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Boatbuilding for the rest of us – Part II

A basic understanding

You don’t have to be a tech expert, but a basic understanding of building methods can help make you a smarter buyer

Editor’s note: This is the second of a two-part series on boatbuilding. This package examines the range of methods and materials used in a variety of boats, including larger vessels and custom boats, as well as high-tech construction. (Last month’s story focused on some of the new processes and materials used to build boats up to around 35 feet.)

How much do you really know about how your boat was built and what materials were used? If your answer is “fiberglass,” you may not know quite enough.

Possessing a basic understanding of boatbuilding methods and materials will help you better evaluate and understand what to expect in terms of a boat’s performance, longevity, strength, maintenance and repair, as well as resale value. Don’t worry — a degree in marine engineering or naval architecture

isn’t required to be an educated consumer. The purpose of this article is help you become a more savvy boat owner or buyer by providing an overview of current building materials and techniques. We’ll help you sort through some of the jargon and acronyms you’re likely to run into. Can you describe SCRIMP in 30 words or less? Do you know what a pre-preg is?

You want to get to the point where you’re able to ask at least a few insightful questions of the dealer or builder regarding the specifics of how your boat was put together, with what materials and why. The goal is to cut through the marketing hyperbole and get substantive answers.

If the builder is touting the addition of carbon fiber to the laminate, find out why it is necessary and exactly where it is being used. Knowing which resins have been used and if they are properly matched to the reinforcing fabrics can be a significant factor in the strength and integrity of the hull. If the hull is cored rather than solid fiberglass, which areas are cored and what does the sandwich consist of? Various core materials can provide either stiffness or flexibility to the finished laminate; it pays to know which material is being used. And how is the core bonded to the outer skin? This is critical, because a failure here could spell delamination.

Most of what I will discuss here is out of sight and, therefore, too often out of mind. After all, it’s not as if you can actually “see” or inspect the bond between the core and the inner and outer skins, for example, although you can certainly learn how to check for delamination in a used boat. So to the degree that we don’t know or understand how the boat is built, we take a leap of faith regarding its construction when we choose boat and builder. We make our choices based on everything from price and reputation to size, styling and accommodations. Doing a little homework on how that boat you’re interested in is put together could help you make a more informed decision. If nothing else, you may wind up better appreciating why two boats of similar size and power are priced so differently.

High-tech materials and processes don’t come cheap. If you want a boat built with leading edge technology, you’re going to have to pay for it. Period. So what are some of the advantages? Yacht designer Doug Zurn says that when high-tech materials are used correctly — and that is the principal caveat in all of these discussions — they will produce a boat that is stronger, lighter, more seakindly and more fuel efficient.

“The boats are more consistent in weight as well as glass-to-resin ratios; they provide a higher level of quality and an overall better product,” says Zurn, head designer at Zurn Yacht Design in Marblehead, Mass. “The consumer has to put in more upfront, but the boats are kinder to the environment and provide better resale value.” And, he notes, “Better-quality hulls offer additional protection from impact.” Obviously, there’s a safety factor in a stronger, tougher hull, as well.

A word of caution: Be wary of overhyped materials and techniques. Some builders promote their use of carbon fiber mainly for its high-tech sizzle, without fully understanding its limitations or even the correct way to use it in building a boat, says J.B. Turner, managing partner of Lyman-Morse Boatbuilding in Thomaston, Maine (see accompanying story). The same holds true with Kevlar. In Turner’s opinion, Kevlar has a limited application in the marine environment.

There is, of course, no right way to build a boat or one best material to use. That’s one of the beauties — and challenges — of building and buying boats. “Good boats can be traditionally built with plank/frame construction, epoxy composites, S-glass composite, aluminum, steel or ferro-cement, as long as all involved parties understand the requirements and processes involved,” says naval architect and designer David Gerr, director of the Westlawn Institute of Marine Technology. “There is nothing wrong with, for example, an old Grand Banks, which is a top-quality boat built in the traditional manner.”

