on the other end of the lever.  

Also by the way,  I would never try to imitate Mahe Drysdale's sculling.  I'm quite convinced that he, and all those other guys, and gals, that I watch in the Olympics are of some other (entirely superior) species than the one i belong to.    

Cheers,

John G

 
 
 

drag computation

Post by thomas.k.car.. » Tue, 19 Mar 2013 23:52:22

Hi Carl, thanks for that expanation, it certainly rings true that the most important factor in rowing well is maintaining length and reducing the time taken to take the catch so thnaks for correcting me on this, its seems more important to focus on what you are doing on the recovery to try and get rate in that case.

On your comments on not wanting to accelerate the boat, perhaps I should have clarified in my post that I mean the boat+rowers, and I am not sure I completely follow your reasoning? From your post I agree with this statement wholeheartedly

"What we do in rowing is try to deliver as much useful power as we
possible in the least disruptive way"

but I dont agree with this

"- not increasing the surge & check on the boat, not bouncing it definitely not accelerating it"

A lot of what I have read online taling about the acceleration/deceleration of he boat suggests that the best crews show a deeper but shorter deceleration of the boat and then accelerate the boat much quicker than a lesser skilled crew (see link), and a paper from Biorow even goes so far to state "it is very unproductive to try to minimise this so-called boat check, which is one of the myths of rowing biomechanics."

http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews11.pdf

So according to this the ideal stroke is one which has the greatest "check" on the boat but for the shortest amount of time, puts the most acceleration on the boat through the stroke, and then could even add some additional acceleration during the recovery from pulling themselves up the slide on the recovery (or actually pulling the boat towards them since the boat is considerably the lighter of the two) whilst making sure the approach to the catch is timed just right to ensure the next stroke starts as above with a sharp deceleration but quick power application.

The main difficulty I would imagine is ensuring the sharp deceleration at the catch was coming from covering the blades (and so aplying a brake on the boat) and not from the fact that the legs were pushing on the stretcher before the blades are completely covered

 
 
 

drag computation

Post by stewie.. » Wed, 20 Mar 2013 01:46:23

Quote:

> Hi Carl, thanks for that expanation, it certainly rings true that the most important factor in rowing well is maintaining length and reducing the time taken to take the catch so thnaks for correcting me on this, its seems more important to focus on what you are doing on the recovery to try and get rate in that case.

> On your comments on not wanting to accelerate the boat, perhaps I should have clarified in my post that I mean the boat+rowers, and I am not sure I completely follow your reasoning? From your post I agree with this statement wholeheartedly

> "What we do in rowing is try to deliver as much useful power as we

> possible in the least disruptive way"

> but I dont agree with this

> "- not increasing the surge & check on the boat, not bouncing it definitely not accelerating it"

> A lot of what I have read online taling about the acceleration/deceleration of he boat suggests that the best crews show a deeper but shorter deceleration of the boat and then accelerate the boat much quicker than a lesser skilled crew (see link), and a paper from Biorow even goes so far to state "it is very unproductive to try to minimise this so-called boat check, which is one of the myths of rowing biomechanics."

> http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews11.pdf

> So according to this the ideal stroke is one which has the greatest "check" on the boat but for the shortest amount of time, puts the most acceleration on the boat through the stroke, and then could even add some additional acceleration during the recovery from pulling themselves up the slide on the recovery (or actually pulling the boat towards them since the boat is considerably the lighter of the two) whilst making sure the approach to the catch is timed just right to ensure the next stroke starts as above with a sharp deceleration but quick power application.

> The main difficulty I would imagine is ensuring the sharp deceleration at the catch was coming from covering the blades (and so aplying a brake on the boat) and not from the fact that the legs were pushing on the stretcher before the blades are completely covered

'Check' is a change (decrease) in speed - acceleration is change in speed per unit time. The check is therefore the integral of those acceleration-time profiles from the zero before the catch to the zero after it, which I believe is lower for the "more skilled" crews.
 
 
 

drag computation

Post by thomas.k.car.. » Thu, 21 Mar 2013 23:12:15

Interesting, I had always associated "check" with an excessive decelleration of the boat at the catch which I I would have thought would show as the depth of the peak on the acceleration curve of the boat (so deeper the spike, the more the check) however you think that check is the area under the curve so the amount that the boat has deccelerated over the stroke