Voyaging laminates

Offshore sailors are a conservative lot, curiously unwilling to risk their lives with gear that has not yet withstood the test of time. All the same, technology tramps onward, and gradually a new breed of sails are finding their way into the inventories of more and more long-range voyaging sailors.

Mylar laminates, once regarded as andquot;throwaway specialsandquot; for the sky’s-the-limit racing crowd, have been in use for more than a decade aboard club racing boats, cruiser/racers, and a few inquisitive voyagers. Early versions frequently deserved their reputation for fragility, but as sailcloth manufacturers and sailmakers have refined their materials and techniques, laminate sails have improved by leaps and bounds. In the most recent single-handed round the world races, almost every competitor used inventories consisting primarily of laminated sails incorporating Kevlar or Spectra reinforcements. Their experiences with these materials have been so positive that it’s worth taking a closer look at these races in the context of ocean voyaging.

The ’90/’91 BOC Round the World Race and the ’92/’93 Vendee Globe Challenge were singlehanded races contested in extremely fast, lightweight monohulls, usually 60 feet in length. The BOC is a four-stage race, while the Vendee is a non-stop circumnavigation with no outside assistance permitted along the way.

Despite being ultralight displacement boats, the contemporary 60-foot singlehander is awesomely stable thanks to extreme beam (up to 20 feet) and as much as four tons of seawater ballast carried in tanks offset far to windward. Great stability imposes enormous loads on sails (as well as the rig and hull structure, of course), so it’s fair to say that the demands of four months of continuous ocean racing aboard one of these monsters would probably equate to several years of serious voyaging aboard a more typical offshore boat.

In both these races, the great majority of laminate headsails came through with flying colors. Laminate mainsails were sometimes a bit more problematical, but not to the extent that competitors would consider going back to woven Dacron. According to Alain Gautier, who was second in the BOC (after blowing out his mainsail with a week to go), the mainsail above the second reef was in absolutely perfect shape even after having done the whole BOC race, but below that reef point, the Kevlar did suffer. In the same interview, Gautier suggested “possibly protecting the Kevlar better in a sandwich construction, principally toward the bottom of the sails.” Whatever the improvements, they must have worked, because less than two years later he sailed Bagages Superior to a comfortable victory in the Vendee Globe, almost a week ahead of the next competitor and with no significant sail problems.

The growing success of laminate voyaging sails and the increasing acceptance of roller furling/reefing go hand in hand. Furling gear is gentle on sails; struggling to bag them on a wave-drenched foredeck definitely is not. “The biggest problem with laminates is shrink or wrinkle retention,” says Fred Schueddekopp of Bainbridge Sailcloth’s West Coast office. “Crumple a piece of paper, then try to flatten it out and you’ll find it’s never as big as it was before.” In other words, these sails tend to get rumpled if you manhandle them on deck or stuff them into sailbags. According to Schueddekopp, there are various approaches that improve resistance to creasing problems, such as multi-ply laminates with lightweight woven fabrics on each outer surface. However, he agreed that the single best thing one can do is to keep sails on furlers.The many faces of polyester

Besides being the principle ingredient of most fiberglass hulls and the proverbial cheap suit, polyester continues to play the starring role in contemporary sailmaking. Dacron is DuPont’s tradename for polyester in fiber form as used for spinning threads or yarns, and ultimately for weaving fabrics. When liquid polyester is extruded and drawn through tiny orifices to form filaments, its long chain molecules are, to a large extent, aligned into parallel arrays. This substantially increases the tensile strength and stretch resistance along the axis of the strands (see accompanying story on synthetic fibers).

Woven Dacron sailcloth resists stretch much better along it’s thread lines than on the bias (45 degrees to the weave). Its stretch resistance along the thread lines is largely determined by crimpandmdash;the difference between the length of a yarn within a piece material and the same yarn removed and pulled absolutely straight. In woven materials, crimp results from the way the yarns zigzag over and under each another. Although it’s possible to construct sailcloth with very little crimp in one axis, this comes at the expense of greater crimp (and stretch) in the other. Sailmakers refer to a fabric woven with large, straighter yarns going one way, and smaller highly crimped yarns running the other, as an unbalanced style. Because the stresses in sails are predictably much greater in some directions than others, unbalanced materials (both conventional sailcloths and laminates) are the norm in contemporary sailmaking.

