drag computation

drag computation

Post by John Greenl » Mon, 11 Mar 2013 04:32:59


Carl wrote>

Quote:
> If we take the analogy of a racing car, would we deduce its aerodynamic
> drag losses by using a dynamometer to measure the power delivered at the  
> drive to the prop shafts?  I think we know enough to say that there are
> significant further energy costs in the generation of smoke, noise &
> heat as a result of wheel spin & of tyre & suspension hysteresis.

Hi Carl,

This really is tricky, but I believe I have it right.  I think your analogy is not exact.  if we measure the force  the oar shaft exerts on the pin,  (and the feet exert on the footplate, as I wrote)  those are the actual forces acting on the boat. Multiply that net force by the boat speed and that's the actual propulsive power. In your car analogy,  you would be measuring the power delivered to the axles, which is larger than the power propelling the car, as you correctly say-  there is loss involved at the tire/road interface, and in bearing friction, etc.  Going back to the boat,  the oar/water interface has a lot of loss, and for a given handle force applied by the rower that loss will certainly reduce the pin force.  So the rower is having to supply more power than is applied to the boat at the pin.  But the pin force is still the actual force that pushes the boat.  The analogy to your car example might be to measure the power applied by the rower to the oar-  that is indeed bigger than the propulsive power to the boat at the pin, because of the loss at the blade/water interface.  

Quote:
> Also, rowing carries its own KERS system.  It's not quite the same as in  
> F1, but it is still a cyclic energy storage & return system, using a
> mobile & quite plastic system of interlinked body masses which we move
> quasi-independently of the boat & at some energetic cost.  Over the
> first part of the stroke we can't help giving a disproportionate amount  
> of energy to the rower, who accelerates faster over the water than the  
> boat does (or they wouldn't move from frontstops!).  In the rest of the  
> stroke, up to the finish, the boat is belatedly catching up with the
> crew.  And during a chunk of the recovery the human KERS system is
> moving relative to the boat to generate the force needed to sustain its
> run despite hull friction.

Yes, exactly, and the measurements of pin force and footplate force take that into account properly.  The exchange of momentum and energy between the rower and the boat- the KERS system, as you say- is very important.  That exchange goes by way of the forces the rower exerts on the footplate and the pins-  those, after all, are the only points of force application between the rower and the boat.  For instance, on the recovery, when the boat needs to catch up to the rower, the rower is pulling the boat back to him by pulling on the footplate-  it's the footplate force that speeds up the boat and slows down the rower. So the rower is applying propulsive power to the boat through the footplate at the expense of his own kinetic energy- he slows down.   That's the Kinetic Energy Recovery System.  

This all seems very complicated if I try to think about the whole system.  What I finally realized is that you can isolate one part of the system- the boat- and consider all the forces that act on it.  There are only the pin force, the footplate force, and the drag forces.  That's all (again, assuming you can neglect the force due to rolling of the seat). Physics says that if you do that correctly- include ALL the forces that act DIRECTLY on the boat-  then you will know how the boat moves. Newton's laws work.  Again, all that other messy stuff, the internal motions and loss in the rower, the lossy stirring of water by the oar blades, aerodynamic drag on the rower, various frictional losses, etc,  all require power from the rower,  but the only power that actually moves the boat is applied through the pins and footplate.  

Or- how about this:  forget all about the oars and the rower.  All the boat "knows" is that something is pushing and pulling on its pins and footplate- that's all that makes it move.

I feel like I wasn't doing a good job explaining-- hope this is somewhat better.

Cheers,

John G

 
 
 

drag computation

Post by Carl » Mon, 11 Mar 2013 05:39:17


Quote:
> Carl wrote>
>> If we take the analogy of a racing car, would we deduce its aerodynamic
>> drag losses by using a dynamometer to measure the power delivered at the
>> drive to the prop shafts?  I think we know enough to say that there are
>> significant further energy costs in the generation of smoke, noise &
>> heat as a result of wheel spin & of tyre & suspension hysteresis.

