Fiber composites

As most serious sailors are aware, a quiet revolution has been sweeping through the fiberglass boatbuilding industry. The old bucket-and-roller approach has been gradually giving way to wet-out/chopper guns, resin impregnators and vacuum-bagging systems. Some yards have adapted closed-mold, resin-infusion technologies, both to improve product quality and to meet tough new air quality standards. Simultaneously, the composite materials themselves have progressed remarkably, with superior fibers, reinforcement styles, core materials, resins and adhesives all playing a part in the creation of demonstrably better boats. This article provides an overview of what’s cooking, both literally and figuratively, in the evolving world of fiber composites.

Perhaps it seems obvious, but it is worth taking a moment to consider why fiberglass has emerged over the past 50 years as the overwhelming favorite for pleasure boat construction. The chief reasons why fiberglass is so popular today are the same reasons that it took off back in the 1960s: the stuff is rugged, waterproof (more or less), rot-proof, corrosion-proof and highly amenable to series production using relatively low-cost molds.

For large commercial vessels, steel and aluminum remain the norms. But as boat size diminishes to 40 feet and less, it is increasingly hard to justify the weight penalties involved with metal. Furthermore, unlike foam or balsa sandwich, a comfortable metal cruising boat will need full interior insulation — a significant extra in terms of construction time and cost. Corrosion control is another key issue with metal boats, as is the challenge of achieving and maintaining an attractive exterior paint job, particularly with aluminum. Certainly, voyagers can easily appreciate the sense of security that comes from “bulletproof” steel or aluminum construction, but there is good evidence that well-engineered composite boats can also perform very well in terms of impact and puncture resistance. Of course, metal remains the best choice for withstanding severe abrasion — a very desirable quality should you ever find yourself aground on a surf-swept shore.

But in all, composite construction dominates the pleasure-craft industry. Moreover, I believe this general approach to boatbuilding will gain additional ground in years to come, thanks to the broader adoption of improved technologies such as those discussed below.

All fiberglass boatbuilding requires molds of some sort — a cost that must ultimately be borne by customers if a builder is to remain in business for long. Consequently, the recent development of improved mold-making methods is potentially of benefit to everyone, builder and buyer alike. Most dramatic is the advent of large, multi-axis robotic milling machines that can automatically carve out a complex hull or deck plug precisely to the designer’s plans. These machines work directly from CAD (computer-aided design) data, thus eliminating the need for tricky, time-consuming lofting procedures. This technology is currently extremely expensive, and it is likely that only the big powerboat consortiums such as Genmar and Brunswick will be able to justify outright purchase. On the other hand, small yards already have the option of sub-contracting this work to a growing number of specialist firms.

When a highly polished mold is required for series fiberglass boatbuilding, it is normal practice to create a robotically-shaped plug (rather than a female mold), because the convex plug surfaces are much easier to hand finish. On the other hand, for limited production, it may be more efficient to mill out a female mold that can be used “rough,” with the final fairing and finishing labor devoted to the part itself.

Single skin vs. cored construction

Traditional fiberglass construction, using alternate layers of chopped-strand mat and woven roving, is actually a very sensible way to build single-skin, moderate-to-heavy displacement boats. Fiberglass is inherently strong and fracture-resistant, but not especially stiff, so it is necessary to build up a generous thickness in order to forestall oil-canning or other disconcerting signs of flexibility. Mat and roving builds thickness quickly using fewer layers and less glass fiber than the newer, unidirectional-type reinforcements discussed below. Mat, in particular, soaks up resin like a sponge, and bulking products such as Coremat do much the same, although with a little less added weight.

Glass fibers are vastly stronger than resin, of course, but most of the old-style fiberglass boats incorporate enough fiber to be perfectly safe despite glass-to-resin ratios of only about 30/70. On the other hand, it is possible to do a great deal better when a relatively thick, low-density core is added to the laminate as a structural spacer between two relatively light fiberglass skins.

Cored-fiberglass boatbuilding dates back to the early post-war years, but misgivings remain widespread, especially when it comes to using sandwich construction below the waterline. Without question, there have been plenty of bad experiences due to inadequate or poorly installed cores.

