Why Nylon Still Rules the Run
Why Nylon Still Rules the Run
Like many club racers, Mike Kirkland of Bellingham, Wash., has only one spinnaker for his Nordic 34, a boat best known as a cruiser. Nevertheless, Kirkland and his crew are keen competitors in local PHRF events, and he credits their lightweight, asymmetric cruising spinnaker from UK Sailmakers for keeping them in the hunt on the downwind and reaching legs. The 980-square foot spinnaker is built from Bainbridge Internationals AIRX 700, a premium .75-ounce material often used on hot one-designs and grand-prix raceboats.
Kirkland paid a little extra for the AIRX fabric, because, as he puts it, "We like good performance. I sail against bigger boats a lot, and we have to try to keep up. A lot of the time we sail in light winds, and with a big-shouldered, poleless sail, weight is important." AIRX has also gained a reputation for toughness that Kirkland admits to testing on occasion. "In one race, the wind came up, and we finished on a beam reach in 25 knots. We did well, and the spinnaker was fine."
Of course, as Kirkland appreciates, a limited inventory places higher demands on the spinnaker(s) aboard, and in turn, upon the fabrics used. Nowadays, its routine for performance-oriented boats to plane steadily at over 12 knots in a 25-knot wind with the spinnaker facing only 12 to 14 knots apparent. So-called .50-ounce materials are easily strong enough to handle the resulting loads and are typically fast as well. However, should the boat round up and stop, that lightweight spinnaker must contend with the full 25 apparent as it abruptly refills. This scenario results in quite a few ruptured spinnakers and, in a sense, extra pressure on sailcloth manufacturers to maximize the burst strength of lightweight products. True, catastrophic spinnaker failures are usually chalked up to "operator error," but most sailmakers are eager to avoid these unhappy incidents and will recommend what they judge to be the best available fabrics, even when the cost is somewhat higher. Heres some information to facilitate a discussion with your sailmaker when choosing fabric for your next chute.
Fibers and Fabrics
Creating a good spinnaker is more difficult than building a mainsail or a jib because the three-dimensional curvature is much more pronounced, and because it will often be called upon to perform effectively in a stalled or partially stalled condition. Airflow patterns around a spinnaker vary dramatically from top to bottom because a nearly horizontal head portion transitions to more-or-less vertical lower sections. Theres also a premium on stable flying shapes because most spinnakers have three unsupported edges.
Laminated spinnaker material hasnt caught on because, with the possible exception of costly Cuben Fiber, cloth manufacturers have yet to perfect laminates that are sufficiently light, durable, and easy to bag. In addition, the problem of "mylar shrinkage" (caused by fine-scale crinkling of the film) is more pronounced in lightweight laminates. By and large, the industry remains committed to paneled spinnaker construction using woven materials.
Good old nylon is still considered the "right stuff" for nearly all spinnakers. Its slightly stronger than polyester (Dacron), a bit less expensive, and its greater elasticity is, in some respects, an asset because it helps absorb shock loads and makes for chutes that are easier to fly in bouncy conditions. Of course, greater elasticity also has a downside—sail-shape distortion in stronger winds.
Nearly all lightweight spinnaker materials are, by nature, "warp-oriented" because the warp yarns (yarns running the length of the fabric) are pre-tensioned during the weaving process. The result is disproportionate crimp (zig-zagging) in the fill yarns (which run at right angles to the warp), resulting in fabrics that stretch more easily in the fill direction. Tri-radial construction takes advantage of warp-oriented styles because it ensures that the major sail loads are aligned with the most stretch-resistant axis (the warp) of the fabric.
Many spinnaker cloths are also "unbalanced weaves" with higher DPI in one axis than the other. DPI (denier per inch) is a figure representing the amount of fiber per linear inch in a fabric. When two figures are given (i.e. 30 x 40), its conventional to give the DPI of the warp first, followed by the fill. For example, the traditional .50-ounce construction is a 30 x 20, with a 30 DPI warp to handle the primary loads and a weaker 20 DPI fill to minimize weight. By contrast, a material suitable for all-purpose and cross-cut spinnakers might be a 30 x 40 construction, with beefier yarns in the fill direction to compensate for their greater crimp. Nearly all spinnaker fabrics have oversized ripstop yarns at intervals in both the warp and the fill to help prevent small tears from spreading catastrophically. Dimension-Polyant Sailcloth pioneered the use of twinned ripstop yarns in their Formulon styles—a method of further boosting tear strength that other manufacturers have begun to adopt of late.
In recent years, woven polyesters have seen some success in spinnakers, although their popularity now appears to be waning. Limited availability of low-denier polyester yarns has impeded development of styles for small and medium-sized raceboats (less than 0.9 ounces per sailmakers yard (SM-oz) or the weight of a piece of material 28.5" x 36"). At the same time, the best nylons have improved so much that polyesters no longer offer a clear-cut advantage in terms of shape-holding ability. Nevertheless, for big-boat heavy weather and reaching chutes, polyester fabrics provide excellent shape-retention, minimal water absorption, and exceptional UV resistance.
|Finishing techniques vary. The cloth is either impregnated, usually with a melamine resin for bias stability, or coated with a urethane finish to reduce porosity.
