How do sails work?

[franksingleton]

Unless I have missed it, quite likely, most (all?) of the above, ignores Newton’s third law. Apply a net force and you get an acceleration. If there was no friction or drag with the surface and no drag through the air speeds would increase with no limit. An ice yacht has little friction between it and the surface so speeds can be great. A foiling yacht has more drag than an ice yacht but far less than a displacement hull, so speeds can be far greater. After that, it is all about Bernoulli’s theorem.

 

[Buck Turgidson]

Unless I have missed it, quite likely, most (all?) of the above, ignores Newton’s third law. Apply a net force and you get an acceleration. If there was no friction or drag with the surface and no drag through the air speeds would increase with no limit. An ice yacht has little friction between it and the surface so speeds can be great. A foiling yacht has more drag than an ice yacht but far less than a displacement hull, so speeds can be far greater. After that, it is all about Bernoulli’s theorem.

It all requires Newtons third law. It's all those molecules of air and water being displaced that provide the action, the boat moving is the reaction.

 

[anoccasionalyachtsman]

I think @Thistle is well on his way now, but just to add that I remember the day that it dawned on a spotty teenager that the wind never really just pushes a sail along, not even dead downwind. There's always a flow across it. Normally luff to leech, but not always.

With a spinnaker oversheeted and stalled it will stay 'inflated' but wobbling all over its surface like a jelly. That's all the turbulence as the air nips round the luff, breaks away and comes back to pummels the front surface. Not much power, wake the trimmer up. Sheet in some more and it'll collapse. No push without flow.

A fun exercise to reverse the flow on the mainsail is to go dead downwind and sheet it back in from 'square' and, at a point not far from where it would start to think about gybing itself, having been from luff to leech the flow will reverse and go leech to luff. You'll see the leech telltales start to stream the wrong way to prove it. (A preventer may be A Good Idea unless you're happy with a few semi-intentional gybes).

 

[Thistle]

A fun exercise to reverse the flow on the mainsail is to go dead downwind and sheet it back in from 'square' and, at a point not far from where it would start to think about gybing itself, having been from luff to leech the flow will reverse and go leech to luff. You'll see the leech telltales start to stream the wrong way to prove it. (A preventer may be A Good Idea unless you're happy with a few semi-intentional gybes).

Another fun exercise - on a dinghy at least, I've never tried it on anything bigger - is to sail backwards around a triangular course.