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Hmmm. I suppose Jeff that you may be correct under really light conditions, a sail with deeper camber will stall because the air cannot "follow" the shape of the sail; i.e. the sail cannot "accelerate" the flow (get it to change direction, thus generating lift force). I''ll have to experiment with this next time I''m out on the Potomac on one of our notorious light air days. My acquired understanding from reading tuning guides is that in "light" air, as opposed to "heavy" air, powering up involves making the sail shape rounder. Steve Colby''s theory is that faster moving air has a harder time following a rounded sail shape and will be more likely to "separate" from the sail, stalling it. But your sentence regarding "powering up" seems absolutely correct: Ease halyards/downhauls, outhauls, backstays to power up in lighter air. This should give the sail a rounder shape for maximum lift. And, as Jeff points out, as the wind builds you reach a point where you have all the lift you can handle and need to depower the sail by flattening it.
Check out the following guides for mainsail trimming and shaping:
Here''s a good guide for those who lack a vang:
Here''s Dan Dickison''s two part article on controls for the mainsail other than the traveler and sheet: