I should stay out of this since I have absolutely no aerodynamic training, but as I understand it (from years of training on the Discovery Channel) the winglet, as well as those little fins that they stick on the top of plane wings were put in place to keep the air from spilling off the end of the wing thus keeping the flow more front to back and there for making it more efficient. Improved air flow/attachment equals less power needed to generate the same amount of lift.

An alternative theory is that the winglets are designed to "intercept" the vortex at an appropriate angle to produce thrust.

Consider a propeller spinning in a straight flow producing thrust. That's easy to understand because we see it all the time.

Now invert it so that a fixed foil is in a spinning flow. This is the case at the tip of a wing with a winglet attached.

The fluid wants to move from the high pressure side to the low pressure side which generates a vortex/twist in the flow. Put a small foil at a slight angle in the way of this twisting flow and you may be able to generate some thrust.

I really don't know. In fact, my previous post was based on what I could learn from Chapter 3.12 of John Denker's online book "See How It Flies", from where I quote the following:





My view is the following:
Sails are different from wings. A sail tip does not rotate/move due to boat motion/waves at speeds that are relatively high compared to the wind speed. IF winglets help keep the vortex in place, they could possibly boost performance.

I don't know, wouldn't winglets increase the drag to leeward? And all the planes you see them on operate at fairly constant angles of attack, with the way cat masts move around it seems like it would add more drag, for example every time the bows go up and down over a wave it would raise and lower the angle, and keep shaking the airflow off. As a longtime hangglider pilot I've seen different tries at the winglet idea, and nobody has been able to make it work, mostly because every time the wing yaws a little it brings one side to too low an angle as the other side goes to too high an angle. One of those ideas that seem good but that don't work in actual use, maybe?

Those things don't exactly bob around much...and how many AC boat have you seen with sail winglets?