A Mainship trawler is another example of a traditionally built fiberglass boat — hand layup, E-glass, polyester resin with a vinylester skin coat, solid glass laminate below the waterline and at the bow overlap, balsa core in the hull sides — that is successful because it doesn’t get overly fancy just for fancy’s sake. It’s hard to justify the cost of exotic materials in boats that average 12 to 15 knots (top speed 27 knots), says Jim Krueger, vice president of operations for Mainship Trawlers. And adding a small amount of carbon fiber just for the sake of marketing is meaningless, he notes.

That’s not to say the company isn’t using technology to improve its boats and keep costs in check. Computer cutting programs, for instance, enable the builder to control exactly how much glass goes into each boat, improving laminate consistency and reducing waste, Krueger says. And other computer programs that show the stresses that every section of hull could experience allows for more precisely engineered layups. Consistent quality without overbuilding the various parts is the goal. The company has been vacuum-bagging some parts for years and has just started using resin infusion on smaller ones. Expect more of that in the future.

That’s just one example. If you’re in the market for a boat, spend the extra time and effort to really understand how the boat was put together. One way or another it will pay off. Whether you’re looking for a production boat or a custom or semicustom hull, choose a quality builder, one who has really mastered the materials and construction processes.

What follows is a look at some of the materials and techniques used in boatbuilding today, from traditional hand-laid fiberglass, carbon fiber, Kevlar and other fabrics to cores, resins and various processes, including SCRIMP, resin infusion and vacuum bagging.


By far the most common boatbuilding material is fiberglass, also known as FRP (fiber reinforced plastic) or GRP (glass reinforced plastic). It consists of a matrix of fiberglass reinforcements and hardened resin. The fiberglass reinforcements are made from extremely fine fibers of glass. Molten glass is extruded into filaments between 5 and 25 microns thick, which are then coated with a sizing that protects against abrasion and helps bond the fibers when they’re wetted out with resin. There are basically two variants of fiberglass.

E-glass — Originally developed for use as stand-off insulators for electrical wiring, E-glass is the most commonly used fiberglass material in boatbuilding and probably offers both builder and buyer the best bang for the buck. It is relatively low in cost, easily produced, and has acceptable resistance to heat and chemicals. It has a low modulus (poor elasticity) when compared to other types of glass fiber. As the fibers are being produced, they are normally treated with sizing and coupling agents that reduce the effects of fiber-on-fiber abrasion, which can significantly degrade the mechanical strength of the individual fibers.

S-Glass — Considered a high-tech material, S-glass was developed for the aircraft industry and is rarely used in boatbuilding. It is identical in chemical composition to E-glass, but the individual fibers are spun finer and are of higher purity with fewer defects, which dramatically improves the strength of individual fibers and the resulting laminate. S-glass is considerably more expensive than E-glass, but its tensile strength is as much as 30 percent higher.


Kevlar is DuPont’s trade name for an aramid fiber spun from a liquid chemical blend. Although available in different grades, Kevlar 49 is commonly used as a reinforcement in FRP composites. It exhibits good resistance to impact, with the lower modulus grades being used in ballistic applications.

Designer Zurn believes the driving force behind the use of Kevlar in boatbuilding is its strength and weight. “As it is resistant to punctures, it is used in hull bottoms and topsides,” he says. Roughly twice as strong as E-glass, Kevlar can provide substantial weight savings without compromising the strength of the composite.

Naval architect Steve Burke, the principal of Burke Design in Bristol, R.I., adds that Kevlar is an excellent material when used in tension, but has no tolerance for being used under compression.

Carbon fiber

Like S-glass, carbon fiber originated in the aircraft industry, with its need for a high-strength, stiff, lightweight material. Controlled variations in the manufacturing process of carbon fiber produces two basic types. High-strength fibers are produced at 2,600 degrees C, while high-modulus (stiff) fibers are produced at 3,000 degrees C. Carbon fiber is extremely costly and, therefore, typically is reserved for use in high-stress areas and for parts that require high strength and low weight.