> Hi Carl,

> This really is tricky, but I believe I have it right.  I think your analogy is not exact.  if we measure the force  the oar shaft exerts on the pin,  (and the feet exert on the footplate, as I wrote)  those are the actual forces acting on the boat. Multiply that net force by the boat speed and that's the actual propulsive power. In your car analogy,  you would be measuring the power delivered to the axles, which is larger than the power propelling the car, as you correctly say-  there is loss involved at the tire/road interface, and in bearing friction, etc.  Going back to the boat,  the oar/water interface has a lot of loss, and for a given handle force applied by the rower that loss will certainly reduce the pin force.  So the rower is having to supply more power than is applied to the boat at the pin.  But the pin force is still the actual force that pushes the boat.  The analogy to your car example might be to measure the power applied by the rower to the oar-  that is indeed

bigger than the propulsive power to the boat at the pin, because of the loss at the blade/water interface.

Maybe I'm being dumb, in which case that won't be the first time!  But
I'll try once more & see if one of us can clear the thinking of the
other - no prizes beyond a better understanding, which is a prize for all:

All that propels the boat & crew is the force on the oar-blades.  But
oars are seriously inefficient devices for connecting with water.  Every
stroke they slip, generate frictional losses & uselessly stir up water &
also mix it with air.  And the oar shafts are elastic, absorbing &
releasing energy as the load on them grows & falls.  The force at the
pin, resolved into the direction of motion as it should be, is ~50%
higher than the force on the blades, because the pin is simply a pivot
point or bearing within what amounts to a larger gearbox.

Adding in stretcher forces (not too easy in reality to resolve axially)
may leave you wrestling with the inertial terms arising from the
relative accelerations of boat & rower, & those between different bits
of the rower (some of them provided by work from the rower which never
appears as forces on pins or blades but is real work - will (I suggest)
knock that simpler model askew.

Taking an extreme example, consider a kid on a swing:  she gets that
swing moving by doing work, none of which can be measured at any point
between *** & earth as a force x a distance.  Well, we don't take it
that far, but we seem to my simple mind to do a bit of that.

As I said, maybe I'm wrong.  But I think it's not as simple as we'd both
like to think it is.  Any further thoughts?

Quote:
>> Also, rowing carries its own KERS system.  It's not quite the same as in
>> F1, but it is still a cyclic energy storage & return system, using a
>> mobile & quite plastic system of interlinked body masses which we move
>> quasi-independently of the boat & at some energetic cost.  Over the
>> first part of the stroke we can't help giving a disproportionate amount
>> of energy to the rower, who accelerates faster over the water than the
>> boat does (or they wouldn't move from frontstops!).  In the rest of the
>> stroke, up to the finish, the boat is belatedly catching up with the
>> crew.  And during a chunk of the recovery the human KERS system is
>> moving relative to the boat to generate the force needed to sustain its
>> run despite hull friction.

> Yes, exactly, and the measurements of pin force and footplate force take that into account properly.  The exchange of momentum and energy between the rower and the boat- the KERS system, as you say- is very important.  That exchange goes by way of the forces the rower exerts on the footplate and the pins-  those, after all, are the only points of force application between the rower and the boat.  For instance, on the recovery, when the boat needs to catch up to the rower, the rower is pulling the boat back to him by pulling on the footplate-  it's the footplate force that speeds up the boat and slows down the rower. So the rower is applying propulsive power to the boat through the footplate at the expense of his own kinetic energy- he slows down.   That's the Kinetic Energy Recovery System.

> This all seems very complicated if I try to think about the whole system.  What I finally realized is that you can isolate one part of the system- the boat- and consider all the forces that act on it.  There are only the pin force, the footplate force, and the drag forces.  That's all (again, assuming you can neglect the force due to rolling of the seat). Physics says that if you do that correctly- include ALL the forces that act DIRECTLY on the boat-  then you will know how the boat moves. Newton's laws work.  Again, all that other messy stuff, the internal motions and loss in the rower, the lossy stirring of water by the oar blades, aerodynamic drag on the rower, various frictional losses, etc,  all require power from the rower,  but the only power that actually moves the boat is applied through the pins and footplate.

> Or- how about this:  forget all about the oars and the rower.  All the boat "knows" is that something is pushing and pulling on its pins and footplate- that's all that makes it move.

> I feel like I wasn't doing a good job explaining-- hope this is somewhat better.

