there is a picture of the powerfoil sail I mentioned on
You have to look carefully to see the battens on both double surfaces.
On the thread there is also some heavweight aerodynamics from the respected Tom Speer. He has run many simulations of various configurations. He says that single surfaces sails work very weel - but only over a narrow apparent wind angle. Writing in 2002 he concluded
"So I think we're seeing a convergence on a sail shape that has a properly formed leading edge, smooth lee side contour, a double luff for 20% to 40% of the chord, and a single surface for the remainder of the mast-sail section. And the purpose of having a double luff is to cut the drag asociated with the windward side separation bubble."
Sounds pretty much like a current race sail no ?
btw Loft sails are pushing an RDM formula setup as a way of extending mast life. I Just wonder if it may also offer an aerodynamic edge having a slimmer leading edge ?