In terms of their chemistry, polyester sheets and films such as DuPont’s well known andquot;Mylarandquot; are virtually identical to the polyester in yarns. However, due to different forming processes, the long polymer molecules are not aligned to the same extent that they are in an extruded filament. Instead, they take the form of a tangled array. Consequently Mylar film has fairly uniform mechanical properties and no “bias”. However, pound for pound, it is less stretch resistant than Dacron fiber. Its usual application in sailmaking is as a component of a laminate where it beefs up the bias of another material. In addition, a Mylar film is often used as a foundation layer so that widely spaced yarns in the form of a loose scrim can be used to reinforce a thin, lightweight sail membrane.

Before leaving the subject of polyester, it should be mentioned that some of the adhesives used in bonding laminated sail materials are catalyzed polyester resins, not dissimilar to the resins used in fiberglass boatbuilding. These adhesives need to be fairly resilient, and therefore contribute little to the overall structural properties of the sail. Although crucial to keeping the laminate intact, they are essentially dead weight in other respects.The rationale for laminates

Conventional voyaging sails are made from relatively heavy (8 to 12 oz.) woven Dacron because this material is moderately priced, resistant to many kinds of physical abuse, and only gradually degraded by UV light. However, for its weight, the typical cruising Dacron is a rather pathetic performer. It’s quite difficult to manufacture heavy, tightly woven, unbalanced Dacrons with large, crimp-free fill yarns running across the panelsandmdash;the styles needed to make stable, long-lasting, cross cut sails. Instead, the great majority of cruising Dacrons available (which probably means the great majority being used) are rather balanced, and loosely woven. Even under modest loads, they stretch considerably along both thread lines, and dramatically on the bias.

Sail stretch is highly detrimental to all around sailing performance because its effects run counter to the principles of good sail trim. In an increasing wind, a sail should ideally become flatter (less cambered) with the draft, or deepest point in its fore-and-aft curvature, further forward. Instead, cloth stretch makes the sail fuller and shifts the draft aft.

The sheer weight of a conventional voyaging sail is also a drawback because weight aloft reduces stability and adds pitching moment, while weight on deck makes sail handling difficult. Having said as much, it’s worth noting that most sailmakers using laminates for voyaging boats under 45 feet have opted for extra shape-holding ability and increased reserve strength, rather than striving for dramatic weight savings. On the other hand, with larger boats, and particularly when advanced, high-strength reinforcing materials like Kevlar and Spectra are used, the weight savings can be substantial.Laminating basics

The beauty of laminating is its potential to create a composite that pools the strengths and avoids the shortcomings of each component on its own. Mylar is a uniform polyester film with no bias weakness, but tears extend across it quite easily once they get started. Polyester yarn, with its highly-oriented molecular structure, possesses two-to-three times greater unidirectional strength. In addition, an array of spaced yarns has valuable rip-stopping qualities. To get the best of both worlds, fibrous reinforcements can be bonded to a film substrate to boost the strength of the laminate in one or more specific directions, to increase its tear strength, and to improve general durability or abrasion resistance.

The earliest laminates consisted of nylon spinnaker cloth bonded to a Mylar film. The lightweight woven nylon was far too stretchy to share sailing loads with the Mylar, it was only there to prevent the sail from tearing catastrophically when it snagged on something.

Over the next couple years, sailmakers and cloth manufacturers discovered that much better results could be obtained if the Mylar was bonded to a lightweight woven polyester sailcloth. With the woven component shouldering much of the load instead of just going along for the ride, performance was greatly improved.