> Cheers,

> John G

Let me try it another way: if we add 1-way friction brakes to the
rotation of the oarlocks, what does that do to your model if we are
unable to quantify the energy that these brakes dissipate?  Does that
differ materially from the energy unproductively dissipated around the
blades in the water?

And then let's revert to a seemingly trivial example - that it really is
possible to move a boat in 1 direction simply by moving the body up &
down the slides in an appropriate manner, with no force on the pins.
There the forward or sternwards progress is due to combinations of
pitching with non-linear variation of fluid drag with speed.

Or was that a red-herring too far for a Saturday night ;)

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, 11 Mar 2013 13:24:39

Carl, thanks very much for sticking with me on this.  This is a good lesson for me-  I didn't anticipate how counterintuitive this would be to someone who has thought deeply and correctly about all these things as you have.  I just got home from playing a performance of Bernstein's West Side Story, which is quite demanding music and I'm too tired to think now, will respond more fully tomorrow.  For tonight, let me just say that the girl on the swing is a great (and not easy) physics exam problem.  It is in fact exactly the work she does, and the work she allows gravity to do on her, that drive the swing, and it is all real work, all of which is indeed forces acting through distances.  Physics is a stern ***,  she doesn't allow any hanky-panky with the laws of classical mechanics.  If we find something in our boat problem that doesn't agree with her, then we'll happily share a Nobel together!!  

Final late Saturday-night thought--   The oarlock brake is a great example.  Who knows how much power it will take- our poor rower may bust a gut moving that oar at glacial speed, but the pitifully small remaining net force that is the difference between the gigantic forward force the oar exerts on the pin and the gigantic backward force the rower exerts on the footplate, as he swears at us for making him do this, is still what moves the boat.  Goodnight!!

 
 
 

drag computation

Post by thomas.k.car.. » Mon, 11 Mar 2013 15:47:57

Although I am massively out of my depth I thought I'd jump in here following on from my earlier comment, I think as Carl is saying in testing/measuring boat shapes and designs you would need to not only take into account the force the rower is applying but also how the movement of the rower and the pitching of the boat comes into play, I would agree since if we are wanting to look at how the shape of the hull affects drag in rowing a boat that is pitching forwards and backwards, or rolling side to side would have the effect of changing the wetted surface area of the boat though the stroke and so the drag

If the weight is moving around in the boat, and the hull is sensitive to this pitching, then I would thought it would show up in the velocity of the shell but also in the acceleration of the hull, if the weight pitches and has a negative effect then you would see more deceleration in the recovery or a reduced acceleration through the drive phase

As mentioned in a separate bread I've actually been using an app on the iPhone that measures the acceleration of the boat, called rowing in motion, and due to using the app I had been thinking recently about how the app is obviously intended for use to check out technique, but that perhaps it could also be used to check out the boat hull performance. They have recently announced they are bringing out a set or wireless strain gauges to measure force at the pin, and also mentions foot stretcher measurements are on the board for future systems

http://www.rowinginmotion.com/cooperation-with-oarinspired/

Using this system I could see that you could use it it measure and calculate your best rig (as changing your gearing would affect how the boat accelerates I imagine) but also how the shape of the boat affects the boat acceleration depending on your individual technique. For example if a certain shape of boat caused a significant amount of drag when the boat pitched forwards and backwards a lot, and the rowers technique caused the boat to pitch in this way, then i would have thought you would see on the acceleration curve a larger deceleration (or reduced height of acceleration) through the stroke and could then recommend that rower either smoothes out their technique or tries a hull that "cushions" the pitching (like the ones with the increased buoyancy at the bows) and then test them again in that boat. Assuming the forces measured per stroke come out equal I would have thought this would be a valid test of the hull performance for that rower

 
 
 

drag computation

Post by Steve » Mon, 11 Mar 2013 16:28:29

While the John/Carl colloquy it's the pins or it's not the pins has a certain fascinating je ne sais quoi, I think it is diverging from what could be something of general value and importance. Up above a few posts Carl tries but does not succeed to focus on the fluid dynamics of the oars.

