Shape & trim
Draft position
How far aft the deepest point of the curve sits — and the clearest sign a sail is worn out.
Draft position is where along the chord the deepest point sits, measured from the luff as a percentage. A mainsail typically wants it around 45–50%; a headsail a little further forward, around 35–40%.
This is the number that tells you a sail is finished. As cloth ages it stretches most where the load is highest, and the deep spot migrates aft. An old sail with its draft at 60% is a sail with a hooked leech: it heels the boat, it won't point, and no amount of string will pull the shape forward again.
It is also the honest test for a recut. If the cloth is still sound and only the shape has moved, a recut can bring it back. If the cloth itself is dead, it can't — and we will say so. See repair, recut or replace.