Monday, April 28, 2008

Beast of Burden

Sun Mar 30

I have a physics question for all you sailing geeks.

We all know the relationship between body weight and boatspeed in a small racing sailboat like a Laser, right? Or we think we do.

In heavy winds, let's say over 25 knots, the heavier guys do better in the races (other things like overall ability being equal). Generally speaking, in these conditions someone of 200 lbs will beat someone of 150 lbs in a Laser because the big guy can hike the boat flat upwind and will still be surfing downwind, while the little guy is struggling all the way upwind and would be better off in a Radial.

On the other hand in lighter conditions, let's say 5-10 knots, the little guy will probably be faster around the course. Upwind he may well be hiking flat out while the big guy will be simply sitting on the side deck and going slower, wallowing in the hole his heavier mass makes in the water. And downwind the lighter sailor will be soooo much faster. It's all to do with displacement and drag and Newton's Second Law, F=ma and all that.

So far so good. Please don't bother to argue with the above, because I know I'm right. I can even look at the results of the racing at my old frostbite fleet and make a very accurate guess of what the wind conditions were like based on the relative finishing positions of two (both excellent) sailors of very different body weights.

The question I have is what happens at the very light end of the wind spectrum, say 0-3 knots? I know that no race committee in their right minds would run races in these conditions... but sometimes they do. I've noticed on a number of occasions that the heavier sailors start doing well again in these conditions.

There was the time my (heavier than me) son beat me in the 2006 Laser regatta at Hunterdon Sailing Club in New Jersey, in a very light patchy easterly wind.

And it happened again on the last day of the Kurt Taulbee clinic in Florida last month. There was one other guy on the clinic who said he weighed 150 lbs while I owned up to 200. (OK, I rounded down, but who's counting?) On the Saturday in 5-10 knots he beat me easily in every practice race. But on the Sunday in a dying gradient breeze fighting the new sea breeze, shifty, patchy, and dying away to zero occasionally, I did a horizon job on him in every race.

So my question is, why? Is it a matter of these heavier sailors actually having very good light air skills (relative to the opposition that day) that overcome any weight disadvantage they may have? Or does weight actually become an advantage again in the very light stuff? Is there some kind of inertia effect which means that the heavier sailor somehow keeps coasting through the lulls, maintaining flow on the sails and foils, until the next little puff comes along?

Answers please. Do no attempt to write on both sides of the paper at once. Credit will be given for references to any physicist other than Newton and for the use of second order differential equations. Bonus points will be given for anyone quoting Stuart Walker and being able to explain the quote in less than 500 words. Anyone referencing Nietzsche, Sartre or Rilke will be awarded a failing grade.


Shopping City Chaplaincy said...

I suspect that you right about the momentum thing but this requires that the helm does indeed keep the boat moving and not try to tack to often.
One of the things that Walker suggests for light airs is that you bang the corners and this would mean keeping on the same tack.
Most of the time in light airs it is about choosing the side of the beat that has the best pressure.
The light weight helm may think that because of his relative light weight that he can take advantage and tack on every shift and gain on the heavy weights. But such a tactic leads to sailing a more central course and Walker says that this is not good. Added to this the inevitable speed loss of every tack it is not suprising that the heavies can do well.

Unknown said...

I think you aere close - not Beast of Burden, but Beast of Bourbon! ;)

Tony said...

I think the lightweight still has the advantage in 0-3 knots, but the advantage is greater in 5 to 10 knots as he or she can get hiking earlier, plane earlier, and catch waves more easily. In under 3 knots, even the lightweight can't do these things, so the advantage is smaller. Meanwhile, strategic advantages for a smarter sailor are greater, because a one knot increase in pressure is worth much more in 0-3 knots than in 5-10 - in the light stuff you get a much greater reward for putting your boat in the right place to get that extra pressure.

So ultimately you were probably sailing strategically better than the lighter sailor, but it was easier to see the benefits of this when it was lighter.

I doubt that the momentum would be hugely different between a 280lb boat and sailor and a 330lb boat and sailor. Slight difference for sure, but not enough to offset the reduction in displacement and drag that the light sailor gets. And I am very distantly related to Ernest Rutherford, so I know my physics - it's in my blood. Like he said, all other sciences are just stamp collecting.

Pat said...

It would seem very hard to make valid conclusions from isolated drifter races given that these tend to be infrequent affairs where luck of the draw / being in the right place when a puff-let hits plays a significant role. And, there are such creatures as light-air specialists and folks who are better than others at setting up boats for these conditions. Plus body positioning, legal kinetics, people with bad tiller-wiggling habits ....

Anonymous said...

In the world of model boats we see this all the time. Our club races US One Meter which has no minimum weight and runs about 6 lbs, I used to use an Int'l One Meter hull intended to be sailed at the class minimum of 8.8 lbs in those races.

If the wind is consistent but light, the light boat wins. Once the light boat is overpowered, the heavy boat takes over. If the winds are really light and puffy, momentum beats acceleration.

We had a course where the windward mark was occasionally in the lee of some trees. As the fleet hit the dead spot, and come to a stop, my boat would drift to the lead (as long as I didn't slow it down with too much rudder.) That used to be my favorite wind direction.

Tillerman said...

Great comments Tillerheads. I'm glad this question created some interest.

The model boating example from that prolific poster "anonymous" suggests that there is something in my inertia hypothesis.

But I also think there's a lot of truth in what Tony, Pat and Tim say. I know that on the Sunday at the clinic I was definitely sailing towards the pressure ignoring other factors. And my son and I did spend a lot of time drifting around New Jersey lakes in next to no wind so perhaps we are light air specialists in spite of our weight.

Kudos to Tim for explaining Walker in a way we can all understand (has anyone seen his latest triumph of obfuscation in this month's Sailing World?) and to Tony for bringing Rutherford into the discussion.

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