In terms of implications for serious rowers and scullers, the fluid dynamics of the blades and shaft are
1) stunningly relevant
2) surprisingly accessible to contemporary experimental and numerical techniques,
yet, as far as I can tell, unless there is a lot of secret knowledge in the vaults of the Canadian, UK, Australian, and NZ high performance programs,
3) generally ignored

With respect relevance, here are four pertinent questions  
i) what is the best blade shape and size?
To choose two extreme cases, It is hard to believe that a choice between the Dreher EHX and the C2 "fat2" is not influenced by blade hydrodynamics.

ii) what are the optimal entry and extraction angles
Kleshnev has assembled some useful experimental data, but there is no theoretical confirmation

iii) does i) interact with ii)
As far as I know, this is has not been addressed.

iv) how are the answers to  i), ii) and iii) related to body dimensions and strength
In the USA, one can find 50+ women scullers with the blades, spread, inboard and outboard of Olympic mens 1x gold medalists.  What's with that?

v) and what about shafts?
C2 small  circular section ("skinnys") vs. Croker larger diameter vs. Dreher asymmetric shaft vs either?

I'm not even sure we understand the qualitative features of the blade hydrodynamics, much less the quantitative ones.  

Take a look at Jim Dwyers' video
http://SportToday.org/
Download it an go through frame-by-frame. To my eye, there are some simply remarkable qualitative features of the fluid velocity field in the region about the blade,  e.g,  quite stunning vorticies coming off the outboard corners in the early part of the stroke prior to separation. Here is something else notable: whereas it is standard to say the flow reattaches at the end of the stroke with the inboard edge becoming the leading edge of the flow, Dwyers' video does not show that.

So, why say this is accessible to experimental and numerical techniques? Because there is a lot of work on insect flight that one can borrow from, both concepts and techniques.

A flapping insect wing is not wildly different than a flapping oar blade. Michael***enson, pioneered the fluid dynamics of insect flight an won a MacArthur for his efforts. He is now at Cal Tech and his lab is here http://SportToday.org/;

If a scientist can figure out the fluid dynamics of flapping fly wings, surely we can figure out the fluid dynamics of flapping oar blades.  (by the way, there is a quite entertaining TED talk by Disckenson here
http://SportToday.org/;and at just about 3 minutes into the talk there is an animation of the vortices coming off a fly wing that looks a lot like Dwyers' video)

 
 
 

drag computation

Post by Leo » Mon, 11 Mar 2013 16:54:02

Quote:

> A flapping insect wing is not wildly different than a flapping oar blade.

Oh yes it is!
The presence of the free surface increases the difficulty of the problem by 3 or 4 orders, before even considering turbulence and viscous effects at the interface.
 
 
 

drag computation

Post by Carl » Mon, 11 Mar 2013 22:31:59


Quote:

>> A flapping insect wing is not wildly different than a flapping oar blade.

> Oh yes it is!
> The presence of the free surface increases the difficulty of the problem by 3 or 4 orders, before even considering turbulence and viscous effects at the interface.

Agreed, Leo.  And I very much welcome Steven's comments - which leave a
lot of work & thinking to be done.

Maybe Steven misreads those vortices?  I think we're seeing air
entrainment, pure & simple, as a separate effect, not the kind of
single-fluid vortices we ought/want to get around a well covered oarblade.

And that's no criticism of the brilliant work that Kim has done, & which
I'd not had the pleasure of viewing previously.

Fascinating, all of it.  And quite a change from Bernstein, eh, John?
Didn't some music critic once devote one of his reports to a study of
Bernstein's personal choreography on the rostrum?

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 » Tue, 12 Mar 2013 03:59:42


Quote:
>All that propels the boat & crew is the force on the oar-blades.  But

oars are seriously inefficient devices for connecting with water.  Every
stroke they slip, generate frictional losses & uselessly stir up water &
also mix it with air.  And the oar shafts are elastic, absorbing &
releasing energy as the load on them grows & falls.

Absolutely, yes.  As with any mechanics problem with more than one moving part,  there are several complementary, correct ways of looking at it.  You're looking at the whole boat+rower system.  The only forces that couple the boat+rower to the rest of the world are the force of the oar blades on the water, and the drag forces.  Those have to balance at a steady pace, or else you'll slow down, or speed up.  

