Sail trim question
A sail no longer acts as a foil the moment it stalls, ie., the air flow is no longer attached to the lee side. This can be detected by telltales (yarns) attached to the sail near the luff on a jib and at the leach of the main. Trim so as to keep the telltales streaming as long as possible. When the boom is out to the max, come up into the wind enough to keep the telltales streaming. In light air,under 10kts, tacking downwind is a good strategy as it allows you to maintain the lee side lift on the sail. If it is blowing hard even a stalled sail will have all the power one can use and a direct downwind course is best. There is a lot more to it than this of course. Good condensed references are the North Sails Performance Racing manuals.