So I had a new main made from Bainbridge cloth (Diax 2) cross-cut for my mystere 6.0 but it’s really flat and simply doesn’t feel as powerful as my old main from North. My battens are the old Sailbones which are solid fiberglass. I can tighten the hell out of the battens and leach line which helps and of course playing with different downhaul settings, but it’s still not very shapely (i.e. very little draft about 50% of chord).

I run my mast fairly 90 degrees perpendicular to the deck, mast pre-bend is 1 ½” from the spreaders and tight diamond wires. I don’t have any means to truly measure wire tension at the moment.

Sail flatness is relatively consistent through the entire sail.

Question: Is it worth trying to file down the battens with a belt sander to achieve better shape, or should I just return to manufacturer for a re-cut? Has anyone else done this with success?

Any help is hugely appreciated.

I would try taking some of the prebend out of the mast before I would touch the battens. Do it with the spreader adjustments. The diamonds should be very tight. Like violin tight. Check with Mike. He should have some sugestions.

If your mast is at 90 degrees upwind, that could be taking a lot of shape out of the main. Try rotating the mast back so the limiter (or luff track) points somewhere between the daggerboard trunk and the rear beam...this is a more typical upwind setting and may give you some more shape.

Working/shaving your battens worked well on stretchy dacron sails - but isn't going to make a significant difference on pentex/mylar sails. The difference between the two shapes you have has more to do with how much curve was shaped into the luff of the sail...also remember that flatter doesn't necessarily mean slower. Sail shapes have evolved a little flatter over the last several years so it may just require some different tuning.

I think he was referring to mast rake ,not rotation as far as the perpendicular thing.
Like the second post said try less spreader rake to get more shape in the luff.

Yes, I was referring to rake in the original post. I always use the limiter and keep the rotator bar pointed just aft of the shroud, but have tried different settings to see if the sail would perform better to no avail.

loosen the battens and unroll the sail flat on the ground
the sail will then be board flat
string a line from head to tack and you will see the luff curve.
when your mast bends/prebends this much - you will have a flat sail.
your mast needs to be straighter than this luff curve to have shape.

Great explanation, thanks all for the suggestions.

It is amazing how a little shape affects performance.

Does this have an influence even if you don't pull the cunningham?

Yes.

word.

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.

Nice description of the A cat trick Rex, thanks!