Those forces are extremely complicated, because they involve the fluid flows, both around the blades, and around the hull and rower.  We want to measure those fluid forces on the hull.  We could in principle get at that by knowing the force of the oar blade on the water, and solving the whole system of the complex motion of the rower and boat that both generate that force, and respond to it, all in order finally to infer the drag force on the hull.   Wow, that's a hard problem.  

But what if we change our viewpoint and look at only one part of the system, the boat itself. The enterprise of physics depends on this sort of trick.  Happily, the laws of classical mechanics tell us how to do this correctly.   Newton tells us that if we can know all the forces on the boat, we can calculate its motion.  We don't need to know how those forces were generated, we only need to know what their magnitudes and directions actually are at the points they are applied to the boat.  Or, for our present purpose, if we know all the forces on the boat but one (the drag), and we know the motion (the boat speed)  we can calculate the remaining force, the drag.  

The big advantage of this point of view for our purpose is that we can just directly measure the other forces, we don't need to get tangled up in the details of how they were generated. Whatever happens at both ends of the oar- whether the blade is churning its way through the water, or otters are swimming along holding the blades in their teeth (sorry,  don't know where that came from, but I like the image);  whether a rower is pulling on the handle or there's a rope on it being pulled by somebody in another boat,  we need for our purpose only to know the effects of all that on the boat itself.  We see that the oar is connected to the boat only at the pin, and we can directly measure the force it applies to the boat there.  If it is a rower pulling on the handle, then he also is connected to the boat, by his feet, and we need to know the force he applies there.  Luckily for us, that's all there is acting on the boat, other than the drag.  We know how to measure the boat speed.  Now we have everything we need to know to carry out our calculation of the drag. To the extent that our measurements are accurate, our calculation will be correct.

Gee, I hope this helps.  I still feel like I am making heavy work of an explanation that should be simple.  Sorry!

Other bits:

Your example of moving the boat without putting oars in the water is a very good one- when you cleverly move up and down the slide just the right way you can generate net motion of the boat.   Now you're applying fore-and-aft force to the boat only through the footplate.   So, again, we measure the force on the boat at the footplate, we measure the boat's motion, and we could calculate the actual drag.  No red herring, or any other color, Newton would really like this one, it's simpler without the oars.

Oh, and just for fun, here's a nice little discussion of swing-pumping with some good video demonstrations, by an old friend of mine and a wonderful teacher,  Bill Case, who teaches at Grinnell College>
http://www.grinnell.edu/academic/physics/faculty/case/swing/

enjoy!!  and thanks, I await your thoughts,

John G

 
 
 

drag computation

Post by John Greenl » Tue, 12 Mar 2013 04:02:48

Quote:

> Fascinating, all of it.  And quite a change from Bernstein, eh, John?

> Didn't some music critic once devote one of his reports to a study of

> Bernstein's personal choreography on the rostrum?

> Cheers -

> Carl

Yup-  that was complex motion indeed!  Unanalyzable, I would have said!

Cheers,
John

 
 
 

drag computation

Post by thomas.k.car.. » Tue, 12 Mar 2013 15:45:49

If the gearing of the blades stay the same between hulls (ie same set of blades, same inboard/outboard) then I would imagine for the purposes of your experiment this would make the losses from the blades equivalent and so would mean you could then focus on the hull efficiency in the experiment

Even better would be if you could use the same riggers on the hulls!

 
 
 

drag computation

Post by andrew.slia.. » Sat, 16 Mar 2013 10:47:26

Maybe now is a good time to jump into the conversation.  Regarding modelling the rowing stroke - specifically the hydrodynamic character of the blade in the water - I have done quite a bit of work, and recently put together a website to display (most of) it.

https://sites.google.com/site/surgingforwardrowing/

Without getting into too many specific details of the flow, or directly answering any of the above posed questions, I'm confident that this research can effectively be coupled with higher-level boat modelling and rower biomechanic modelling to design a comprehensive rower-oar-boat system.

So if there is a desire to collaborate on such a model, or if anyone has any questions related to the topic, don't be a stranger.

