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.