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:
[color:"blue"] Winglets encourage the vorticity to be shed nearer the wingtips, rather than somewhere else along the span.[/color] This produces more lift, since each part of the span contributes lift in proportion to the amount of circulation carried by that part of the span, in accordance with the Kutta-Zhukovsky theorem. In any case, as a general rule, adding a pair of six-foot-tall winglets has no aerodynamic advantage compared to adding six feet of regular, horizontal wing on each side.
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.