“Carbon fiber is both extremely stiff and strong in both tension and compression while being exceptionally lightweight,” says Boston BoatWorks founding partner Mark Lindsay, who has done extensive building with carbon fiber. “Sailboat masts need all of those attributes, and carbon is the best material available for that use. For extremely lightweight racing hulls it is also the best material. However, most boats have enough weight in equipment and accommodations that the extra cost of carbon fiber isn’t justifiable. Using a small amount of carbon fiber can be a problem, since it is so stiff that it takes the entire load first. If there isn’t enough of it, it breaks.”

Because carbon fiber is so stiff, when it fails, it fails dramatically. Carbon fiber can be used in place of E-glass and can have from three to 10 times the strength and stiffness, creating much stronger laminates.


Reinforcing fabrics are the structural backbone of fiberglass boats and are selected based on the stresses anticipated. The direction of the fiberglass strands and their orientation in the boat is very important. Once selected, they need to be in a form that can effectively be used by the builder.

Chop, chopped strand mat

Also referred to simply as mat, chopped strand mat comprises glass fibers cut into short pieces called chop — typically 1/2 to 2 inches long — and randomly clumped together with a binder called seizing, which results in a mat-like material. When CSM comes in contact with the styrene in polyester and vinylester resins, the seizing dissolves. (CSM also is available as “stitched” mat, which contains no binders and, as such, can be used with epoxy.)

CSM is soft, thick, bulky and somewhat sponge-like when wet with resin. Since the pieces of chop are relatively short, they produce low-strength reinforcements. CSM is good for bonding to layers of other types of glass fibers and cores, creating a reinforced filler cushion between uneven layers of woven roving (see below). When used alone or as filler material, CSM typically results in a layup of uniform thickness, but due to the fibers’ random direction and short lengths, it isn’t particularly strong when used alone.

Fiberglass chop that’s not formed into a mat is applied with a chopper gun, which further chops the strands and mixes them with catalyzed resin as they are sprayed under pressure (see below). As the mixture hits the mold, it does so in a random pattern of varying thicknesses. Many boatbuilders apply a layer of CSM or chop immediately after the gelcoat to block print-through of the woven roving. By increasing the rigidity of the gelcoat that has been applied to the mold, CSM also serves to prevent the prerelease of the composite from the mold.

Woven roving

A commonly used reinforcement, woven roving is a heavy, coarse fabric woven from bundles of glass fibers that have been flattened. Because the long bundles are woven and run at right angles to each other, woven roving forms a strong reinforcement and provides good impact resistance. Its strength in the direction of the fibers, however, is reduced compared to knitted fabrics. An all-roving laminate, properly laid up, can be nearly twice as strong for the same weight as a standard combined roving/mat laminate.

The woven fabric’s higher profile — where the bundles cross — means that more resin must be used to fill the voids between layers of reinforcement, thus creating a more resin-rich and brittle laminate. Without the soft, spongy chopped strand mat laid up between the woven roving layers, it’s difficult to make the comparatively hard, flat plies of roving adhere to each other reliably. It takes great skill to produce a high-quality, all-roving layup, and few builders go to this extra expense.

Knitted reinforcements

To circumvent the shortcomings of woven roving, knitted reinforcements were developed in the mid-1970s. Also referred to as unidirectional or stitched fabrics, knitted reinforcements have fiber strands that are dimensionally similar to those used in woven roving but lie flat and run in a single orientation, held together with a light scrim or stitching. This unidirectional orientation allows the fiberglass strands to be fully loaded in tension because they aren’t crimped like the interlocked fibers in woven roving. Unidirectional reinforcements typically are used in the tops of hull stringers and for keel and stem strengthening.

