Most are good for a 20 percent reef
According to my sailmaker (Greg Koski of Doyle in Cleveland) and other references I have heard, a foresail can be furled up to 20 percent and still maintain "reasonable" shape. A foam luff can improve the shape and perhaps extend the amount of furling before the sail becomes baggy.The problem is that sails are carefull designed to hold a precise airfoil shape. This assumes that the luff is straight, and the sail curves away from the luff in a designed way to maximize the drive, and minimize the drag of the wind on the sail. When we roll the sail on a straight forestay/furler, the curve is increase beyond the designed shape. This "baggy" shape decreases drive (for the amount of sail deployed), increases side forces (meaning more heel), and increases drag. This is doubly important, since typically a flatter sail shape is better in higher winds.By the way, when we reef the genny, we need to move the sheet fairlead forward to maintain the right sail shape through the top of the sail (or twist).DavidLady Lillie