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.
It's completely my ignorance that i didn't think of a sail as an aerofoil, in my lightbulb moment my brain thought, "fabric catch wind" in a very neanderthal voice.
I really appreciate everyone's comments however i think with yours the nail is sufficiently hit on its head.