I'll apologize in advance if the response is delayed... the lack of funds available to work on rowing research full time means I'm generally busy with other work commitments

Andrew

 
 
 

drag computation

Post by thomas.k.car.. » Sat, 16 Mar 2013 21:48:28

Quote:

> Maybe now is a good time to jump into the conversation. Regarding modelling the rowing stroke - specifically the hydrodynamic character of the blade in the water - I have done quite a bit of work, and recently put together a website to display (most of) it. https://sites.google.com/site/surgingforwardrowing/ Without getting into too many specific details of the flow, or directly answering any of the above posed questions, I'm confident that this research can effectively be coupled with higher-level boat modelling and rower biomechanic modelling to design a comprehensive rower-oar-boat system. So if there is a desire to collaborate on such a model, or if anyone has any questions related to the topic, don't be a stranger. I'll apologize in advance if the response is delayed... the lack of funds available to work on rowing research full time means I'm generally busy with other work commitments Andrew

An interesting site, thanks for creating and sharing it! At the risk of thread jacking however there are some parts I am unsure about and would like clarifying, mostly on the pages talking about blade shape and blade efficency on this page

https://sites.google.com/site/surgingforwardrowing/rowing-research/eq...

Now although I agree with the fact that it is possible to design a blade that is more efficient that another (using the amount of slip of the blade as a measure of its efficiency) like concept have done with the big blade, I struggle with the connection between the more efficient blade being the faster blade, especially an absolute value of 1% or 2 seconds

The issue I have is that I see boat speed being a combination on the force the rower is able to apply, and the way that the rower applies that force (and how the boat/blades accelerate in response to this force). If you run the assumption that the rower has a finite amount of force they can apply, then to me it seems the more efficient the blade the slower the rower would be able to accelerate the boat past the blades connection with the water (as a rower would say it "feels" heavier), which means a reduced acceleration of the boat which means a slower overall boat.

Yes you can correct the "heavy" feeling by making the blade outboard shorter but then if that is the solution then why cant you just make the standard smoothie blade shaft longer in response to its reduced efficncy compared to the fat smoothie

Unfortunatly ive not really seen a discussion which has clarified this for me which makes me think more that focusing on boat shape is a more effective focus that on blade shape, especially if a rower has a "favourite" shape or type of blade already and the fastest gearing for them is found from testing

 
 
 

drag computation

Post by Carl » Sat, 16 Mar 2013 23:58:33


Quote:

>> Maybe now is a good time to jump into the conversation. Regarding modelling the rowing stroke - specifically the hydrodynamic character of the blade in the water - I have done quite a bit of work, and recently put together a website to display (most of) it. https://sites.google.com/site/surgingforwardrowing/ Without getting into too many specific details of the flow, or directly answering any of the above posed questions, I'm confident that this research can effectively be coupled with higher-level boat modelling and rower biomechanic modelling to design a comprehensive rower-oar-boat system. So if there is a desire to collaborate on such a model, or if anyone has any questions related to the topic, don't be a stranger. I'll apologize in advance if the response is delayed... the lack of funds available to work on rowing research full time means I'm generally busy with other work commitments Andrew

> An interesting site, thanks for creating and sharing it! At the risk of thread jacking however there are some parts I am unsure about and would like clarifying, mostly on the pages talking about blade shape and blade efficency on this page

> https://sites.google.com/site/surgingforwardrowing/rowing-research/eq...

> Now although I agree with the fact that it is possible to design a blade that is more efficient that another (using the amount of slip of the blade as a measure of its efficiency) like concept have done with the big blade, I struggle with the connection between the more efficient blade being the faster blade, especially an absolute value of 1% or 2 seconds

> The issue I have is that I see boat speed being a combination on the force the rower is able to apply, and the way that the rower applies that force (and how the boat/blades accelerate in response to this force). If you run the assumption that the rower has a finite amount of force they can apply, then to me it seems the more efficient the blade the slower the rower would be able to accelerate the boat past the blades connection with the water (as a rower would say it "feels" heavier), which means a reduced acceleration of the boat which means a slower overall boat.

> Yes you can correct the "heavy" feeling by making the blade outboard shorter but then if that is the solution then why cant you just make the standard smoothie blade shaft longer in response to its reduced efficncy compared to the fat smoothie

> Unfortunatly ive not really seen a discussion which has clarified this for me which makes me think more that focusing on boat shape is a more effective focus that on blade shape, especially if a rower has a "favourite" shape or type of blade already and the fastest gearing for them is found from testing

Let me add a few thoughts here:

1. Force does is not the only consideration since force alone does not
move the boat.  What does move the boat is Work, which is Force x
Distance (through which that force application travels.

