Voyagers are slow to adopt 'high-tech' laminate sailsJan 1, 2003
More than 20 years have passed since 1977 America's Cup when the first experimental Mylar laminate genoa was hoisted aboard the 12-meter Endeavor and a "revolution in sailmaking" got seriously underway. After that milestone, the innovations came fast and furious: vertically cut sails, radial panel layouts, Kevlar and other super-strong fibers, and, underlying it all, a whirlwind of computerized design and fabrication techniques.
Highly publicized recent developments include the individually laminated sails produced by North's 3-DL process and Sobstad's rivaling Genesis approach. There's no doubt that today's "high-tech" sails are lighter, more stretch-resistant, and in most cases significantly faster than otherwise comparable sails built using less sophisticated methods. Nevertheless, the majority of voyagersparticularly those with boats under 50 feet LOAcontinue to buy woven Dacron sails that are almost identical to the sails of 20 years ago. The question is, why?
After talking to quite a few sailors and sailmakers about voyaging sail choices, I've concluded that the reasons why relatively few voyagers are so far opting for laminate sailcloth and modern radial layouts is a 20% to 50% price premium coupled with nagging concerns about durability and reparability. This is not to say that a conventional Dacron sail will necessarily have a longer service life, or that a high-tech sail will be difficult or perhaps impossible to repair at sea. However, few of the new-generation voyaging sails have been in use for even six years, and, despite very positive indications from the sphere of long-range ocean racing, the longevity issue has not been settled to the satisfaction of the voyaging community.
As for reparability, highly effective adhesive-bonding systems for laminated sailcloths do exist, but the industry has not yet made these products available to amateur sailors in a user-friendly form. In principle, it should be easier to glue a patch to a damaged sail that to sew one on, but successful repairs under field conditions will require tolerant adhesives that bond tenaciously at low temperatures and in the presence of moisture.
Some four years ago, I reported on new sailmaking technology for Ocean Navigator (Issue No. 56, September/October 1993). At that time, I somewhat rashly suggested that laminate voyaging sails could be the norm by the turn of the century. With just two years to go, my original prediction looks pretty shaky, but I nevertheless believe that laminated sails will eventually prevailmuch as fiberglass hull construction has largely supplanted wood. The bottom line is that contemporary laminated materials, if used in conjunction with appropriate design and fabrication technology, can produce sails that are superior in just about every way. Unfortunately, voyaging sailors are still being turned off by an overly narrow focus on the speed advantages of high-tech, without a corresponding emphasis on long-term shape retention and overall durability. At the same time, a sizable sector of the sailmaking industry is happy to build sails the old-fashioned way as long as there's a market for good, economical sails and personal service.
So at present, the technology gap between the latest Gran Prix racing sails and the average voyaging inventory is wider than ever. Nevertheless, the serious voyaging sailor will want to keep abreast with leading-edge developments, if only to prepare for making objective and intelligent purchasing decisions a few years down the road. In this spirit, here is an overview of the state of the art in sailmaking.
A diverse industry
Even in the computer age, cottage industry sailmaking remains viable because there's still a strong demand for low-tech Dacron sails that can be built economically without a heavy capital investment beyond some hand tools and a good sewing machine. Due to the characteristics of standard weaving equipment as well as market demand, almost all the heavier polyester fabrics suitable for voyaging sails are fill-oriented styles with the largest, straightest yarns extending across the width of the roll. These fill-oriented styles are only appropriate for conventional cross-cut sails. With few exceptions, if you want the inherently superior shape retention of a bi-radial or tri-radial panel layout, you'll need to abandon all-woven materials in favor of laminates. These more complex layouts can only be constructed efficiently with the aid of computerized sailmaking tools.
Since the early '80s, computerized design and panel cutting have played an ever-increasing role in sailmaking. With a computer-aided design system feeding information to a full-scale plotter or automated cutter, the speed and accuracy of sail productionbe it cross-cut, bi-radial, or whatevercan be greatly improved. What are the drawbacks of computerization? Specialized hardware and software are expensive, and learning to use, update, and maintain these tools is often a major challenge.
The big-name players in sailmaking today are either multinational franchise operations or working partnerships formed by like-minded groups of previously independent sailmakers. When an independent elects to either purchase a franchise or share resources with other lofts, one of the principle motivations is to gain affordable access to very costly, sophisticated design and production systems. But, ironically, as long as the average voyaging sailor isn't buying high-tech, the sailmaker who invests too heavily in technology is at risk of getting priced out of this important market sector.
