"The construction of this boat is truly state of the art," says the boat salesman to his prospect. "Vinylester resins, uni-directional fibers with triaxials in the stress points, and PVC coring up to the waterline. Carbon fiber caps on the stringers, the bulkheads are honeycombed, and it has Kevlar in all the right spots." He smiles hopefully at the potential new owner.

boat construction

Boat show salesmen know all the right buzz-words, when it comes to basic boat construction. But, do you?


Two revolutions have taken place in the boat-building industry in the past few decades. The first, of course, was the shift from wood to fiberglass construction, but the second is much more subtle, as the bewildered buyer above has just found out. There has been a transition from conventional fiberglass methods to a host of exotic materials and techniques which can befuddle even the experienced yachtsman with buzzwords and jargon.

The concept remains essentially the same, however. Fiberglass Reinforced Plastic is still a fiber material set in a binding substance of resin. In the very early years, fiberglass was literally fibers of glass, but that soon changed to fibers of various synthetic plastics. For many years, fiberglass construction has been from cloth, roving, mat, and resins you can buy in any hardware store. Today, however, a degree in chemical engineering is useful when reading the brochure for a new boat.

Some of the advances came from the efforts of the builders of offshore racing powerboats and ocean racing sailboats to produce lighter, stronger and therefore faster boats, but the real impact came with the energy crisis. Since fiberglass is a petrochemical product, it made sense to boat builders to investigate ways to make boats lighter without any loss of strength, because every pound of fiberglass eliminated saved the builder several dollars. For powerboat builders, a light but strong boat was also more fuel efficient at a time when buyers were worried about gas lines.

speed boats

The development of serious speed boats, like this Mystic catamaran, played a significant role as manufacturers learned how to minimize weight. It was the cost of construction, however, that spurred on this trend among boat-builders of all types.

Much of the research and development actually came from the aircraft and aerospace industries, where strength and light weight have always been a prime concern, and the trickle down of technology has become such a waterfall that many boat owners are unaware of the changes taking place.

Let's look through the fiberglass process and define some of the new language.


There are three types of resins (polyester, vinylester and epoxy) listed below, and each has a place in the boat-building world. The important factor is for the builder to correctly match the resin to the type of reinforcing material being used so that the strengths are matched. For example, a vinylester resin is ideal for S-glass but, when used with E-glass, the reinforcing material will fail before the resin.

Polyester: This is the resin commonly used for boatbuilding today, and most boat owners are familiar with it. It is inexpensive and generally all-purpose. It has poor stretch (elongation) properties so it is not used on modern high-performance boats, but it is perfectly adequate for most boats. The most common polyester is an orthophthalic base, but the newer isophthalic based polyesters are gaining in popularity. The isophthalics are more resistant to water and chemicals, are more abrasion resistant, and have higher impact and fatigue (flex) performance. Most modern gel coat finishes are made with isophthalic resins, and it's likely that isophthalic resins will become the common boatbuilding resin in the future.

Vinylester: An alternate to polyester, vinylesters are claimed to have better stretch characteristics than polyesters, so they more closely match the strengths of the various exotic reinforcements. Vinylester has good water resistance and fatigue properties, but it is also more expensive than polyester resin. One important feature of vinylester is that it has excellent secondary bonding strength, so bulkheads or stringers added to a cured hull will be stronger than on a polyester hull.

Epoxy: This is the high-performance resin, with a matching price tag. Epoxy resins have had a reputation for being hard to work, since early epoxies were thick, but many modern epoxies are quite liquid. Epoxy will adhere better than any other resin to a wide range of materials, which makes it ideal for attaching cores, stringers, or other items.

Fiberglass Cloth

Woven fabrics using continuous strands are the most common cloths, with weights ranging from 4 to 15 ounces per square yard. Heavier weights, usually called roving or woven roving, consist of untwisted yarns of fiber in weights that range up to 48 ounces per yard. The finished roving resembles a coarse burlap and, like all cloths, has good bi-directional strength; ie, in the N-S and E-W directions. The lighter cloth weights can be found in a variety of weave patterns, such as twill, satin and matt, for different purposes.

fiberglass cloth

Two examples of fiberglass cloth; the fiberglass at left is significantly thinner and lighter, but provides less stiffness.

