Luff Curve

Here's a shot of a Presto 30 mainsail in production on our loft floor.  I've laid a batten along the luff to show just how much luff curve there is in these sails.  

And here's the same sail as represented in Sail Pack:

Luff curve is one way that we add shape to an otherwise flat sail, but we also use luff curve to match mast bend.  If there isn't enough luff curve for a mast then you get ugly wrinkles extending from the clew to the middle of the mast as the sail distorts to fit the spar.

A Presto 30 mast is an unstayed, carbon tube and very straight.  However, it changes dramatically as the wind picks up, bending to leeward and opening up the leech.  Trimming the mainsheet tensions the leech and bends the mast even more.  Too little luff curve and the sail quickly turns itself inside out.  So . . . a curvy sail for a bendy mast.

Check out Thorfinn Expeditions and the Presto 30 in action here: