Mast Head T Foil?

1) Lift and span are constrained. Here winglets will definitely reduce induced drag. 

2) Lift and root moment are constrained. Here a wing with winglets will not be significantly better than a straight wing, as long as the straight wing can have a slightly larger span.

Root moment is the same as heeling moment, which is definitely a constraint on a sailboat. So winglets on a sailboat generally don't make sense. They would make sense only if there is a severe mast-height constraint in the rules, such that case 1) is more relevant."

Tom Speer has noted on the same forum;

and in another post;

and;

And in reply to someone asking why would Boeing use winglets if they didn't work, Tom (former Boeing designer) replied;

I love reading Tom and Mark's stuff, because what these guys who are actual aerodynamic gurus says actually matches what we see in real life. In contrast, there are legions of guys who appear to have read a webpage on basic aero theory and then come up with silly ideas and blame their lack of acceptance on rating rules or supposedly conservative sailors.

And apologies for going off on a tangent, but while looking for the Speer/Drela stuff I came across yet another person saying "if the Wright Brothers had listened to the scientists and experts, they wouldn't have learned how to fly". What a load of rubbish - when Wilbur decided to get involved in flying the first thing that he did was to write to the Smithsonian and get all the available scientific info sent to him. Even before the famous first flight, he was asked to give lectures to qualified engineers and was supported by a major name in engineering, and both brothers used the scientific research of Smeaton, Lilienthal etc in their design. They are classic examples of people who learned from the state of the art science and then took it further through application of the scientific method, but they get used as an example of the opposite approach. It's probably a self-generating myth; those who hate research don't actually do any research about the Wrights because they prefer to rely on their anti-intellectual BS, therefore they will never find out how wrong they are.

Surely the same general idea applies to sailing - we can learn more by starting from the viewpoint that the state of the art in any field (such as dinghy design) is valid technology and learning from it, rather than saying "most of the stuff out there is cr&p and it's easy to see how it could be done better".