"Axel Boldt" wrote...

I'm trying to understand how a boat can sail against the wind.
Explanations on the internet usually go like this: "The sail has an
airfoil shape, the air goes faster on one side than on the other,
creating lift by Bernouilli's principle." But I know that for flying
planes, the wing's airfoil shape is not essential (otherwise planes
couldn't fly on their back): it's the angle of attack that matters. So

An airfoil creates lift when the air flows across it. The amount of lift
depends on the speed of the air flow, the shape of the airfoil, and the angle of
attack.

An airplane can fly on its back because the wing airfoil is rigid. It can
create lift in the "wrong" direction if the angle of attack is high enough in
the proper direction. Competitive acrobatic aircraft use airfoils that are
nearly symmetric (same curve on top and bottom) for better performance
up-side-down. Few wings are actually flat on the bottom; they just have less
curve than on the top.

Similarly, a sailboat can tack upwind by positioning the sail so the wind blows
on the opposite surface, while positioning the sail over the hull so there is a
net lift force in the desired direction. The sail then acts like the bottom
surface of the airpalne as it fills on the opposite side. The sail assumes the
same shape on the other side, and behaves like a symmetric airfoil.


I'm wondering if the same is true for sailing: would it be possible to
sail against the wind with a sail that's a rigid flat surface, not an
airfoil-shaped piece of cloth?

An airplane uses the engine to pull the airplane through the air, so lift can be
created by a less efficient surface. A sailboat depends on the wind to provide
the propulsive force, so it is much more difficult. While you might be able to
make some net upwind progress with such a device, it would be slow and
inefficient.