A better understanding of core installation procedures at the shop floor level, as well as improvements in the core materials themselves, have lately gone a long way toward improving the reliability of cored hulls. But by the same token, building a cored hull will always require more skill and care than a single-skin one, because the skin-to-core bonds must be reliably strong and void-free. Good, consistent shop procedures are crucial to success; and vacuum bagging (discussed below) can be particularly helpful, because it is usually the best way to achieve strong, uniform clamping pressure over a broad area.

Done properly, cored-composite construction is the cat’s meow, because it can exceed the physical properties of single-skin construction, while saving considerable weight. The bending stiffness of a panel increases roughly as the cube of its thickness; and it makes little difference whether the panel is a homogeneous slab or two thin, strong skins with a generous layer of lightweight coring sandwiched between. As for impact resistance, many cored-composite panels, especially those incorporating resilient synthetic foams like Airex and Core-Cell, can outperform the average single-skin laminate. When subjected to concentrated impact loads (as when a yacht collides with a semi-submerged shipping container), these relatively tough, flexible foams serve to cushion the blow, allowing the inner composite skin to survive intact (or remain waterproof).

By comparison, the more rigid core materials such as end-grain balsa, the cross-linked PVC (polyvinyl chloride) foams, and the more exotic honeycomb materials such as Nomex represent a trade-off: reduced impact-resistance, but greater stiffness, shear strength and/or compressive strength at a given density. Nevertheless, with proper engineering, cored hulls built with these products can also withstand collisions at least on par with single-skin construction, while simultaneously achieving worthwhile gains in their other characteristics. The chief downsides of cored construction are a requirement for greater expertise on the part of the builders, and, of course, added expense.

A materials smorgasbord

Once a boatbuilder has committed to cored construction (whether for decks and interiors only, or for entire boats), it usually makes sense to simultaneously move up to a higher class of reinforcing products commonly known as unidirectionals. Conventional woven cloths or rovings follow wave-like paths as they zigzag over and under one another. This “crimp” degrades structural performance, because when a woven reinforcing material is subjected to load, the individual fibers will straighten as well as stretch, resulting in greater deformation.

In unidirectional materials, all the fibers lie parallel to one another, and are essentially crimp-free. But unlike fiberglass cloth or woven roving, a simple “uni” consisting of a flat sheet of parallel fibers tends to fray and come apart when cut or handled. To facilitate use, it is common practice for composite materials manufacturers to stack up crossed unidirectional layers, often along with a thin layer of chopped strand, all stitched together with fiberglass thread. These so-called bi-axial rovings are now widely used throughout the industry. And where lightweight reinforcements are needed, low-crimp twill weaves can be substituted for conventional glass cloth for a considerable gain in structural performance.

Nearly all the fiberglass used in boats is a grade known as E-glass (electrical glass). A more expensive variety known as S-glass (structural glass) is available for critical applications, but in boatbuilding, its superiority is rarely considered sufficient to justify the higher price. Besides eliminating crimp, unidirectionals offer closer fiber packing, resulting in much improved fiber-to-resin ratio — typically around 50/50 instead of the usual 30/70. Eliminating a significant portion of the deadweight resin represents a major overall improvement, and for many projects the further gains that would stem from moving up from unis to even more costly exotic fibers are impossible to justify.

But of course, there’s always a call to go the next step. The lightweight aramid fibers such as Kevlar and Twaron are often used in race boats as a step up from fiberglass, particularly when carbon fiber has been banned as an economy measure. Sometimes, these materials are also incorporated in the underbodies of voyaging boats as an anti-collision barrier. Their effectiveness in the latter application is a matter of debate, because a resin-bonded laminate incorporating layers of aramid is by no means the equivalent of the stack of loose Kevlar plies in a bulletproof vest. All the same, several boatbuilders now offer this feature, and some, such as France’s Dufour, claim impressive results in simulated collision testing.