2.  The force on the blade is the force that _you_ apply (mediated by
the leverage of the system).  The water around the blade only reacts by
delivering an equivalent opposing force.

3. The work done is the maximum work available to move the boat.  Sadly,
a fair chunk of the work you do gets wasted/dissipated.  When this
happens it is lost from the propulsive process and diverted into moving
& stirring up water, plus entraining air into the water.  This wasted
energy in part creates the puddle.  The puddle is a monument to the work
which did _not_ move the boat.

4. You can like the puddle, the moving of chunks of water & the
sternwards slip of the blade to wheel-spin & tyre smoke in a car making
a hasty getaway - it's a dead loss.

5. When we discuss blade or propulsive efficiency we are not talking
about Force; we are discussing how much of the Work (= Force x Distance)
that we put into the stroke ends up actually moving the boat - what's
left after the wasted work (puddle, etc) has been taken away.  We
express that as a percentage: 70% propulsive efficiency means that 30%
of the work we put in simply went to waste.

6. Better propulsive efficiency is one big area, largely ignored, where
rowers have the greatest potential to go faster - did they but know how
to achieve it.

Which brings me to a point which I believe Andrew has looked into at the
suggestion of others, and on which he had some interesting preliminary
results a while back (so I believe?):
What he seems not to address in that link, while looking closely at
blade shapes (which might be a rather secondary element in the overall
business of blade efficiency) is the Z-axis of the stroke - the vertical
component.

My own experiments, rational engineering argument, feedback from
thoughtful scullers, observation of champion scullers' techniques &, I
believe, some work done by Andrew (but I'm open to correction) confirm
the how deep you row your blade makes a really significant difference to
overall propulsive efficiency.  Put simply (& I have done this here a
few times!) somewhat deeper than is generally promoted is somewhat more
propulsively efficient.

In short, the rowing stroke is not 7 should not be treated as a
pre-ordained 2-D process with the 3rd dimension conveniently eliminated
from what we do & discuss by application of a widespread & dogmatic "no
deeper than this" rule.

Why is deeper better?
1. Because a deeper blade is entirely surrounded by water.
2. So you do not therefore entrain air behind the blade to disrupt the
essential tensile connection between blade & water & increase wasteful
slip & puddle-making.
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 period.
4. Unless you go _very_ deep (I wouldn't want to quantify that here,
please), you don't incur relevant countervailing losses due to looming -
the loom is thin, has a relatively low drag coefficient, is at worst not
moving fast through the water & at times any immersed portion may well
be moving astern.

Let's see what that brings :)

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 01:00:35


Quote:


>>> Maybe now is a good time to jump into the conversation. Regarding
>>> modelling the rowing stroke - specifically the hydrodynamic character
>>> of the blade in the water - I have done quite a bit of work, and
>>> recently put together a website to display (most of) it.
>>> https://sites.google.com/site/surgingforwardrowing/ Without getting
>>> into too many specific details of the flow, or directly answering any
>>> of the above posed questions, I'm confident that this research can
>>> effectively be coupled with higher-level boat modelling and rower
>>> biomechanic modelling to design a comprehensive rower-oar-boat
>>> system. So if there is a desire to collaborate on such a model, or if
>>> anyone has any questions related to the topic, don't be a stranger.
>>> I'll apologize in advance if the response is delayed... the lack of
>>> funds available to work on rowing research full time means I'm
>>> generally busy with other work commitments Andrew

>> An interesting site, thanks for creating and sharing it! At the risk
>> of thread jacking however there are some parts I am unsure about and
>> would like clarifying, mostly on the pages talking about blade shape
>> and blade efficency on this page

>> https://sites.google.com/site/surgingforwardrowing/rowing-research/eq...

