I agree with all the posts here and would add some of my experience in this regard. When I got my first 2-up performance catamaran (an N5.5) it came with some really old rags, and the sail was very blown out. The result of this shape was that the sail was extremely full chord and powerful in light air, fast on the reach, but hopelessly uncontrollable and would not point. Old sails are powerful, but not tunable. The compensation for me was to add much more pre-bend to the mast, which served to flatten the old worn out sail. Obviously, if new rags were to be run up the mast, the effect would have been to flatten the new sail drastically, and it would have been similarly de-powered as in your case. ( In my case I bought an f18...)

The A cat guys do something to check the tune which is kind of a handy trick-they rig the main out of the track, rig the sail with the boat on its side and honk the mainsheet and downhaul on maximally. They then see if the luff curve of the main matches the mast prebend, moving mast rotation accordingly. A very good match of the curve to the mast would then logically represent the maximum flattening of the sail, and a good match of the curves of the luff and mast representing the ideal maximum adjustment to take advantage of the dynamic range of the luff curve in the sail. They then adjust the spreaders to get it as even as they can as a starting point for their sail tuning. Then they let it all slack and note the chord depth of the sail and adjust a bit either way. Try it. It's easy and very interesting to see how the luff curve works for sail shape, and gets you thinking about the sail at the least.


Nacra F18 #856