To provide the strength required in varying orientations, fabric suppliers can provide biaxial and triaxial reinforcements, where two or three layers of bundled strands are stitched together in either 90- or 45-degree orientations to each other. These can be supplied with fiberglass mat prebonded to one side. Having the fabrics prestitched in this orientation can save the builder production time in laying up the hull.

Fiberglass cloth

Fiberglass cloth resembles woven roving but with a very fine weave and low profile. It is often used as a surface layer to smooth out the roughness of mat and woven roving. Due to its high cost, it is used almost exclusively in small boats and for finish work.


Resin is the agent that holds the laminate together, bonding the reinforcements, filling voids between strands and fibers, and creating a solid composite that allows the reinforcement fibers to absorb and distribute loads. Resin locks the fiberglass in a matrix, so the closer it matches the physical properties of the glass, in addition to how well it adheres to other, cured fiberglass, the better the end product.

Orthophthalic polyester

Orthophthalic polyester resins are among the most commonly used in boatbuilding. They are the plain vanilla resins that have been around for a long time, are relatively cheap, and are easy to use. They tend to be brittle, however, cracking under high load rather than stretching, giving and then returning to original shape. Ortho-polyester elongates about 2 percent of its length before cracking, with a tensile strength of 9,400 psi. When used with thinner, high-modulus fibers and modern fabrics that flex, the resin cracks before the fibers develop their full usable strength.

Orthophthalic polyester resins also offer poor water resistance because of the microscopic voids left when the styrene it contains is released into the atmosphere as it used. So laminates made with this resin are the least solid and, as such, least resistant to osmosis. However, this problem can be addressed by proper quality control during the building process.

Isophthalic polyester

Isophthalic polyester resins generally have higher mechanical properties than ortho-polyester. Isophthalic resins have somewhat better elongation and much better resistance to chemical attack, blistering or degradation by oil or pollutants. They adhere better to previously cured fiberglass, creating superior secondary bonds. Isophthalic resins have better chemical resistance than ortho-polyester resins and are more resistant to osmosis. In fact, iso-resins are sometimes used as a barrier under the gelcoat to prevent osmotic blistering.

Isophthalic resins elongate 2.5 percent before cracking and have roughly the same tensile strength of ortho resins.


Vinylester resins are far superior to polyester, with excellent fatigue and impact resistance. On average, vinylester resins elongate 5 percent before failure, with a tensile strength of 11,800 psi, allowing the fabric reinforcement to absorb maximum loading and increasing its impact resistance and ultimate strength. Vinylester resins adhere well to cured fiberglass and are good for secondary bonds.

Vinylester is more solid and provides superior osmosis resistance, which protects against osmotic blistering. Most of the excess styrene in vinylester resin cross links with itself during cure, so it doesn’t flash off and leave those microscopic holes in the layup. Some production builders use a vinylester skin coat to resist blistering. Vinylester resins are more resistant to chemical attack and blistering than any of the polyesters.

Also, vinylester has a high peel strength, which means that the bonds between layers of reinforcing fabrics are stronger and resist peeling apart.


Epoxy is the best resin used in boatbuilding and by far the most expensive. Although there are various epoxies available, they are all tough, strong and elongate before failure better than other resins. Epoxy also acts as a strong glue, adhering well to properly prepared, previously cured fiberglass surfaces. In fact, it is often recommended for making repairs to damaged polyester or vinylester components. It is stronger and has higher elongation than vinylester, with a tensile strength of 12,500 psi.

Because of its gap-filling and elongation properties, epoxy has the highest peel strength of any resin system. Epoxy layups can be made without mat, which allows the highest fiber content possible and produces the highest mechanical properties. A carefully vacuum-bagged biaxial S-glass epoxy laminate can have a flexural strength of 85,000 pounds. Epoxy contains no styrene, so it is the most blister-resistant of all resins.

Due to the combined cost of materials, labor and design, epoxy laminates are expensive and are usually best suited for cutting-edge raceboats.


