Fascinating. Take a second look.
The difference between that "sail" and a conventional sail is that this one generates lift, too. The vectors on a conventional sail are horizontal and down, as opposed to horizontal and up.
The lift would cancel the torque efficiently if it didn't pass through the center of gravity.
(And I'm no engineer or physicist. Might be wrong, but never in doubt!)
Anyway, it looks like a great idea.
Arf