Below: in the picture you can see some small battens just behind the mast of my 26S and these create a flat spot on the leading edge of the main sail. The top two battens also have been stiffened near the leading edge to continue the flat spot just behind the mast for the full length of the sail.
Why do this? This is just an experiment but it "sure" seems to improve the main sail performance on reaches where L/D is important. This is the third type of sail I have tried this on now. This idea is not based on some theory but rather came about somewhat by accident. I was using windsurfing sails on an ice boat and broke a batten just behind a camber inducer near the mast. I beefed up all the battens near the camber inducers and the mast by wrapping glass tape/ epoxy around the battens. This created a full lenght flat spot just behind the mast and I felt the sail had better performance after this change. I also did this change to a very old main sail used on a C15 that had a tendency to form an "S" shape just behind the mast in higher winds. The leading edge battens solved this problem and also seemed to make the sail work a little better. I then tried this mod on my 26S main sail (shown here) and "could swear" it made a difference.
Why it makes a difference.. I really don't know. However the leading edge of the sail is where the highest and lowest wind velocities occur (and also the highest and lowest pressures) and with the non rotating mast and single surface sail, it may be that the flat spot aids in keeping the air flow locally attached in the area of the of sail where the highest lift occurs. But this is just a complete guess on my part. However.. I have sailed with this mod a few times now with other boats and I do not plan to remove the mod.. Unfortunately I don't race so I cant be certain that it makes a difference - but I think it just might..
Below: The top two battens have been stiffened on either end creating a "parabolic taper". The batten length just behind the mast has been stiffened for about 1 foot length.. The batten is stiffened by wrapping fiberglass tape / epoxy around the area that is to be stiffened and finally wrapping electrical tape which both tightens the wrap and also squeezes out excess epoxy.
Below: This sail has only two full length battens at the top. For the lower portion of the sail, I stiffened the leading edge by sewing on four battens ranging in length of about 1 foot at the top to about 1.5 foot at the bottom. These added leading edge battens are all fiberglass and are salvaged from old sails..
Below. The leading edge battens were always placed near a sail slug as this prevents binding when the sail is raised or lowered. The batten is first cut and then "smoothed" on either end to prevent chafing. Two inch wide nylon strap isthen completely wrapped around the batten (i.e., the nylon strap is over twice the length of the batten) and then the strap and batten is simply sewed to the main sail.