Its really not as simple as dacron vs kevlar. Okay, dacron is fairly simple. But even with dacron, different varieties exist, with the latest being "racing dacron." The "kevlar" sail is of course two layers of mylar film with kevlar strings in between.
For racing, the issue is not so much how many seasons until they don't hold their shape, its how many knots. In a gust, the dacron sail is going to stretch more than the laminate sails, and recover less quickly. The laminate sail will hold its shape, or at least deform much less. As the dacron sail looses its shape, the draft will move aft and slow you down, even if you properly retrim for the new apparent wind. If you want to actually have a chance of winning, get the laminate sails. If you don't care so much, then why bother getting new sails at all.
Once you decide you want laminate sails, there are a million things to decide. My first question is, who is making the sail cloth? Hopefully that's the first thing you asked the sail maker. Then you want to know how the threads are being laid out by the designer. If they are just thrown in every direction, you get good stretch resistance but lots of weight. The threads should be laid out by the designer (actually, by his software) to align with the anticipated load paths, so you get maximum stretch resistance where the sail is prone to stretch, with the least amount of thread. Less weight.
Then there is the matter of what the threads are really made of. What kind of kevlar? Is it all the same or do they toss some other type of thread in there? Why? Get the sail maker to explain all this to you.
With the main, find out how much prebend they are designing in and make sure your mast is set accordingly.
Keep in mind, many of these nifty threads are UV sensitive. You can extend the life of the sail considerably by adding a taffeta inside the laminate. This cuts the UV from one side, and will do more than you would expect to slow wear.
When you make these decisions, remember that the #1 hits the mast when you tack, but the #3 probably does not, or at least not nearly so much. So, the #3 can use slightly more brittle strings (maybe a little carbon) but the #1 needs more durable strings. The #3 is probably used in heavier wind, so you want it stronger. The #1 in lighter wind, maybe very light wind, so you want it lighter.
Once you have these, take good care of them. Flake them carefully after each use. Don't let salt dry on them. For cruising, use the old ones.
If you like, let us know what the sailmakers plan is and we can give you questions to torture him with. And whatever happens, make very sure he comes out to measure the boat before he gives you the quote!
Scampi 30 MK-IV
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