The most important rule of sail trim is to avoid turbulence on either side, __especially__ the leeward side. Laminar flow on the leeward side means the sail is

The most important rule of sail trim is to avoid turbulence on either side, __especially__ the leeward side. Laminar flow on the leeward side means the sail is deflecting air on that side as well, generating a "pull" in addition to the "push" on the windward side. This is the reason why sails usualy have short strips of canvas on the leeward side - to see if there some turbulence or not. Turbulence depends on sail convexity, it can be changed too, not only sail angle!