I’m thinking if the sail was flat and taught the air would push around the sail and you would lose a lot of energy through resistance or like wind moving laterally. Whereas the traditional sail kinda cups and holds the wind force and creates more pushing force towards the center rather than the edges.
Cause airflow is like fluid dynamics right? Imagine spraying a hose at a wall vs a sheet hanging from four corners.
But I’m high and kinda drunk so who TF even knows.