Originally Posted by obelisk
Thanks for the great explanations. When I posted, I was thinking of end plate effect as pertaining to a keel, but lancelot brings up a good point about a loose footed main: Is the radius of the boom on an attached foot main enough to limit or eliminate the end plate effect?
In theory, a boom ought to act as an end plate & limit induced drag around the bottom of the sail. In practice, booms are nearly always taller than they are wide, to limit beam deflection. So they are not ideal shape for an endplate. You could build in a T-crosspiece for that purpose, but the added weight & windage might cancel any benefit. A bolt-roped foot + boom reduces air leakage; but it also limits draft (power) down low & harms sail shape. Some people believe a shelf foot is a good compromise, and the 'shelf' certainly extends laterally the end plate. But you almost need a flattening reef in any kind of wind.
And nothing you do at the sail's foot prevents induced drag at the top of the sail. & Pretty much the entire aft 60% of your sail is pulling the wrong way when close-hauled anyhow, so obsessing over air leakage around the foot overlooks larger realities. We're trying to use a lifting
foil for forward thrust
. The miracle is that it works at all!