In 1980, Dimension Sailclothandmdash;a company founded just three years earlier that had rapidly grown to become a dominant player in the sailcloth fieldandmdash;introduced a new type of laminate called a “tri-ply.” This laminate was reinforced with scrim – a woven material characterized by big yarns, wide gaps between the yarns, and very little crimp. Hopelessly loose and sloppy on its own, scrim works well when sandwiched between two Mylar films that are bonded to each other. Unlike taffetas (such as conventional woven sailcloth), scrims are relatively inexpensive to produce. This is the case because conventional fabrics, particularly tightly woven lightweight styles with high thread counts, come off the loom very slowly, often at rate of only a few yards per hour. Scrims can be manufactured many times faster.

However, taffetas do have certain advantagesandmdash;notably an ability to “hold” stitching that aids in the construction of strong seams. They continue to be used in many of today’s laminates, sometimes in combination with scrim-type reinforcements.High tech fibers

Polyester is by no means the strongest material that can be used in sails.

Kevlar is an aramid fiber manufactured by DuPont in the U.S. Similar products are now available under different trade names from Europe and Japan. Aramid fibers are several times stronger than polyester for their weight, and they stretch very little under load. Unfortunately it is also several times more expensive, and fairly susceptible to fatigue, particularly if the aramid fibers get a chance to rub against one another.

Spectra is an Allied-Signal tradename for a special variety of polyethylene in which the long chain polymer molecules are highly oriented and extensively cross linked. Its tensile strength is comparable to Kevlar, while its light weight and good fatigue resistance make it seem very promising for sailmaking. If there’s a fly in the ointment, its a tendency to creep, or elongate gradually under high sustained loads. Allied’s 1000-grade Spectra is said to be more creep-resistant than their 900 grade, and successful sails are being made that incorporate this material.

The other difficulty with Spectra (besides its high cost), is its resistance to bonding with virtually every known class of adhesives. One way to circumvent the problem is to sandwich a Spectra scrim between two layers of polyester material (either Mylar or taffeta) that will bond securely to each other.

Another approach is to combine a Spectra warp with a polyester fill in a taffeta weave, so the glue will get a good grip on at least some of the fibers. The laminators appear to have gotten a pretty good handle on this problem, and Spectra-containing laminates appear to have a bright future for voyaging applications.

Lightweight woven materials (including the taffetas and scrims used to manufacture laminates) are almost always warp-oriented fabrics with stronger yarns and lower crimp in the yarns aligned with the length of the roll. This is the case because it’s easy to set up a loom with plenty of tension on the warp yarns, but much harder to obtain high tension on the fill threads being carried by the shuttle as it crisscrosses the warp. It follows that it is easier to keep a loom running if the highly-tensioned warp yarns are larger and stronger than the fill threads because they’ll be less likely to break and halt production.

In the great majority of sails, the highest loads follow stress pathways that run roughly parallel to the leech. For this reason, warp-oriented materials lend themselves to sails with the panels oriented vertically as opposed to the more traditional horizontal or andquot;cross cutandquot; arrangement. However, because the stress trajectories arch into the body of the sail from each of its three corners, radial construction is the optimal way to keep the strong warp yarns of laminates closely aligned with the actual loads.

Radial construction is inevitably a compromise because the added complexity and fabric waste adds complexity and cost to a sail. Computerized sail design is more or less essential, and computerized panel arranging (nesting) and plotting/cutting techniques are desirable. Nevertheless, it’s the preferred way both to take advantage of the built-in strength of these modern materials and to minimize loading of potentially vulnerable seams.Comparing laminates

The raison d’ecirctre for laminate sails is to improve on conventional Dacron in terms of shape-holding, reserve strength, weight, and increased useful life. Since laminate racing sails are built with the same goals in mind, the distinction between voyaging and performance laminates is an artificial one. However, if I had to pick a single feature that characterizes a typical voyaging laminate, it would be the use of at least two taffeta layers, one on each outside surface of the material.

