Laminated sailcloth has evolved rapidly over the last several years in the direction of lower stretch, lighter weight and higher stability. At Neil Pryde we primarily use the DIAX laminates from Bainbridge/Aquabatten who have led world-wide development of laminates by being the first to incorporate high modulus Kevlar 49 in precisely formed scrims which most effectively convert modulus into stretch resistance. These Kevlar 49 warp ribbons are stabilized transversely with Kevlar 29 and Spectra and on the diagonal with Vectran. The result is a lightweight, low stretch material that allows Neil Pryde to build lighter sails which hold their shapes better through extended working ranges.
With a continuing string of successes on the race course, the DIAX range has expanded to include polyester scrim styles as well as taffeta-backed styles in both Kevlar 49 and polyester which add an extra margin of durability to these goods and apply best to the Club/Racer. As a lightweight high performance fabric, Kevlar DIAX is engineered with several features that make it an intelligent alternative to other fabrics or to 3DL or other "molded" sail construction techniques.
There are important differences between the processes of creating DIAX versus other materials, so an overview of the engineering behind DIAX should help bring some of the facts into focus and leave the fiction behind.
Key Components of Lamination
The components of all these high-tech materials are structurally neutral until they are combined through the process of lamination. Bainbridge's lamination technology has been developed through years of fabric and sail evaluation. The materials are bonded on a specially built laminator specifically designed to make lightweight, precision laminates. Extreme high pressure (up to 20x the PSI used for 3DL) assures a permanently bonded fabric which well not delaminate with repeated impacts.
The adhesive they use has a proven track record of durability as well. As a two-part thermoset, it not only mechanically adheres but also chemically cross-links with all the components to guarantee structural integrity of the laminate. Once bonded, the laminates cure for seven days to maximize adhesion before the fabric is cut into sail panels.
Weight to Strength Ratio
Weight aloft impacts the overall performance of any yacht, so sail weights have been getting a lot of attention lately. Minimizing fabric weight without sacrificing strength or durability is a key component of the DIAX line of fabrics. The high pressure laminating technique combined with a high strength adhesive means reduced glue weight and thereby keeps the fabric weight low. The DIAX Vectran diagonal grid also reduces the overall film content but still maintains high stability; yet another way to reduce overall fabric weight. When a DIAX 180-K that contained the same amount of fiber as a like sample of 3DL, the DIAX is almost 5% lighter. America's Cup programs which used both conventional DIAX sails and 3DL found no weight difference overall and actually built lighter sails (up to 10% lighter) with a special AC DIAX style using Kevlar 49!
Fabric efficiency, which is another term for a material's strength to weight ratio. It is imperative to have fabrics that exhibit the same characteristics from roll to roll and at Neil Pryde we use this relative gauge to determine where and how we will use a fabric. Bainbridge controls the efficiency of their fabrics with high pressure laminations that helps keep glue weight to a minimum and they use an advanced scrim formation technology to optimize stretch resistance in the fabric. The Kevlar 49 fibers are precision aligned without twist or crimp so they instantly go into tension when the sailcloth is loaded up. This minimizes low load elongation and maximizes high load strength. By eliminating the initial stretch in a fabric the final flying shape will more closely reflect the Neil Pryde design. The elimination of crimp ultimately helps the higher load extension which, combined with the fabric's light weight and high stability, gives you the best "performance for weight" of any sailcloth available.
Load Mapping of Sails
Load paths in sails are complex. They may vary considerably between a sail set up for a full, powerful shape and the same sail flattened out with twist to de-power in higher winds. Aside from the primary load paths within a given sail, there are also a wide array of secondary loads. DIAX's unique tri-directional fiber array is specifically designed to handle these loads by increasing fabric stability. Neil Pryde works closely with Bainbridge in using the computer program RELAX to help identify what is occurring in sails. This has led to significant developments in our design process and panel orientation.
Paneled Sails
There are three really outstanding features of panel sail construction, as opposed to molded sails. They are:
1. The ability to "step down" in thread density,
2. the ability to step down in film thickness both of which maximize the weight to strength ratio, and
3. the ability to recut or fine tune a sail.
In addition to being strong, light and durable, DIAX used in paneled sails is an excellent value for the owner of the sail. It is no secret that all sails need some recutting at various stages in their working lives to correct shape changes. With a sail made from panels of DIAX , it is a simple and direct process to alter the curve in an already existing seam and retune the sail shape for better speed. The result is a longer effective working life for the sail and a better value for the boat owner. In the case of molded sails, effective recuts are only achieved by slicing across the primary yarn paths, thereby negating any ideas of continuous fibers being a benefit.