>> Now although I agree with the fact that it is possible to design a
>> blade that is more efficient that another (using the amount of slip of
>> the blade as a measure of its efficiency) like concept have done with
>> the big blade, I struggle with the connection between the more
>> efficient blade being the faster blade, especially an absolute value
>> of 1% or 2 seconds

>> The issue I have is that I see boat speed being a combination on the
>> force the rower is able to apply, and the way that the rower applies
>> that force (and how the boat/blades accelerate in response to this
>> force). If you run the assumption that the rower has a finite amount
>> of force they can apply, then to me it seems the more efficient the
>> blade the slower the rower would be able to accelerate the boat past
>> the blades connection with the water (as a rower would say it "feels"
>> heavier), which means a reduced acceleration of the boat which means a
>> slower overall boat.

>> Yes you can correct the "heavy" feeling by making the blade outboard
>> shorter but then if that is the solution then why cant you just make
>> the standard smoothie blade shaft longer in response to its reduced
>> efficncy compared to the fat smoothie

>> Unfortunatly ive not really seen a discussion which has clarified this
>> for me which makes me think more that focusing on boat shape is a more
>> effective focus that on blade shape, especially if a rower has a
>> "favourite" shape or type of blade already and the fastest gearing for
>> them is found from testing

> Let me add a few thoughts here:

> 1. Force does is not the only consideration since force alone does not
> move the boat.  What does move the boat is Work, which is Force x
> Distance (through which that force application travels.

> 2.  The force on the blade is the force that _you_ apply (mediated by
> the leverage of the system).  The water around the blade only reacts by
> delivering an equivalent opposing force.

> 3. The work done is the maximum work available to move the boat.  Sadly,
> a fair chunk of the work you do gets wasted/dissipated.  When this
> happens it is lost from the propulsive process and diverted into moving
> & stirring up water, plus entraining air into the water.  This wasted
> energy in part creates the puddle.  The puddle is a monument to the work
> which did _not_ move the boat.

> 4. You can like the puddle, the moving of chunks of water & the
> sternwards slip of the blade to wheel-spin & tyre smoke in a car making
> a hasty getaway - it's a dead loss.

> 5. When we discuss blade or propulsive efficiency we are not talking
> about Force; we are discussing how much of the Work (= Force x Distance)
> that we put into the stroke ends up actually moving the boat - what's
> left after the wasted work (puddle, etc) has been taken away.  We
> express that as a percentage: 70% propulsive efficiency means that 30%
> of the work we put in simply went to waste.

> 6. Better propulsive efficiency is one big area, largely ignored, where
> rowers have the greatest potential to go faster - did they but know how
> to achieve it.

> Which brings me to a point which I believe Andrew has looked into at the
> suggestion of others, and on which he had some interesting preliminary
> results a while back (so I believe?):
> What he seems not to address in that link, while looking closely at
> blade shapes (which might be a rather secondary element in the overall
> business of blade efficiency) is the Z-axis of the stroke - the vertical
> component.

> My own experiments, rational engineering argument, feedback from
> thoughtful scullers, observation of champion scullers' techniques &, I
> believe, some work done by Andrew (but I'm open to correction) confirm
> the how deep you row your blade makes a really significant difference to
> overall propulsive efficiency.  Put simply (& I have done this here a
> few times!) somewhat deeper than is generally promoted is somewhat more
> propulsively efficient.

> In short, the rowing stroke is not 7 should not be treated as a
> pre-ordained 2-D process with the 3rd dimension conveniently eliminated
> from what we do & discuss by application of a widespread & dogmatic "no
> deeper than this" rule.

> Why is deeper better?
> 1. Because a deeper blade is entirely surrounded by water.
> 2. So you do not therefore entrain air behind the blade to disrupt the
> essential tensile connection between blade & water & increase wasteful
> slip & puddle-making.
> 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 period.
> 4. Unless you go _very_ deep (I wouldn't want to quantify that here,
> please), you don't incur relevant countervailing losses due to looming -
> the loom is thin, has a relatively low drag coefficient, is at worst not
> moving fast through the water & at times any immersed portion may well
> be moving astern.

> Let's see what that brings :)

> Cheers -
> Carl

And I should've added to the 4 last points above that

5. If you properly bury the top of the blade you add considerably to the
length of blade edge now subject to shear forces, thus improving
"connection" & reducing slip
6. Standard experiments going back many years confirm that fluid drag on
partially to wholly immersed plates increases with depth as they go
further below the surface.

None of this should be in the least bit surprising.  But still those who
row deep get told "it's wrong".

Funny business, this rowing game ;)
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