Shape & trim
Draft (camber)
The depth of the curve in a sail — how much belly it has.
DutchZeilbuikour Dutch loft →
A sail is a wing, and draft is its depth: how far the deepest point of the curve sits from the straight line between luff and leech, usually given as a percentage of the sail's width at that height.
Deep draft — say 15% — makes power at low speed and in light air, but adds drag and heel. Flat draft — nearer 10% — makes less power but points higher and heels less. That is why you flatten a sail as the breeze builds: halyard, cunningham, outhaul and backstay are all draft controls.
Draft doesn't come from cutting a curved outline. It comes from building curvature into the seams between panels (broadseam) and into the luff round — flat cloth, shaped into a wing.