designing a fast rowboat

I have attached two graphs which show Michlet predictions compared to Wellicome's experiments on a full-size (approx. 18m long) rowing shell.

The first graph shows predictions using two different skin-friction lines - the ITTC 57 and the (immodestly named) LL08 line. No form factors have been used.

The second plot shows predictions using empirical form factors derived by Scragg and Nelson. I used Prohaska's method to convert their form factors for the ITTC line to ones that can be used with the LL08 line.

I'm not sure which version of Michlet people are using these days, but versions after 8.07 include these empirical form factors as an option in the input file.

Leo.

 

Here are a few references as a start for interested nerds:

Brearley, Maurice and de Mestre, Neville J.,
``Modelling the rowing stroke and increasing its efficiency",
3rd Conf. on Mathematics and Computers in Sport,
Bond University, Quensland, Australia,
30 Sept - 2 Oct. 1996, pp.\ 35--46.

Cabrera, D., Ruina, A. and Kleshnev, V.,
``A simple $1^{+}$ dimensional model of rowing mimics observed forces and motions"
{\em Human Movement Science\/},
Vol. 25, No. 2, 2006, pp.\ 192--220.

Caplan, Nicholas and Gardner, Trevor,
``Modeling the influence of crew movement on boat velocity fluctuations during the rowing stroke"
{\em Int. Journal of Sports Science and Engineering\/},
Vol. 1, No. 3, 2007, pp.\ 165--176.

Chandler, R.F., Clauser, C.E. McConville, J.T., Reynolds, H. and Young, J.W.,
``Investigation of inertial properties of the human body",
AMRL TR 74-137,
Wright-Patterson Air Force Base, Ohio, USA,
(NTIS No. AD-A016 485),1975.

Clauser, C.E. McConville, J.T. and Young, J.W.,
``Weight, volume and center of mass of segments of the human body",
AMRL TR 60-70,
Wright-Patterson Air Force Base, Ohio, USA,
(NTIS No. AD-710 622),1969.

de Leva, Paolo,
``Adjustments to Zatsiorsky-Seluyanov's segment inertia parameters"
{\em J. Biomechanics\/},
Vol. 29, No. 9, 1996, pp.\ 1223--1230.

Macrossan, M.N.,
``The direction of the water force on a rowing blade and its effect on efficiency",
Uni. of Queensland, Mechanical Engineering Report No. 2008/03,
April 2008, pp. 21.

Macrossan, M.N. and Macrossan, N.W.,
``Energy efficiency of the rowing oar from catch to square-off",
Uni. of Queensland, Mechanical Engineering Report No. 2008/05,
May 2008, pp. 24.

Parolini, Nicola,
``Computational fluid dynamics for naval engineering problems",
PhD Thesis,
\'{E}cole Polytechnique F\'{e}d\'{e}rale de Lausanne,
France, 2004.

Penwarden, A.D., Grigg, A.F. and Rayment, R.,
``Measurement of wind drag on people standing in a wind tunnel",
Building Environment, No. 13, 1978, pp.\ 75--84.

Scragg, Carl A. and Nelson, Bruce D.,
``The design of an eight-oared rowing shell",
Marine Tech., Vol. 30, No. 2, Apr. 1993,
pp.\ 84--99.

Wellicome, J.F.,
``Report on resistance experiments carried out on three racing shells",
NPL Ship T.M. 184, 1967.

 

Thanks Leo. I'll have a look at that lot when I get a chance.

Yup.

You can't. Best you can get is parabolic. Linear is not possible for displacement hulls in this sort of speed range. Shells are already very close to a parabolic drag curve and really fast recreational boats aren't that far off it.

Wellllllllllllllll, in practice sculls go fastest when the length of the stroke is increased, and of course doing that decreases the stroke rate. I'm not sure why this is so. It could be a human factor rather than a hydrodynamic factor. Maybe one of Leo's suggested articles has the info in it.

To reduce the variation in speed I think you'd want to increase the mass of the boat relative to the crew, since the variation is largely caused by the reciprocating motion of the crew. However I think the added wave drag and skin friction would make things worse. Competition sculls are always built to the minimum weight, so I doubt there's any advantage in going heavier.

There's an exchange of momentum even with a fixed seat. It's just not as pronounced. I'd guess that instead of the situation with shells (where the peak speed is around 50% greater than the minimum) a fixed seat boat would have a much smaller speed variation (say around 20% or maybe even less).

 

It could be because they don't.
Rowing shells tend to go faster at higher stroke rates.

 

I thought I remembered reading somewhere that longer strokes worked better on the whole.

 

You might have read somewhere that some nebulous measure of "efficiency" or "effectiveness" is worse for higher stroke rates but, in general, the speed of the boat is greatest for high stroke rates. A good example is the 45+ stroke rates at the start of Olympic races.

