Cuts & construction
Broadseam
Tapering a seam so flat cloth becomes a curved wing.
Here is the trick at the centre of sailmaking: sailcloth is flat, and a sail must be curved. Broadseam is how you get from one to the other.
Where two panels join, the seam is not a constant width. It is tapered — wider at one end than the other — so that joining two flat panels forces a fold of shape into the cloth. Do that along every seam, in a controlled amount, and the assembled sail is a three-dimensional wing. Take out more, and the sail is flatter.
It is the same principle a tailor uses to make a flat piece of fabric fit a shoulder. Along with luff round, broadseam is where a sail's draft actually comes from — not from cutting a curvy outline.