DCPD (poly dicyclopentadiene) resin blends are used by boatbuilders because of their lower styrene levels. DCPD alone is too brittle for boatbuilding, but it can be blended with orthophthalic, isophthalic or vinylester resins to help builders meet EPA emissions levels.

Because of its lower styrene content, and subsequent reduction in shrinkage, DCPD is often used in the first layer of fiberglass to minimize reinforcement print through. DCPD resins can be difficult to repair, as the resin cross links quickly and completely, impeding the chemical linkage necessary for secondary bonds.

Construction methods

The ideal fiberglass layup will keep the resin content to a minimum, since resin alone adds weight without adding strength. Keeping the fiberglass-to-resin ratio consistent is a constant concern for boatbuilders. The more fiber you add in proportion to the binder, up to a certain point, the stronger the composite will be. Advanced fabrics, materials, resins and construction techniques allow builders and designers to produce stronger, stiffer, lighter hulls, though at a higher cost. When building composite boats, the important factors are the strength and compatibility of the components and the weight of the finished laminate. The builder must be qualified and experienced in each of these areas.

Sandwich construction

Fiberglass is strong but not particularly stiff for its weight. To compensate for that, many boats are laid up using sandwich construction. A sandwich structure consists of two high-strength skins separated by a core material such as end-grain balsa, closed-cell foam, plastic honeycomb, impregnated paper honeycomb, plywood or solid wood. This construction method, when properly engineered and fabricated, increases stiffness and thickness without incurring the weight penalty that comes from additional laminate layers or framing. The core acts like the web in an I-beam, where the web provides the lightweight separator between the load-bearing flanges.

There are many ways to build a good boat using core materials, but the materials must be properly matched and carefully constructed. “It cannot be overemphasized how critical careful attention to detail in bonding and installing cores is to a successful hull,” says naval architect Gerr.


End-grain balsa, such as Nida-Core BalsaLite, has been widely used as a core material since the 1960s. With end-grain balsa, the grain runs from fiberglass skin to skin. It is resistant to compression along the direction of the grain and contributes significantly to the stiff nature of sandwich panels built with it.

Balsa’s strength and stiffness make it a good core material for decks and other rigid surfaces, but shear values are reduced significantly where thickness is increased beyond a half-inch. Being a stiff material, impacts are readily transmitted from the outside to the inside skin, which can cause the end grain to split and the inside skin to delaminate without detection.

Balsa is susceptible to rot in the presence of moisture and oxygen. The void that develops between the inner skin and core after an impact can collect condensation and result in water damage to the core.

Due to its low resistance to water vapor and humidity, balsa core is dependent on proper manufacturing and repair techniques. The importance of keeping the balsa dry cannot be overemphasized. Elaborate precautions must be taken before drilling holes through any component cored with balsa.


Compared to balsa, foam cores are lighter and more resistant to rot. However, they are more expensive. Structural foams are available in a range of densities to suit different applications. Molecular cross linking, or the interlocking of molecules, is a major difference between the foams. The greatest shear strength and stiffness are offered by foams with the most cross linking, such as PVC, but they can be brittle. The stiffness of PVC foams does make them a good choice for decks and other rigid surfaces. Divinycell and Klegecell are types of PVC foam.

Linear foams such as Airex offer the least cross linking and, therefore, have the least shear strength. However, they are the most ductile and can yield and distort further, absorbing much more energy before failure. In a collision, a ductile core will absorb more energy than a brittle core, limiting damage to the fiberglass skins, the core itself, and the skin/core bond.

A compromise between these two categories is SAN foam, commonly referred to by the trade name Core-Cell. Its stiffness is close to that of PVC foam, but its ductility is close to that of Airex. Core-Cell tends to stretch as it fails, helping to keep the hull intact after impact.

