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Certainty and Binarisms in Design.

Post #13

"If we start being too certain about anything it means we're missing something"; Iain MacGilchrist, neurologist and author of The Master and His Emissary.

Do we understand the physics of what lifts our boats to windward and allows us to fly on foils? How can we be certain and is it even a good idea to be certain?

Most of us used to think that mainsails with a parabolic head were the most efficient plan form. I imagine the logic behind that was that if the Spitfire's parabolic wing was the most efficient then that's the planform shape we should use for our sails. There was (and still is) a perfectly sound argument for the parabolic wing planform being the most effective way to minimize tip loss and for a long time no one saw any reason to argue with it. But there is more to the tip vortex than the planform of the wing or the sail. Fluid dynamics have moved on and so have our sails.

And then we have what I call "binarisms".

Skinny hulls are faster than fat hulls.

Flat rocker is faster than a rounded rocker line.

Multihulls are faster than monohulls.

Of course none of these things are definitively true in the absence of some specific parameters which can spoil the party by narrowing down the circumstances where any of these things are observable in the real world.

Have you ever tried to figure out exactly what foods to eat to keep healthy, how much of them to eat, and how they interact with the various organs of the body? Yes, it's complicated. And sailing boats are like that too, Everything you add, subtract and adjust has an effect on something else in the system.

I've heard some pretty simplistic and sometimes bizarre explanations for how a wing generates lift. Some of them are published in Youtube videos as being scientific fact. In fact there are three primary effects at work in generating lift; the Bernoullie effect (conservation of energy) the Newtonian explanation (conservation of momentum - which is simply the force of the oncoming air against a tilted surface), and the Euler equations which determine that we also have to take into account the conservation of mass. All three of these effects interact in complex ways and are further affected by a bunch of nuisance effects like turbulence, viscosity and the tip vortex.

Richard Dawkins would have us believe that the Bernoullie effect is the big one for yachts and aircraft with some assistance from Newtonian physics. Some specialists in fluid mechanics would suggest this is far too simplistic.

So can we be certain that Dawkins has it right? He's no amateur in matters of lift and flight. Maybe he's just trying to keep it simple for those of us who are not so deeply invested in the complexities of fluid mechanics?

Well for now I'm prepared to believe that Newton, Euler and Bernoulli were three pretty smart guys and the physics that lift our sails and wings are a complex interaction of the forces described by all three.

At the same time I'll take advice from McGilchrist and be prepared to be just a little bit uncertain, especially if someone has a much simpler explanation that is plausible when subjected to the scientific method.

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