Carbon-fiber reinforcements are the lightest and stiffest materials in widespread use. And as their use expands into the mass markets for sporting goods and vehicles, prices have been trending downward. For high-end boatbuilding, it is increasingly attractive to move up to carbon for significant weight savings. Obviously, this is not for everyone, however. Cost is still at least four times higher than E-glass. Also, carbon laminates — even those that incorporate “low-modulus” carbon — tend to be more brittle than those made with more flexible fibers. Electrical conductivity can also create engineering challenges in certain instances. Often the most cost-effective use of carbon fiber in boatbuilding is for highly loaded components such as chain plates and stringer caps, while the big areas are laid up in E-glass. The force grid alternative

Force grid — a marketing term coined by now-defunct Ericson Yachts during the 1980s — describes a molded fiberglass hull liner featuring large, criss-crossed channels that become structural box beams once the liner is bonded to the inside of the hull. Variations on this theme are now the norm among production sailboat manufacturers, and many power yacht manufacturers as well. These structural hull liners not only stiffen and support the outer skin, but also anchor the interior with molded slots to accept plywood bulkheads and furniture modules. This approach can greatly reduce building times, especially when robotic woodworking machinery is used to mass-produce accurate, pre-finished cabinetry.

In recent years, a growing number of the world’s largest sailboat manufacturers — high volume builders such as Beneteau and Catalina — have found that unidirectional and bi-axial reinforcing materials can be worth using, even for single-skin hull construction, provided the thinner outer shell that results is adequately supported by a grid-type sub-structure. The higher cost of these reinforcing materials on a per-yard basis is frequently offset by reduced fiberglass and resin consumption. At the same time, by paring down the weight of the boat, performance and stability improve. As an added bonus, using flat bi-axial rovings instead of traditional woven ones helps minimize the problem of “print-through” — a common cosmetic flaw in which the texture of the underlying reinforcements becomes visible on the surface of the gelcoat due to resin-shrinkage during cure.

Moving up the evolutionary ladder of fiber composite boatbuilding triggers what engineers sometimes call a positive design spiral. In essence, a lighter boat not only becomes easier to propel, it also sustains lower stresses while underway. This frequently means that even more weight can be pared off (the cycle peters out as the gains become cost-prohibitive).

For now, at least, the most cost-effective improvements in fiber-composite boatbuilding result from the shift from woven to unidirectional reinforcements, and from single-skin to cored construction. Switching from fiberglass to aramid or carbon reinforcements is generally more expensive relative to the improvements achieved. The same applies to changing from solid balsa or foam coring to the aramid (i.e., Nomex) or aluminum honeycomb products, although Nida-Core, an extruded polypropylene honeycomb, may be an exception to this rule.

Upgrading the laminating resin may be worthwhile, particularly if the change can open the door to a superior fabricating technique such as resin-infusion molding (discussed below) — a process that requires exceptionally long gelation times. Polyester resins remain the norm in production boatbuilding, primarily for cost reasons. However, moving up to vinylesters can substantially improve the strength and fatigue-resistance of a glass laminate, primarily because these more elastic resins can stretch as much as glass fibers, while polyesters are comparatively brittle. Vinylesters also offer superior resistance to osmotic blistering, so budget-conscious production builders sometimes use them for the “skinning layer” just beneath the gelcoat, before switching over to a general-purpose polyester for the bulk of the hull laminate.

Epoxies are premium resins due to their superior adhesive and mechanical properties. Naturally, they are also by far the most expensive. They are normally used in conjunction with vacuum-bagging techniques that consolidate the fibers, minimize resin consumption, and boost fiber-to-resin ratios into the 65/35 range.

Some epoxies gain extra strength and chemical resistance when post-cured at substantially elevated temperatures. For building large parts such as boat hulls, this can be a daunting undertaking, because it means constructing mammoth ovens equipped with precise temperature controls. Oven curing is an absolute necessity for boatbuilding with epoxy pre-pregs — premium reinforcing materials that come from the factory already impregnated with just enough high-grade resin to coat the fibers. The epoxies used for pre-pregs must be formulated to cure only very gradually at room temperatures, because it often takes hours to put down all the laminations for a large, complex part like a boat hull. Moreover, these materials must be shipped and stored frozen to prevent premature curing. On the plus side, pre-pregs are generally easier to work with than wet-pregs, especially with honeycomb cores. Fiber-to-resin ratios as high as 70/30 can sometimes be achieved.

