Durability
The cloth in dacron sails, which is what it looks like you have, tends to last a long time. If you look up the actual type of plastic (polyester? polyethylene?) that it is on an EcoNazi website, you'll see that it probably takes 150 years to decompose naturally. What happens BEFORE it decomposes is that it stretches. Sailmakers don't want cloth to stretch, because then it loses the shape they (and you) want it to have. To keep the sail from stretching, they use special "high modulus" woven cloth, they arrange the panels of the sail so minimize the stretch factor, they use stronger (heavier) cloth than they might otherwise, and they use cloth that has been treated with a resin coating that "fixes" the threads of the fabric so they don't move - much like a hair stays immobile after it dries in your last coat of varnish. What happens is that the sail luffs, making cracks in the resin each time you tack, and the cloth stretches a little. The halyard gets pulled extra tight on a windy day, and the cloth stretches a little. You head off to a reach before the crew eases the sheet, so the sail's pulled in too tight and it stretches a little. The breeze picks up above what the sail was designed to handle, but the harbor's only 10 minutes away, so it stretches a little. The sail still "looks" fine; the cloth could be made into shirts that would last 50 years or more. It's still relatively clean, and it still pulls the boat. Sails made out of burlap would pull the boat too. People don't use burlap if they can avoid it because it stretches a lot and wears out too fast. (Woven plastic grain bags -- is it polyester? Hmmmm. -- are better than burlap because they don't stretch as much and last longer than burlap and many other natural-fiber cloths.) If you're racing, the stretch after pehaps two hard seasons in SFO Bay might be enough to have a noticeable effect on your results. A boat like yours but with new sails might beat you every time. If you're cruising, you probably won't notice that it takes you four minutes longer to get from A to B than it would have if you had new sails. And you probably wouldn't care. Until they start to chafe through, get worn out and frayed, they'll work. 20-year old dacron sails will still work. So would sheets of plywood. It's a question of what you want in the way of performance and how much you want to balance that with cost. and pu