The shape-life of a sail is more problematic, since it deteriorates gradually with every hour of use, and the effect on performance is much harder to judge than that of a sail which won’t stay in one -piece. Sails which stretch too much, become too full, and will not retain a critical airfoil shape (with a distinct rounded entry and flat, straight exit), cost you in more subtle ways. Let me read your mind; I know what you are thinking. “I’m just a cruising sailor, I don’t care about performance.” Actually, you do, it’s just that “performance” is based on a different set of criteria. Yours is not the quest for another tenth of a knot of boat speed or one degree of pointing. But it is critical to control heel. Full, stretchy sails, rob power in light air, but more critically, they create heel and weather helm just when we want control. Also, lets face it, at some point, we all have to sail upwind; (usually at the least convenient moments). After all, a bathtub with a sheet can go downwind. One of the real luxuries of a good cruising boat is the ability to go upwind when necessary, and for most cruising boats this goes against the grain of much of their design criteria. If sails are not shaped properly, and their materials and structures are not designed well enough to resist stretch, the boat will not be able to go upwind effectively.