By and large, these aerospace-style technologies have been mainly used for cost-is-no-object Gran Prix race boats. Nevertheless, a few mainstream builders have lately begun to explore the possibilities. For example, Fairport Yachts of Ohio has recently introduced their CandC 99, a true racer/cruiser featuring heat-cured epoxy and bi-axial E-glass laminates over foam and balsa coring. This 32-footer carries a comprehensive voyaging interior, yet displaces only 9,265 lbs.

Beyond weight savings, going high tech may bring additional benefits that will help to tie up the total package. For example, although even fully-cored yachts need an internal framework of bulkheads, ring frames, stringers and so forth, a substantially stiffer hull shell may make it feasible to down-size and reposition these internal elements, thus freeing up valuable interior space. In small, light-displacement boats, this can be critical, because reduced displacement obviously means less volume below the waterline. It is ironic that modern, weight-saving construction exacerbates the yacht designer’s age-old problem of providing interior headroom while retaining an attractively low, sleek profile. Vacuum bagging and resin infusion

Although often regarded as high tech, vacuum bagging is really a very straightforward procedure. Basically, a tough plastic sheet is arranged over the top of a freshly prepared laminate and sealed around the perimeter of an airtight mold. A vacuum pump then evacuates the space between the bag and the part, causing the atmospheric pressure to squeeze the laminate tightly against the mold surface. In theory, a 100 percent vacuum produces a clamping pressure of about 15 psi, but half this is entirely adequate for most applications. The vacuum is maintained until the resin has cured.

In boatbuilding, vacuum bagging is a huge help in achieving reliable, uniform core bonding. It can also improve the overall quality of laminations by compacting the fibers, eliminating voids and extracting excess liquid resin. True, there are added shop costs involved with vacuum equipment and throwaway materials such as bag materials, sealant tapes and bleeder fabrics. Nevertheless, vacuum bagging is gradually spreading through the industry as more and more builders become familiar with the procedures and benefits.

Resin-infusion molding is a derivative of vacuum bagging. First, dry reinforcements and core materials are carefully positioned within the mold and temporarily secured as necessary. Second, everything is bagged up and the vacuum applied. Only at this point is resin introduced — literally sucked into the mold through a series of small tubes controlled by valves. These valves are opened in sequence so a “resin front” progresses slowly and uniformly from one end of the mold to the other. Following an appropriate cure cycle, the vacuum is released and the part is de-molded as usual.

Done right, resin infusion produces virtually void-free laminates with excellent fiber-to-resin ratios. It also eliminates the problem of noxious styrene emissions — a major issue for many U.S. boatbuilders who still use polyester or vinylester resins in conventional open molds. With resin infusion, nearly all the styrene monomer remains under wraps during the cure cycle, so it mostly ends up chemically bound in the finished polymer where it belongs. Certain major builders such as Tillotson-Pearson in Rhode Island have now switched their entire production over to resin infusion, in part for quality reasons, but also to keep up with strenuous air quality standards. Open molding a sunset technology?

For those builders who, at least for now, have elected to stick with open molding, the future is a bit cloudy. As a rule, the Environmental Protection Agency and Occupational Safety and Health Administration favor the maximum achievable control technology. Both take a dim view if firms choose to keep on doing things the old way when safer alternatives exist.

To help alleviate the regulatory pressure, suppliers have formulated low-VOC polyester and vinylester resins. Unfortunately, they are sometimes not on par with their predecessors. Epoxies are, by nature, low-emission resins, and more boatbuilders may eventually go this route to meet air-quality standards while upgrading laminate performance.

Resin infusion is undeniably promising, but many boatbuilders are hesitant to take the leap because they envision a steep learning curve and substantial financial risk. If everything goes smoothly the results are outstanding; but a vacuum failure, premature resin gelation or various other problems have occasionally resulted in the total loss of hulls and decks worth tens of thousands of dollars.

The potential for large-scale disasters increases further when boatbuilders venture into the rarified world of oven-cured materials such as pre-pregs. Nevertheless, there is little doubt the boatbuilding industry as a whole will continue to move in the direction of more sophisticated composites and fabricating methods. Indeed, superior technology is likely the key to developing new boats that can continue to sell against an ever-growing armada of good used ones. Because after all, as the saying goes, “fiberglass is forever.”

Contributing editor Sven Donaldson is a marine technical writer and former sailmaker based in Vancouver.

By Ocean Navigator