Debunking Dodgy Sailing Theories

Well without getting over complicated both work in the same way, the flow over the surface needs to be exhausted, so it is the same as the propellor blade in water, the sail in air, both fluid flow over a foil generating a force propelling a boat forward.

Sails generally were slow acting foils, as speeds have increased, apparent wind increases, foils have twisted more and become higher in aspect ratio.

For a while windsurfing drove that increase in twist, but it has passed on to high aspect cat foils and even fixed wings have to have twist in order to function.

Old school dinghy sails had some twist but they were pin heads, so getting 'twist' used to be a complicated affair. With the advent of higher aspect (fatter headed) sails the twist is easier to dial in.

But twist is there for more than just the perceived variation of wind speed at different heights, that's another old wives tale. The primary reason is to increase the speed of the flow by exhausting without turbulence at the leech or tip vortices (like you try not to get with glider wings).

I think if you do the vector maths, the twist required would be less when you have a large apparent wind component which is probably why cats such as the ACs dont appear to need much twist in their wings.

(EDIT: Im not a cat sailor, nor have I sailed with a wing sail - just considering the mathematical point of view which as we all know is often not as simple as we think).