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Sail Cloth and Sail Durability

The question I’m so often asked is how long will my sails last? Predicting the life of a sail is difficult as so many factors come into play from the sails owners’ usage to mother nature.  The single most important contributor to durability is sail cloth. For the racing boat, this is the difference between winning and losing but for the cruising boat it is being able to foot faster, heel less and to pass that point in one tack. It is additionally the sail’s ability to maintain shape in time, something so difficult to judge when the sail is new. It is clear that well-engineered cloth lasts longer. The most economical sail probably doesn’t use the cheapest cloth.

Peter Mahr, “Mr Mylar,” approaches the subject of sail life in the following way: “The best way to measure sail longevity is not in years of ownership, but in hours of use. Racers log hours on their sails to track the aerodynamic life of their sails (how long the designed shape will last)… recreational sailors should do the same. 

A charter boat often changes its sails after about three years which amounts to roughly two thousand hours. At this point they are fairly shot. This brings up the issue of performance life and ultimate life. Performance life can be enhanced by recutting, easing shrunk bolt ropes, changing tapes. Ultimate life is when the structure of the sail itself starts to fall apart.

Firstly, one needs to assure that the cloth weight chosen in any given type corresponds with the load expected. Sail cloth tends to degrade fairly linearly and that whilst cloth resistance is greater than load the sail will perform without deformation or rupture. In time cloth resistance drops and can fall below load which will mean shape change and cloth failure. As a simple rule if your sail is stretching, you’re pretty close to failure. If your sail is fairly new the cloth is just not adequate.

For each sail you should be given an estimated Load facture, this number should be equalled by the cloth resistance. Any margin is security in time and in extreme conditions. We use a notion Adjusted Modulus which is based on the total modulus of the various components in a material, modified by empirical assessments of performance, expectations and usage. Load varies from boat to boat, with wind strength, hull stability, and importantly with the aspect ratio of the sails. This is how most old riggers were so kind on their sails.

Racing sailcloth has undergone a revolution over the last 10 – 20 years. Cruising sailcloth has as well, taking the best ideas developed for racing and adapting them for a different task. Not only are today’s woven polyester fabrics more stable and stretch-resistant than ever, cruisers can also choose from lighter, stronger polyester/mylar laminates and ultra durable high performance Spectra laminates that combine superior performance and long sail life. Today’s cruising sail has a lot more racing heritage than most sailors realize!

Here is a quick introduction to each cloth. Be aware that some styles mix in a lot of polyester to reduce cost. Traditional polyester sailcloth is woven very tightly to produce a stable, durable fabric without excess fillers or laminates. The fact that the fibres shrink with heat is used as a means of assuring the tightest weave possible in the better woven cloths. In the sail, the stronger, straighter fill fibres are oriented to handle the heaviest loads along the leech.

Polyester laminates are the next step up in performance from woven polyesters. They combine Mylar® film with polyester or Pentex  cloth in two or three plys. For laminates, the taffeta cloth can be woven more loosely because the Mylar film provides diagonal stability. Aramid  & Spectra laminates offer maximum performance and minimum weight, though at a higher cost. Aramid and Spectra® fabrics are actually a weave of large Aramid or Spectra warp fibres and polyester fill fibres. The weave is then laminated to a Mylar film. Carbon is increasingly used, mostly as an additional low stretch fibre in some cloths featuring an additional grid of yarn applied in a criss-cross pattern on top of the laminate to improve durability.

Spinnaker cloth is most commonly woven from Nylon®, but within the last few years, small diameter polyester yarn has also been used. The polyester cloth stretches less for its weight than Nylon, though its appearance is virtually the same but can make more unstable sails in rough waters.

Obviously here in the Caribbean, UV degradation can be significantly accelerated if you don’t cover your sails. Equally, in our waters, leaving a sail up and flogging in a squall for 30 minutes can equal 50 hours of “normal” sailing. Consistently flying a sail over its designed wind range can also age a sail well before its time.

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