The wind makes no distinction between a racing boat and a voyaging boat. Air flow reacts to sails (and other obstructions) in strict accordance to the laws of fluid dynamics, and the optimal three-dimensional flying shape for a voyaging sail for a given set of conditions will be virtually identical to that of a racing sail. In principle there should be some advantage to tweaking the shape of the voyaging sail for a wider groove (so that it can better tolerate the steering inaccuracies of inexpert helmsmen, windvanes etc.). However, in practice, the flying shapes of a great many voyaging sails are actually less forgiving than their racing counterparts, as well as considerably less efficient.
The single most important feature of a sail's flying shape is its draftthe depth of the curvature as a ratio of the distance from luff to leech. The difference between a flat sail and a full one is surprisingly subtle; it's generally in the 3% to 4% range. Effective upwind headsails are typically between 9% and 12% draft, while mains range from 8% to 11%. These are average values, with the majority of sails deliberately built slightly deeper toward the head than the foot (to suit the vertical pressure gradients over the surface of a working sail).
It's pressure differences between the windward and leeward sides of the sails that drive a boat forward and create a heeling moment. All else being equal, a deep sail will generate greater forces, so in strong winds a flatter sail shape and/or smaller sail becomes necessary to avoid being overpowered. Trim adjustments such as altering mast bend, headstay sag, luff tension, lead/traveler position and sheet tension allow a sail to function more efficiently across a range of wind conditions. But sailmaking has not progressed to point that one sail can handle everything from a zephyr to a full gale, and it very likely never will.
The flying shape of any sail is determined by a combination of built-in curvature and fabric stretch. But because the stretch component works to make the sail grow deeper in building winds (when flatter would be preferable), cloth manufacturers and sailmakers constantly strive to minimize it. On the other hand, serious racing sails need to be as light as possible so they will lift more easily in zephyrs and contribute less to speed-robbing weight aloft when the wind pipes up. Nowadays, space age fibers are routinely substituted for inexpensive, but stretchy, polyester in the quest for lighter, less distortion-prone sails. All these advanced fibers are many times more costly than polyester, and, with materials typically representing 30% to 40% of the price of a finished sail, their use can lead to some breathtaking price quotations.
So, for many voyaging sailors, the high-tech alternatives to good old Dacron look like non-starters for price reasons alone. But bear in mind that modern laminates constructed largely or exclusively out of polyester can still offer a substantial performance improvement over woven Dacron, but with a much smaller price penalty. Furthermore, by overbuilding with laminates, it's feasible to construct a very robust voyaging sail with superior shape-retention in varying wind loads and still come in below the weight of woven Dacron.
The skipper who claims that sail shape is unimportant because "we're only voyaging" could be making a serious mistake. Not all voyages are easy trade-wind passages, and windward efficiency can be crucial to storm avoidance or clawing off lee shores. A typical cross-cut headsail made of fairly soft voyaging cloth is almost always a major compromise: too flat, particularly near the luff, for optimal light air performance, but nevertheless prone to becoming excessively full with the draft moving aft when heavily loaded toward the top of its wind range. Because the lion's share of a sail's driving force is generated over the forward 25% to 30% of its surface, even subtle deviations from the optimal curvature in this area will have a disproportionate negative impact on performance. A stretchy genoa that suffers from a starved luff and tight leech will encourage the crew to oversheet the main (to reduce backwinding). The excessive weather helm that plagues many voyaging boats can often be traced to this problem.
Computerized voyaging sails
Due to the obvious expense, only a few Gran Prix racing sails are currently designed at a high level of sophistication (see sidebar). But by and large it probably doesn't much matter because the lessons picked up from racing are routinely transferred to club racing and voyaging sails. By observing which sail shapes and construction approaches are proving most effective in top-end racing applications, many sailmakers have systematically improved the sails they build for mainstream customers.
The limited-edition sail design software programs used by most computerized lofts are considerably less refined and more idiosyncratic than, say, Windows '95, so extensive hands-on experience and a fairly high level of computer literacy is a must. By the same token, it's nearly impossible to offer radial panel sails without access to computerized design and full-scale plotting facilities. Otherwise, these complex sail layouts are simply too laborious to build competitively.
Even with the help of efficient panel nesting programs, fabric waste for radial construction is around 15%about three times higher than for cross-cuts. And, as mentioned earlier, the great majority of woven sailcloths are fill-oriented styles intended specifically for cross-cut construction. The bottom line is that if you want the superior shape-holding potential of a radial sail you'll probably have to pay extra for more costly laminated sailcloth, additional assembly time, and greater fabric waste. The total price premium ranges from about 15% using all-polyester laminates to 50% or more when high performance reinforcements are involved.
