less than a 10% increase in windward performance compared to the board being up or down

Random thoughts without any hard data to back them up: I sailed Umiaq for a while with the centerboard either jammed up or missing. Although I wouldn't care to put a number on it, It made a big difference when beating to windward. Enough that I was really glad to get it working again. It's possible that the boat seemed a little more tender without the board, but I can't feel any difference up or down (although I'm far from being an expert). There are some quite large boats whose designers decided it was worth the trouble of putting a lightweight centerboard inside a ballast keel. Having said that, they could be right. I'm wondering what you would compare... Velocity Made Good? Angle of leeway? Readout on the Fun-o-meter? Come to think of it, 10% might be quite a lot -- beating your nearest competitor by 6 minutes after a one hour race would definitely earn the centerboard's keep! For an idea of the forces involved, consider a Laser with a centerboard about the size of the board on an M-15. Sail on a beat with the board up, and note that you're going sideways. Now push the board down and note the immediate difference: You're not going sideways any more. It won't be anywhere near as obvious when the board is combined with a keel, but that would give you a feel for what the board can contribute. Giles Morris