Dacron sails have been around so long and have proven so successful that this fabric has become the gold standard of sailmaking. Dacron is DuPont's trade name for the fiber polyethylene terephthalate or PET. In the world of sails, though, it has come to mean the generic fabric produced when PET fibers are spun into yarns and woven into sailcloth. By now there are literally hundreds of variations on this theme - different weights and twists of yarn in the warp and in the fill, different weave tightness, and different finishing processes from resin baths to calendaring. PET's advantages include low stretch, high strength, good abrasion and flex tolerance, and low cost. The only real drawback is its long term sensitivity to UV exposure, which weakens the fibers over time. In most cases, UV exposure is the ultimate cause of death for PET sails. 

Woven PET sails proved so successful that it was inevitable someone would attempt to use the film version of this molecule in sails. (DuPont's name for PET film is Mylar!). Although the film-only sails were interesting, they had plenty of problems, not the least of which was that any tear, no matter how small, led to catastrpophic failure. While woven fabrics were soon laminated to film to stop tears, it eventually became clear that the oriented fibers should do the load carrying, leaving the film as a laminating medium and pressure barrier, rather than the load carrying element in sails.

Super Fibers Arrive 

The first true high modulus fiber to make an appearance in sails was DuPont's aramid Kevlar. This fiber was so stretch resistant it was virtually imposible to weave tightly. However, woven loosely (or not at all) and laminated to or between sheets of Mylar it has proven so successful for sails that it is still the number one choice for racing sails today. Ounce for ounce it has about 8 times the stretch resistance of PET. 

Kevlar and other aramids that followed are not without their own weaknesses. They are very sensitive to UV degradation, especially in combination with flex. Kevlar sails are definitely for racers and other performance junkies. 

The next "super modulus" fiber to appear in sails was Allied Signal's anisotropic melt polymer Spectra. This fiber has even higher modulus than the aramids and is so light it actually floats (as you might expect from a near relative of polypropylene). Moreover, it's resis tance to UV is on a par with PET. But Spectra had two problems that militated against its use in sails. First, it is so slippery that it is very difficult to laminate. Second, it is subject to creep, a gradual elongation under consistant high load. After some false starts though, these problems have been mostly solved - the slipperiness by special acid bath scouring and the creep by using much more of the fiber than modulus and tenacity considerations would dictate. By keeping the load low on each yarn, creep is kept to a minimum. 

There are several other "super modulus" fibers that have found their way into sails, including Vectran, Certran, PBO Xylon and Carbon fiber. Vectran is finding increasing favor with the mega yacht community, while PBO and carbon sails are seen on the front row racers (although carbon is illegal in IMS sails for some reason). 

Good New Polyester 

The latest fiber to make a big splash on the sail fabric scene is a new form of polyester called polyethylene naphthalate or PEN. Marketed under Allied Signal's trade name "Pentex", this fiber should be a big hit with racers and cruisers alike. 

In appearance and almost all other properties, PEN is very similar to PET. It is equally tenacious (resistant to breaking), tolerant of flex and abrasion, and UV resistant. Its big advantage is that it has about 2.5 times the initial modulus (resistance to stretch) of PET. 

The result is a fiber that seems destined to be cast in a starring role in the world of sails. Introduced in sail fabric by Challenge Sailcloth, Pentex has already found favor with thousands of PHRF and one design racers. They love the light weight, low stretch sails,but , either for budgetary or rule-related reasons, have been precluded from using the non-polyester super fibers. Pentex film-scrim-film and film-scrim-taffeta laminates found almost instant acceptance. 

Meanwhile, Pentex is finding its way into more cruise-oriented applications, especially taffeta-film-scrim-taffeta constructions. 

The Road Ahead 

The exciting news for sailors is that there is no sign that the discovery of new fibers that make better sails is over. In fact, all signs indicate that if anything it is accelerating. Material science on the molecular level is one of the hottest areas of predicted technological growth in the coming decades, and we can expect fibers to be a significant part of that growth. 

The sailmaking wish list is for super strength, low stretch, light weight, perfect tolerance of UV, flex and abrasion. The only problem with building sails of such a fiber is: they might be the last sails we ever build!

Maine Sailing Partners
www.mesailing.com
1-888-788-SAIL