Further clarification...the air that flows off the tip of the wing (or sail) does so from the high pressure (windward) side to the low pressure (leeward) side, thereby reducing the pressure difference between the sides...and therefore the lift force...which is exactly the opposite of what you want to achieve...
This effect is known as "Tip Loss". It also occurs on our rudder/center/dagger boards.
I just wanted to clear up what a winglet does. It simply makes the wing more efficient. It does this by forcing the air to flow over the end of the wing as it does in the middle of the wing, straight back. Without the winglets (and the same hold true for our sails) the air near the tip spills off toward the end instead of straight back. On our cats we lose at the top and the bottom.
I think a float that was airfoil shaped could work on square top sails. I don't think it would need to provide lift. Thats the last place you want to create drag.