The Crucial Finish
The characteristics of spinnaker fabrics, particularly the lightweight materials, are greatly influenced by the way the basic weave is finished. Indeed, under a microscope, modern .50-ounce materials look surprisingly like scrim-type laminates with gaps between the spaced yarns bridged by a thin layer of polymerized coating material.
Nearly all spinnaker fabrics are either resin-impregnated or polymer-coated. The specific formulations are trade secrets, but urethanes are by far the most common finishes. Silicones are sometimes used for small-boat spinnakers where an extremely slippery, water-repellant finish is more important than maximum stretch resistance.
A phase of the finishing process entails subjecting the fabric to high pressure and elevated temperature by passing it between massive hot rollers. Heat shrinking tightens and stabilizes the weave, while the pressure forces the polymer deep into the fibers and literally crushes the material flat. Ideally, the finished cloth will be completely airtight; because zero porosity means maximum drive.
Not surprisingly, sophisticated finishing has the greatest positive impact on the characteristics of the lightest spinnaker cloths. Furthermore, as the denier goes up, the crimp in the yarns increases—another reason why heavier fabrics often appear to "under-perform" relative to their actual weights. On the other hand, theres still no substitute for substantial yarns in the warp and the fill if you want a robust chute that can withstand both strong winds and prolonged, rough treatment.
Even the most elaborate tri-radial spinnakers cannot perfectly mirror the actual stress paths in a working spinnaker. Consequently, the bias, or off-axis stability of the fabric, makes an important contribution to a smooth, fast shape. As a general rule, a firm, highly resinated finish will improve the stretch resistance of a spinnaker cloth, particularly on the bias. It also tends to lock the fibers together, thus decreasing tear strength. Conversely, a softer, more elastic finish does less for cloth stability, but allows the threads to shift a bit and resist tearing. Luckily, this tradeoff is not an absolute, and cloth manufacturers are getting better at combining low stretch and tear resistance. According to industry-standard 20- and 40-pound tests, Bainbridges premium 600N AIRX spinnaker cloth exhibits less than half the bias and fill elongation of the same manufacturers 0.5 oz. Stablekote material. Better yet, AIRX is 30-to 40-percent better in tear tests (impaled on a nail and loaded to failure). Both weigh in the normal range for half-ounce cloths.
As most sailors know, the quoted weight of a spinnaker fabric is nearly always somewhat less than the actual weight (see illustration). Traditionally, it was the weight before the resination and heat shrinking. The actual finished weight of most so-called .75-ounce materials will come in at around 1.0 SM-oz; while the lightest 0.4-ounce. styles actually weight about 0.7 per SM-oz.
Racing spinnakers are weight sensitive, and a lighter sail is often a faster sail, right up to the moment it breaks. In an effort to extend the safe upper range of lightweight spinnakers, some sailmakers employ "step-up constructions" with slightly heavier, stronger materials along the luff of an asymmetric, or to ease the transition between corner patches and the body of the sail. To reduce racing costs (and trauma), many one-design classes have established minimum spinnaker cloth weights—for example 1.4 oz./10.76 sq. ft. for the J/24 or Melges 24. Sailcloth manufacturers understandably strive for materials that come in close to the minimums, and since spinnaker fabrics often vary significantly from one production run to the next, individual lot testing is important for critical applications.
The four major sailcloth suppliers that dominate the market each offer several complete series of spinnaker fabrics geared to specific applications. At one end of the spectrum are budget, "all-purpose" materials; however, most racing sailors will probably favor the various premium products. Differences in material costs typically have only a modest impact on the final price of a spinnaker, but the resulting differences in performance and life expectancy can be quite dramatic.
Ultralight spinnaker laminates such as Cuben Fiber may eventually have a rosy future, particularly for big-boat programs, because these constructions can more readily incorporate todays advanced, high-strength fibers. Cuben Fiber Corporation is a pioneer in the field, producing laminates as light as 0.25 SM-oz. Specially-treated Spectra fibers are laminated between polyester or Tedlar films, using an autoclave to generate enormous clamping pressures during a lengthy, high-temperature cure. These limited production materials not only cost at least 10 times more than "ordinary" spinnaker fabrics, but come in fairly small sheets, which means more seams and greater waste. All the same, they are increasingly popular for large multihulls and performance keelboats longer than 80 feet.
On the other hand, theres little doubt that conventional woven spinnaker materials will continue to find favor for most racing and cruising applications. Unprecedented competition among manufacturers is currently driving product evolution, so if you havent "talked spinnaker cloth" with your sailmaker in a while youll be pleasantly surprised.