Fibers, fabrics, and films
Polyester fiber, best known in North American under the familiar Du Pont trade name Dacron, is widely used in both woven sailcloths and laminated materials. Polyester in film formcommonly known as Mylaris the best-known film for sailmaking.
An advanced polyester-type fiber manufactured by Hoechst Trevira under the trade name Vectran has been used with some success by Hood Sailmakers for all-woven voyaging sails. Offering the strength and stretch resistance of Kevlar plus better chafe resistance, its chief drawbacks are greater susceptibility to UV rays and high cost.
More recently, AlliedSignal has introduced another "super polyester" known as PEN (polyethylene napthalate) that has superior strength and more than twice the stretch resistance of ordinary Dacron. PEN fiber is expensive and difficult to obtain, but it offers a loophole for building faster sails in one-design classes that restrict or ban laminated materials. An experimental PEN fiber jib helped Brazil win the Star gold medal at the '96 Olympics. However, some racing authorities have already moved against it. The competitive J-24 class has moved to disallow PEN sails in the interests of economy. Cost considerations aside, a specialized sailcloth using PEN for the warp yarns and ordinarily polyester in the fill might be good approach for building radial panel sails from all-woven material.
PEN film has been used as a high-performance alternative to Mylar in the last America's Cup, with a claimed overall fabric weight savings of 10% to 12%. However, the thinner, stiffer film proved more susceptible to shrinkagea laminate problem caused by extensive, small-scale creasing of the material when the sail is repeatedly handled. Thick film laminates (over 1.5 mil) and dual film sandwich laminates seem considerably less prone to shrinkage, so the best way to capitalize on the superior physical properties of PEN film may eventually be to reduce the amount of fiber reinforcement required.
Fibers in the aramid familyKevlar, Technora, and Twaronare standard issue for racing sails today because they offer an excellent combination of immense tensile strength, light weight, and very low stretch. Their relatively poor resistance to abrasion, fatigue and UV exposure can be improved by laminating them between layers of film, or between a film and a lightweight woven polyester taffeta. PEN film has superior sunblocking characteristics and looks like a good choice for racing laminates with sun-sensitive aramid or Vectran fibers.
On the other hand, there's another family of super-strong, super-durable fibers that resist UV as well as, or better than polyester. The ultra-high-modulus polyethylenes are widely regarded as the best available choice for endurance racing or performance voyaging applications. Best known is AlliedSignal's Spectra, but DMS's Dyneema, and Hoechst Trevira's Certran are fundamentally similar. Stronger than aramids for a given weight and far superior in terms of fatigue, chafe, and UV resistance, they would appear nearly perfect for sailmaking were it not for its tendency to creep or stretch permanently when subjected to high, sustained loads.
Creep is a serious problem for minimum-weight racing sails that are frequently loaded at close to their limits. This is the primary reason why aramids rather than high-modulus polyethylene remains the racing norm. But for a reasonably over-built voyaging sail, creep should not pose a concern. When a sail is designed conservatively, the sustained working loads it sees will be just a fraction of the ultimate breaking load. Momentary shock loads caused by knockdowns or other mishaps do not cause creep and permanent deformation of the sail cloth.
Spectra and its relatives are slippery and hard to gluea challenge when it comes to laminating. A popular solution for voyaging laminates is to sandwich a loosely woven scrim containing widely spaced Spectra yarns between a Mylar film and a lightweight polyester taffeta. A secure polyester-to-polyester bond occurs in the spaces between Spectra fiber bundles.
For leading-edge racing sails, even aramids are becoming old hat. The 1995 America's Cup boats used carbon fiber in some sails. And, more recently, an even more stretch-resistant synthetic called PBO (phenylene benzobisoxazok) has hit the scene. While both of these materials have spectacular physical properties, they are also very expensive. In addition, they show a discouraging vulnerability to wear and tear. Carbon fiber is brittle, while PBO fatigues easily and suffers considerably from UV exposure. Neither looks promising for voyaging sails.
Full-length vs. panels
Seams are potential weak points, particularly if the strongest load-bearing yarns are interrupted in a heavily stressed portion of the sail. With the advent of super-strong fibers, sailmakers have focused on boosting the strength of their seams by using wide overlaps and adhesive bonding to supplement conventional stitching. Sobstad Sailmakers has developed a particularly interesting dovetailed seam that is used in assembling their innovative Genesis sails.