Dual taffeta laminates were introduced about seven years ago, primarily for marketing reasons (although the construction has since proven to have some practical advantages). Voyaging sailors strongly prefer a flexible cloth with a soft hand, which, in the past has tended to disqualify the “firm” coated Dacron styles that have been widely used for racing sails. Of course, for sails that are set from furling gear or captured between lazy jacks to stack nicely on top of the boom, softness has little real impact on ease of handling. But even if it’s mainly psychological, most buyers favor sails that look and feel like cloth, rather than ones with the texture and appearance of stiff plastic bags.

Perhaps the greatest practical advantage of having taffeta on both faces of a laminate is improve resistance to film creasing and “shrink”. The outer woven fabric layers helps prevents the Mylar film inside from being forced into the tiny, tight radius bends which result in localized weakening and permanent deformation. The taffeta should also increases abrasion resistance. Thus, dual taffeta construction is particularly worthwhile for mainsails and headsails that are handled frequently on deck.

At the same time, there’s no good reason to disqualify all “single-sided” laminates – those with exposed Mylar on one surface – for voyaging applications. They tend to be a somewhat less costly than dual taffeta laminates possessing comparable mechanical properties, and are often quite satisfactory for smaller boats and those with roller furling gear. Being shiny on one side, they have a andquot;high techandquot; look that some sailors like and others loath.

Even film/scrim/film styles can be fine for small boats, medium-sized furling jibs, and large, light weather headsails. These constructions are also less costly than the dual taffeta laminates, and potentially offer the highest strength-to-weight ratios, as well as good tear resistance. If they have a drawback, it’s the greater difficulty involved in constructing strong, long-lasting seams because stitch holes tend to elongate in the gaps between reinforcements. And again, some sailors object to their high tech, semi-transparent appearance.

In the early days of laminates, frequent bonding and delamination problems gave “Mylar sails” a fairly bad reputation. Today, these problems have become very rare, at least over the life span of most racing sails. On the other hand, voyaging sails tend to be used much longer than their racing counterparts because most voyagers are justifiably less concerned about andquot;ultimately perfectandquot; sail shape. At present, laminates simply don’t have a long enough track record to ensure that delamination will never be a problem. Although there are now reports of five- to seven-year-old cruising laminates still going strong, some Dacron sails still seem okay after 15 years or more. Only time will tell.

With conventional woven sailcloths it’s an unfortunate fact that weaving and finishing techniques that increase dimensional stability and stretch resistance usually reduce tear strength. With loosely woven material, the yarns tend to shift, so localized forces end up being shared by neighboring yarns. In highly stabilized fabrics, the yarns are immobilized so they tend to break one after another.

At least to some extent, laminates tend to share this problem, although steps can be taken to reduce it. Selecting resilient laminating adhesives that don’t “lock up” the reinforcing yarns too much is one approach. Incorporating scrims also helps because the large, widely spaced reinforcements have a rip-stopping effect. It’s quite possible that the extraordinarily high strength of sails reinforced with Kevlar or Spectra is an asset when it comes to avoiding or localizing damage.

Most laminate sails can be repaired using conventional techniques. Self-adhesive sail repair materials adhere very well to exposed Mylar and can be used to reinforce the edges of a tear so that stitching will have more to bite into.Guidelines for buying laminate sails

The old headache of significant variability from one lot of sailcloth to the next has certainly not gone away with the advent of laminates, even though the major manufacturers have succeeded in sharply reducing the proportion of andquot;bad batchesandquot; with practice and improved equipment.

Certain flaws in a laminated materials are easy to detect. For example, it’s not uncommon for a scrim to be displaced during the laminating process, so some yarns follow wiggly paths instead of straight ones. In effect, this flaw introduces crimp, and hurts the stretch resistance of the material.

Not surprisingly, some other defects are harder to spot; so the very best way to ensure that one is getting top-grade material is to learn to interpret laboratory sailcloth tests. This way, one can evaluate the data for the particular lots of material being considered for sails. Of course, what’s generally more practical is to find a sailmaker one can trust who will obtain this information and make the selection after consultation. As with most purchases, there are tradeoffs to be made, and they frequently relate to price.