Boatbuilders often use a combination of different materials for different applications. You can find boats with solid fiberglass bottoms and balsa cored topsides, Airex cored bottoms with Divinycell topsides, and boats made entirely from balsa, Core-Cell and Divinycell sandwich panels. Closed-cell foam cores like Core-Cell will not absorb water like honeycomb and balsa.


Honeycomb cores are extremely light, high-end materials that originated in the aerospace industry. They can be processed into both flat and curved structures, and deliver the lightest possible composite. Honeycomb cores can produce stiff, light laminates, but due to their very small bonding area are almost exclusively used with high-performance resin systems, such as epoxies, so that the necessary adhesion to the laminate skins can be achieved.

Plastic honeycomb, like Nida-Core, has been used in decks, bulkheads, even hulls. Nomex honeycomb is made from Kevlar-based Nomex paper and is increasingly being used in high-performance components because of its high mechanical properties, low density, and long-term stability.

Hand layup

Hand layup is the most widely used and one of the most successful methods of laminating a hull. However, traditional layups can be heavy, and in powerboats that means larger engines and increased fuel consumption.

With hand layups, the inside of a female mold is coated with a mold-release agent or wax, which allows the cured fiberglass laminate to be removed without damage. That’s followed by a layer of sprayed-on gelcoat and a skin coat of chop or hand-laid mat to serve as a print blocker. Then, alternating layers of resin (either sprayed, brushed or rolled) and fiberglass cloth of varying types and weights are laid down until the desired thickness is reached. The resin must be carefully catalyzed so that adequate working time is provided.

Chopper gun layup

With a special applicator gun, short glass fibers are mixed with resin and sprayed onto the surface of the mold, then rolled out by hand. Since the fibers are short, run in random directions and are laid down without precise thickness control, chopper gun layup typically produces a hull that is less dense and not as strong as a hand-laid hull. The technique is very quick and low cost, and produces boats relatively inexpensively. It is well-suited for mass-produced hulls, where weight and performance aren’t critical.

Chopper gun layups must be thicker in order to approach the strength of hand layups, resulting in a heavier hull without achieving the strength and longevity of a good hand layup. Due to their high resin content, chopper gun layups have the lowest mechanical properties.

Vacuum bagging

An enhancement to hand layup, vacuum bagging is a construction technique that uses atmospheric pressure as a clamp to hold adhesive, cores and laminates securely together until the resin cures. The process involves sealing the wet layup in a membrane and applying a vacuum to remove air and excess resin and eliminate voids.

The result is an extremely dense, even layup that is considerably stronger than an open molded hand layup. Done properly, vacuum bagging can create a lightweight hull or component while ensuring proper bonding between the plies and material within the stack.

Unlike mechanical clamping, which applies uneven pressure to concentrated areas and can damage fragile core materials, vacuum bagging applies firm, uniform clamping forces throughout the assembled laminate. This enables the use of a wide range and combination of laminating materials in addition to providing a superior bond, which results in thinner, more consistent glue lines with fewer voids between the layers.

Since vacuum bagging removes excess resin, the process can create laminates with a glass-to-resin ratio of 60-to-40, decreasing weight and improving the laminate properties over open-mold hand layup. Vacuum bagging also captures the volatile organic compounds released when the resin cures.

Vacuum bagging, however, holds potential pitfalls for the uninitiated builder. When done poorly, it can add cost, create complexity and reduce productivity. Attention must be paid to the many variables that are involved with the process. Although vacuum bagging isn’t an inherently difficult process, builders unfamiliar with its intricacies can run into difficulties that can filter down to the end user — you, the boater.

For example, the choice of resin is critical. It needs to cure slowly enough to get all the plies down prior to cure, yet not so slow as to allow the resin to bleed back out. And there needs to be enough tack to hold all the components in place. The bag or envelope must be fully collapsed and providing pressure before the resin begins to gel.