The Genesis process begins with the lamination of a series of custom panels to make up each individual sail. The number, size, and angular orientation of yarns, as well as the thickness and type of films selected for each part of the sail, are varied in accordance with computer-generated stress predictions. When the panels are run through the laminator, the edges are left open so the layers can be interlaced during final assembly. Wide overlaps ensure 100% load transfer from yarn to yarn once the seams are bonded under heat and pressure.
In North Sail's equally ingenious 3-DL process, the films comprising each surface of the sail are glued together before the reinforcing yarns are added. The films themselves are pre-shaped using conventional broadseaming, but the method used to add the reinforcing yarns is unique. First the lower film is draped over a lattice-type male mold that has been pre-set to the correct three-dimensional curvature. Second, a computer-controlled gantry travels back and forth over the sail surface, following the predicted load paths, and laying down yarns into uncured adhesive as it trundles along. Next, the upper film is carefully stretched over the mold, and the laminate consolidated under vacuum. Last, the bonding adhesive is cured by the application of radiant heat.
The 3-DL approach produces one-piece sails with continuous fibers and no load-bearing seams. They are visually spectacular, with gracefully arching reinforcements easily visible through the transparent film. Unfortunately, the manufacturing process is time-consuming because the yarns are laid down quite slowly and there are many steps involved. In effect, a 3-DL sail is like three separate sails rolled into one. Rival sailmakers claim that paneled construction can be even lighter than 3-DL because it allows greater freedom to vary fiber density and film thickness and because North's low-pressure laminating technique requires a heavier adhesive layer.
For several years, Sobstad Sailmakers has been fighting a court case against North Sails, claiming that the 3-DL approach infringes upon patents that protect the fundamentals of the Genesis process. Despite recurring rumors of an imminent decision, nothing is certain except that the mounting legal costs will do nothing to make high-tech sails more affordable.
Sail cloth manufacturers are, of course, keenly interested in maintaining the demand for paneled sails and have begun focusing more attention on improved seaming methods. Dimension Polyant recently introduced Composite Sail Technology (C.S.T.), a hot-melt gluing system for strong, reliable seaming. Research is also underway on even more advanced adhesives that will combine extreme shear strength with reduced peel strength (so seams can be opened for recutting or repairs).
There's also a third approach to building load-path reinforced sails that may ultimately be of greater significance to voyagers than either Genesis or 3-DL. Since the mid-'80s, Ulmer Kolius has been refining their Tape-Drive sails, and, more recently, other sailmakers have been working along similar lines. A lightweight paneled sail is assembled using either a taffeta/film or a scrim/film laminate. After assembly, the sailmaker applies long lengths of unidirectional fiber tape along the primary load paths. The special reinforcing tapes can incorporate almost any fiber: Spectra, Kevlar, polyester, Vectraneven fiberglass. This approach looks promising for voyaging sails because it allows relatively inexpensive laminates and simplified panel layouts to be used without compromising shape retention.
How much of a gamble?
Readers who sense a pro-laminate bias in this report are pretty much on target. Having followed the development of laminate sails since my own sailmaking days in the early '80s, I believe this can no longer be considered an experimental, throw-away technology. Round-the-world solo races like the Vendee Globe represent three-month, accelerated wear testsin many respects the equivalent of years of normal voyaging. The latest generation of Spectra-reinforced laminates pass with flying colors.
It's less certain how laminated voyaging sails will react to long-term UV exposure (e.g., an extended tropical sabbatical). Spectra yarns will withstand sunlight at least as well as Dacron, but there are indications that UV promotes shrinkage in Mylar racing sails. Some voyaging laminates sandwich the film between two layers of woven taffetaseffective for sun protection, but not particularly weight efficient. At present, the best all-around construction for voyaging laminates is probably a taffeta/scrim/film combination using a relatively thick Mylar (1.5 or 2 mils) for shrink resistance. The woven taffeta adds tear-resistance and stitch-holding ability. However, as glued seam technology improves, watch for a shift toward sandwiched scrim voyaging laminatesalready a trend in racing.
Some boats are much better candidates for laminate sails than others. Big, stiff boats with high-aspect rigs impose particularly high loads on sailcloth. In these cases, a laminate sail utilizing high-strength fibers is easy to justify over much heavier two-ply Dacron construction.
On the other hand, a small-boat voyager with a minimal budget will have a hard time rationalizing anything except a conventional Dacron sail. Fortunately, there are still excellent woven fabrics available, and sailmakers who will build economical, long-lasting sails from them.
Contributing editor Sven Donaldson, a former sailmaker, is a marine technical writer based on the West Coast.