The variety of styles and configurations of laminates available today has become bewildering, even to sailmakers, but it is possible to pick out a few guiding principles:

1. Almost all modern laminates have their strongest yarns and lowest stretch in the warp direction. This makes them ideal for radial construction, and to a lesser extent, vertical or scotch cut layouts, but inappropriate for traditional cross cut sails. Some recent laminates incorporate “off-warp” yarns to help handle loads that are not aligned precisely with the warp.

2. The Mylar film used in most laminates suitable for voyaging sails is 1.5 or 2 mils thick. Some materials sometimes use two plys of 1 or 1.5 mil Mylar with a scrim between them, but most voyaging laminates contain a single film. Thicker Mylar often improves the bias noticeably and sometimes the fill, but rarely has much effect on the warp. It often results in a perceptibly stiffer fabric.

3. A few “cruise laminates” have been manufactured that contain no film -only plies of taffeta and scrim. The rationale here is to take advantage of the superior properties, pound for pound, of lightweight fabrics while staying with an “all woven” material. In reality, this is an artificial distinction because any practical laminate is at least 15 percent adhesive. Styles lacking a film component have tended to be unstable on the bias unless heavily resinated, like stiff Dacron racing sails.

4. Scrim-type reinforcements are either laminated between two films or between a film and a taffeta. They typically have virtually zero crimp in both thread lines, and are less expensive than taffetas for a given fiber density. The large, very strong yarns are good at containing rips and punctures. On the debit side, film/scrim/film constructions that lack a taffeta layer tend to hold stitching poorly and have a shiny, semi-transparent, “high tech” look that some sailors dislike. Most scrim-reinforced laminates made specifically for voyaging sails also incorporate at least one layer of taffeta.

5. Taffetas used in voyaging laminates, with the exception of a few light, low-performance varieties, have unbalanced constructions that heavily favor the warp. High-performance styles for large boats may combine an almost crimp-free warp of Spectra yarns (and occasionally Kevlar) with small, highly crimped polyester fill yarns to tie the weave together. Taffetas are often used on both faces of a voyaging laminate because they look like traditional Dacron, hold stitching well, and resist abuse somewhat better than exposed films.

6. Adhesives used in laminates should contain fungicides to prevent mildew – most likely to be a problem when sails are furled away wet. Mildew is probably more of a risk with multi-ply taffeta materials, and less so for laminates where the reinforcement is sealed between two layers of film.Taking a chance?

At this stage in the game, buying a laminate sail for voyaging is still a bit of a gamble because, although early durability problems appear to have been solved, these materials haven’t been around long enough to make absolutely sure of their long-term durability. In terms of UV resistance, it doesn’t seem to make much difference whether polyester takes the form of a film or a woven fabric. Spectra withstands sunlight about as well as polyester; Kevlar a bit less well.

In the final analysis, if good sail shape when close hauled and close reaching is an important factor, laminates are worth considering. However, if one sails before the trade winds most of the time, cares little about windward ability, and wants to be certain sails last as long as possible, it’s better not to experiment with laminates – at least not yet. If one elects to start out with one laminate sail, the best bet is a genoa or, for cutter rigs, a Yankee. Ideally, the sail should be set from a roller furler. The larger the boat, the more cost effective these sails are likely to be.

Although sophisticated laminates and radial-style panel layouts may still seem daring and avant garde to many voyaging sailors, the fact is, the leading edge of sailmaking technology has long since moved on to much wilder things, such as molded, panel-less construction.

I suspect that by the turn of the century, laminated sails will be the norm. Once that’s happened, it may not be much longer before the sailor who insists upon woven Dacron will be regarded as an anachronism, much like the old salts from past generations who still swear by tarred hemp and cotton canvas.

Sven Donaldson, a former sailmaker who lives in Vancouver, has written numerous magazine articles and four books, including Understanding the New Sailing Technology, published by Dodd Mead.

By Ocean Navigator