I doubt that anyone would even qualify for Olympic class if they can't average at least 32 strokes per minute for 2000m.

All the best,
Leo.

A late edit.
See:

PROPULSIVE EFFICIENCY OF ROWING
Valery Kleshnev
http://www.biorow.com/Papers_files/1999PropulsEff03.pdf

 

When all is said and done, it's the variation of opinion on just about every boat-related subject that makes it so fascinating ...

it's nice to see theory validated by experience, and just as much fun to see it totally fooled! I wish I had discovered boating years ago ...

 

The variation of the drag with speed of a rowing shell is not simple.

The attached graphs for a rowing pair (hull length about 10m) show drag divided by speed squared. Thus, if the drag varied with the square of the speed, the traces would be horizontal lines, and they very definitely are not.

As you can see, the total drag depends on the location of the rowers in the shell and on what they are doing. During the "pull phase" (from catch to release) when rowers are pulling on the oars and moving bow-wards, boat speed is slowest. Shell speed is greatest during the "recovery phase" when the rowers are moving back towards the stern of the hull, with their oars out of the water.

The other two graphs show the variation of the drag components, namely viscous resistance, air drag, and wave resistance. The air drag trace is relatively horizontal compared to the other components but that is because I have assumed that air drag varies with the square of the speed and the frontal area of the boat, crew and equipment. Even then, it's not dead flat because the frontal area changes during the stroke.

Leo.

 

Hmm. This is getting bloody complicated. Methinks I should just build a boat.

 

real world

Rowing in the real world that I encounter, several things become obvious. Conditions dictate stroke length and frequency. In calm water a long pull with a snap at the end gives me the greatest speed. In very rough water, short pulls at a higher rate yield better speed. A heavy boat in rough water seems to punch through the waves better while very light boats get into a start/stop mode where they almost stop between strokes. On a calm day, the light boats are much harder to beat. To me, the GPS is the final authority. The caveat being that you need to be VERY aware of the actual conditions at the moment compared to the previous conditions. The length of your boat also influences things in ways not often considered. In a nasty chop, a shorter boat will row easier as it gets down in between the waves rather than pounding through. The other part of that is in rough going no oneis at hull speed so the short boat isn't giving anything away at that point

 

yes, reality can trump theory anytime!

As a paddler of the double-bladed persuasion I have some advantages over a rower that offset his greater power; my drive stroke rate is double my cadence - which can be far higher than a rower's - and of course unlike the rower I face forward and can see the waves coming. My usual stroke rate of 45 is ludicrously slow compared to a racing paddler, but even I can easily maintain 60 indefinitely - well an hour or so, see last para.

I can hold the boat while it climbs up the front of an oncoming wave and speed up to surf down the back, effectively taking energy from the waves. In these conditions it's mostly the headwind drag that slows me down. I rarely experience pounding in a kayak, short ones are noisy in a small chop but can dodge the bad stuff and behave like corks, and long ones just slice through - the worse ones are broad in the beam with too much primary stability for comfort.

My boats are all shorter than your average rowing hull so I can often surf. Surfing back home (only an idiot sets off with the wind) is almost restful; unless, that is, conditions are really crappy and the waves are moving faster than I am, but that's more of a survival situation than getting the best out of the boat. Ignoring issues of speed, a short boat seems less stressful than a long one in those conditions.

However, while I am goofing off using the minimum effort possible to get from A to B, a decent rower in a good shell is soon out of sight over the horizon. I use the old age excuse when that happens ...

 

That's the spirit!
I have seen many Olympic class rowers who are brilliant when conditions are dead flat, but they struggle in windy or choppy conditions. True champions can overcome adversity and they are versatile enough to be able to cope with a variety of real-world conditions. That counts for far more than a percentage or two of reduced drag.

Leo.

 

Oh I agree with all of the above. Rough water rowing can be a very enjoyable challenge. I also enjoy using shore currents for added boost. Even when you have a strong ebb tide you can often find a back eddy somewhere if you look for it. Playing all the conditions like that makes rowing more fun IMO.

The percentages of drag are interesting though. If you can get a slight advantage that's there all the time then why not grab it?

Re the light boat/heavy boat thing: I'd always go for a light boat. You can easily ballast a light boat to be heavier, but you can't make a heavy boat lighter.

 

Of course, but you should also look to what advantages you can grab elsewhere. Increasing your fitness, and finding a stroke rate that suits you is also very important. Some stroke rates might not be ideal if it leads to unusual internal muscle and organ compression, which in turn can lead to less than optimal (i.e. out of phase) breathing patterns.

I don't want to stray too far into the unethical, but in non-Olympic events there might also be a range of legal performance enhancing medications that some will consider.

 

Mind you, for all this talk of winning races that's not the primary reason I want a fast rowboat. The things are just fun to cruise around in.