your post implies that luff curve is something that was done in the past. It should be noted that cutting curve into the luff and leech remains an important part of modern sailmaking, even on the most advanced sail designs. It is not just the domain of "cheaper" sails.
Nono -- you misunderstood. I said that in the past
, luff rounds were there primarily to create draft. In the days of manila cordage and cotton canvas. Nowadays, draft is induced at the seams (or in the case of 3DL, etc) in the panel material itself. But sails still
have rounds, for lots of good reasons: to accommodate spar prebend and flex. To add area, particularly untaxed
sail area. To lay against the deck (headsail) or boom (loose-footed main) & provide a seal -- end plate effect. To generate the 'knuckle' in free-flown sails like asyms & Code Zeros that give those sails their tremendous drive. And so on.
Some polytarp experimenters still use luff & foot rounds to create draft, and that's a fine approach for very small boats or lower tech materials. If you are cutting a leg-of-mutton sail out of Tyvek for your blow-moulded sit-a-top ... yeah, probably don't need to mess with broadseaming.
Matt: Sailrite also prints some inexpensive booklets
on making various types of sails. The information is a bit dated, but it's a practical approach for the home sailmaker. If you really want to get precise, the Sailcut CAD/CAM program is available free on Sourceforge
. It will let you design a sail based on a few critical dimensions. The software will fill in the fields to generate a middle-of-the-road thing, or you can tweak parameters like draft depth, luff round location, roach, and so on. Helps to know a bit of theory going in. We've built three sails using Sailcut. (There's a spinnaker version of Sailcut out there, too - rather harder to find,