Many resins have a pot life of about 30 minutes, though some offer work times of up to two hours. Even so, that time limit is extremely critical in vacuum bagging. Large projects can easily approach the two-hour mark, and even small, seemingly simple projects can quickly turn frantic when a pesky leak in the vacuum seal cannot be found. Also, depending on when the bag is applied, the amount of resin removed can vary from part to part.

Equipment problems, such as the vacuum pump or suction line causing loss of pressure, can create voids or a dry laminate. Fillers need to be made to keep the bag from “bridging” inside corners and other areas. If not properly engineered, completed parts may not be stiff enough, due to the thinner cross section of the vacuum bagged part.

Resin infusion

Resin infusion is a process that uses a vacuum to draw resin into a dry laminate. While vacuum bagging certainly improves on hand layup, there are still issues involved in working with a wet reinforcement fabric — for example, the laminate begins in an oversaturated state.

With resin infusion, also referred to as VIP (vacuum infusion process), materials are laid into a mold dry, and the mold is sealed. Vacuum is applied, and the laminates are compacted before resin is introduced. Once a complete vacuum is achieved, resin is drawn into the laminate through carefully placed tubing. As a result, only the required amount of resin is used. This helps reduce weight and increase the finished product’s strength by bringing the fiber content to as high as 60 percent, which maximizes the properties of both fiber and resin.

Resin infusion provides another valuable benefit: unlimited setup time. Because the vacuum is applied while reinforcements are dry, there is no resin clock to work against. After the bag is applied, leaks can patiently be sought out. If something is sitting improperly, simply release the vacuum and readjust. No time constraints are introduced until the resin is infused.


SCRIMP (Seemann Composites Resin Infusion Molding Process) is a patented resin transfer process that is similar to resin infusion. There are several variants of SCRIMP, each with its own patent. Like resin infusion, SCRIMP involves laying up the fiber reinforcements, core materials and various inserts in the mold while dry. However, a patented flow medium, or “shade cloth,” is then laid over the laminate, followed by the vacuum bag, which is sealed to the tool or mold. (The shade cloth is a plastic fiber mesh that is removed from the laminate after cure.) The part is placed under vacuum, and the resin is introduced through an inlet port and distributed through the laminate through the shade cloth and a series of channels, saturating the part.

SCRIMP compacts the dry fibers, allowing parts made with this process to have high fiber volumes, typically about 60 percent by weight. Other benefits include improved part-to-part consistency — for a more precise fit of bulkheads, stringers and other structural components — nearly zero emissions in the workplace and the environment, and a more reliable structure.

“You don’t typically see bond failures, as there are no secondary bonds to deal with,” says Eric Sorensen, author of “Sorensen’s Guide to Powerboats” and a Soundings contributing writer. “You don’t see tabbing failures that can occur with open molding, often in the landings and stringers.”


Today’s high-end builders use machines called resin impregnators that provide precise control of the glass-to-resin ratio and the speed that wetted out fiberglass, or wet-preg, is produced. Using a series of rollers, an impregnator passes the dry reinforcement material through a pool of catalyzed resin, compresses the fibers and squeezes out excess resin.

“Wet-preg gives us the same resin content as the very high-tech aerospace approach of using premanufactured pre-pregs that are stored in a frozen state and cured at high temperatures under vacuum,” says Boston BoatWorks’ Lindsay. “The major difference is that our cost is much lower, making a very technical process available to the average consumer. Wet-preg makes much lighter parts than SCRIMP or resin infusion, and makes it easy for us to use much higher performance epoxy resins as opposed to vinylester.”


Preimpregnated reinforcements, or pre-pregs,are delivered to the boatbuilder wetted out in partially cured epoxy resin and ready to be laid up. They are more costly than traditional reinforcement materials, must be stored in a freezer until used, and must be postcured by putting the hull or part in an oven. Only a few custom builders use this method, but the advantages include consistent glass-to-resin ratio, effective bondingto honeycomb cores, and a strong, lightweight structure, The materialis easy to handle and, since it uses epoxy, doesn’t emit styrene.