No need to simulate this with cloth. Of course cloth would be the simplest and most accurate way, but you can achieve this by other means.
Just use shape animation. Place the sail in different shape, and save them as keyshapes. You can animate them with an expression, using a slider for each shape.
I don't think that more than 3 shapes are need. Why? Because the sail is locked at its top (the whole lenght) and bottom (corners only), so it doesn't allow lots of mouvements.
Another option is to use a two 2D or 3D chains. I don't favor this technique, because is would require several chains (at least four, to get an accurate animation), and it would be hard time to give a natural smooth fold to the sail.
Hope this helps
Bernard Lebel