Iso,

Calling Laminate sails "Mylar" is a bit of a misnomer. The Mylar that compromises either side of a laminate sail does carry some load but most of the stretch resistance is carried by the scrim material that is inbetween the two Mylar outsides. The most common laminate ('mylar') sail material that folks are buying these days is Pentex and it will likely be what a sail maker like Calvert or Smyth would recommend for your mainsail. In the Pentex, there is an open crosshatch of Pentex fibers inbeded between two clear (usually) layers of Mylar. This kind of sail is usually opaque in color but can be tinted. Pentex is not much more than a stronger dirivitive of Dacron that is more suitable to be made into individual fibers. You can also get other kinds of mylar laminate sails that have Kevlar, Carbon Fiber, or other exotic fibers woven or in combination in place of Pentex. Pentex is largely affordable and very widely used for beach cats. Most all laminate sails are more expensive, but stronger and longer lasting than Dacron sails.

However, the Pentex and other Mylar laminate sails do have a weakness; they don't like to flex and are brittle (as someone mentioned before). The reason you see most jibs made from Dacron is that the constant abuse the jibs make as they tack and gybe past the mast and diamond wires would quickly break the stronger, but more britle scrim material. In John's case on his Mystere, his jib does not overlap the mast and mainsail and therefore does not go through the same kind of abuse. A laminate jib is more applicable in this case and the same is true for the likes of the Inter 20 or any other boat with a smaller, non-overlapping, jib. Dacron material, while not quite as strong or long lasting as a Mylar laminate, will handle bending and flexing better and last longer in a typical jib application.

Also of note: most laminate sails are built using a tri-radial construction so that the fibers can be better oriented along the stress lines. A laminate material is not strong when stretched at 45 degrees to the scrim pattern. As to the contrary, because Dacron material is stronger on the bias, these sails don't suffer much from being cut from large rectangular panels and a large cost savings can be had by using this construction method. See Murray's catalog for sails to see the different kinds of cuts.