Okay hear me out and this may sound like the ramblings of a lune but i just thought the following.
Surely a perfectly flat tight sail would more effectively convert wind energy into forward momentum rather than a full sail which would first have the wind hit it at a slight angle pushing the sail in that direction rather than perfectly straight forward.
I realise there are practicalities in operating a very high tension sheet but this lives purely in the hypothetical realm.
Does this make sense or have i been drinking drain cleaner again.
Ta Lemmers…
A lot depends on the angle you are heading relative to the wind and how strong and variable the wind is and how easily you want to be able to steer or hold your course.
The simplest case that I think you might be wrong is going down wind, especially in light air, you want the sail to catch the wind like a bag and direct it toward a central point to add all the vague forces into one direction instead of just twisting the sail one way or another. Like I didn’t think you actually want the wind hitting the outer edges of the sail straight on as this would just move the sail, not the boat.
Stiff, even hard not cloth sails are useful to go into the wind at a slight angle, where they are optimized in shape like an airplane wing, and they even talk of the force generated as lift.