Brett Robson wrote:
>> I
>> said that the wind doesn't push the boat. You're right that it is
>> high pressure air pushing under the airfoil, which is not fully
>> countered by the lower air pressure above the air foil, that moves
>> the boat. But that is a force that is (roughly) at a right angle to
>> the direction of the wind. So when I say the "wind" isn't pushing
>> the boat I am not only practically right, I am technically right.
>> The movement of the wind above and below the airfoil merely creates
>> the pressure differential through a phenomenon known as Bernoulli's
>> Principle. The wind itself doesn't do the pushing.
>
> Where does the energy come from?

The kinetic energy (vibration) of the molecules in the air, which vibrate
from heat and strike the underside of the airfoil. Because the air molecules
are "stretched out" from the curved flow of the air across the top of the
air foil, the total kinetic energy of molecules striking that side isn't
enough to counterbalance the force under the airfoil. But this is all quite
different from saying the sail is "pushed" by the wind which is blowing
(roughly) at a right angle to the sail. We both basically understand what is
going on here from the perspective of physics. You were simply wrong to say
that I was technically wrong by saying that the wind doesn't push the sail,
unless for some reason you aren't making a distinction between "wind" and
"air."

> You probably learnt that at highschool.

Nope. I had to learn it to get my private pilot's license actually.

> However the Bernoulli
> Principle/Effect is a specific application of Newton's laws of
> motion. Bernoulli only explains what you see and doesn't explain
> what is happening at all. It is actually particles (ie air
> molecules) hitting the back of the sail that provides the force.

Yes, and those molecules are hitting the bottom of the sail because of their
kinetic energy (vibration from heat), not because of the wind. If you were
to stretch yourself out inside the curved underside of the sail, closest to
the mast, do you think you would feel the force of the wind hitting you?

> A similar effect is molecules of air striking the inside of a
> ballon that keeps it inflated.

Hmmm. So if it is the "wind" pushing the sail, then where does the "wind"
inside the balloon come from?

>> And for the sake of completeness, there are times (such as when you
>> want to go exactly in the direction of the wind) that you might let
>> the sail out at a full right angle to the boat and allow the wind to
>> push you, but it is by using Bernoulli's Principle and treating the
>> sail as an airfoil that you can get the greatest speed in a sailboat.
>>
>
> If you ignore Bernoulli and concentrate on Newton you will
> realise that the only difference is that on a reach you have a
> partial vacuum on the windward side, but on a run you have to
> push the air away from the front of the sail.

I completely disagree that "Newtonian physics" is the best way to teach
someone the principle of how an airfoil works. And the author of the
textbook I read when studying for my pilot's license apparently disagreed
with you as well. Newton is only needed to understand how pressure
differentials cause a push in one direction or the other, a concept most
people generally don't have trouble with. It is understanding how the
pressure differential arises in the first place, and consequently how to
create it using a sail (or a wing) and the flow of the air/wind around it,
that is the most important (and not entirely intuitive) concept to learn for
a would be aviator or sailor.

Jeff