E-Glass: This is the most commonly used fiberglass cloth in boatbuilding today. You can buy E-glass at a marine hardware store, and bond it with polyester resin. It is made from molten plastic spun into fine fibers that are then either woven into cloth or loosely gathered into roving.

S-Glass: This is high performance fiberglass cloth from the aircraft industry. It is three to five times more expensive than E-glass, but it is also much stronger. Developed by Owens-Corning, it has 20 to 40 percent higher tensile, impact and flexural strength than E-glass. There are two types of S-glass: S-1, which meets critical aerospace standards and is blindingly expensive; and S-2, which is used in the marine industry. In Europe, S-glass is called R-glass.

Mat: Mat is usually made of E-glass, and consists of random 2- to 3-inch fibers held in place by a binder that is resin soluble. Mat is used primarily for building thickness (and stiffness) into fiberglass layups. Mat resists "print-through," where the weave of roving shows in the outer layer of the hull, but it also soaks up a tremendous amount of fiberglass and is low in strength for it's weight.

Uni-Directional Fibers: This is one of the advances in reinforcing materials. It consists of strands of fiber running in one direction only, held together by single fibers that are glued or sewn laterally, much the same way that a bamboo fence is held together by a few wires. Obviously it has very high directional strength, so it can be used in areas where the loads are specific. Because it is not woven, there are no kinks and it is easier for workers to "wet out" with resin since it doesn't hold air like a cloth fabric.

Bi-Axial Fibers: Technically, most cloth is bi-axial, but the modern definition means a fabric made from layers of unidirectional cloth that are not woven through each other. One layer simply lays atop the next layer to produce a kink-free band of fiber.

Tri-Axial Fibers: This is a layered material similar to bi-axial cloth, except that the fibers are oriented in three directions, often at 120 degrees to each other to spread the loads.

Non-Traditional Fibers

Carbon Fiber: These fibers of graphite provide superb stiffness as well as high tensile and compression strengths and are often used in conjunction with S-glass or other exotics to provide acceptable impact resistance, which is otherwise quite low. Carbon fiber is very expensive, but it can outperform metal in many situations. Many companies produce carbon fiber worldwide and the usages depend upon the carbon content of the fiber itself, with some being intended for high strength applications and others being aimed for high modulus (stiffness) situations. It is the most expensive type of fiber reinforcement available, costing as much per pound as 100 times common E-glass. Second only to Kevlar in specific strength, carbon fibers are superior to any other fiber in stiffness.

Kevlar: The trademarked name for a DuPont product, it is used to refer to aramid fibers of which Du Pont is the sole producer worldwide. An aromatic polymid (usually shortened to aramid), Kevlar is a form of nylon that was originally developed in the mid l960s as "Fiber B" to reinforce radial tire belting. It's unique properties soon were put into other uses, and the public usually thinks of Kevlar in terms of bullet-proof jackets. There are, in fact, two forms of Kevlar available. Kevlar 29 is used for lines, cables, and flak jackets, while Kevlar 49 is used as a reinforcement fiber in plastic composites. On a strength to weight comparison, Kevlar has the highest specific tensile strength of any commercial fiber. It is five times stronger than steel, and twice as strong as E-glass, which allows a Kevlar hull to maintain the same strength as an E-glass hull at a fraction of the weight. Impact strength is also a Kevlar forte, and it is able to withstand repeated impacts as well as resist the tendency of other reinforcing fibers to allow cracks to spread. The negative side to Kevlar is a marked weakness in compression strength, so it is often used in conjunction with other fibers that balance that trait.

Nomex: A chemical developed by DuPont, Nomex is most famous for it's fireproof qualities, and it is used in fire-resistant suits for firemen and race car drivers. It is an aramid, which is turned into a paper-like substance for use in honeycomb.

Hybrids: These are reinforcing fabrics that combine two or more different types of fiber. One common hybrid is a mixture of Kevlar with carbon fiber. The Kevlar provides high impact resistance, while the carbon fiber supplies the stiffness. Combinations of S-glass, Kevlar and carbon fiber are also available to optimize certain properties at minimum cost.

Core Materials

Core materials are often used to reduce weight and increase stiffness. Some builders core the entire boat; others construct with solid fiberglass from the waterline down and coring above; and still others use some mix of coring and solid glass throughout the boat.


You can see the core (with a checkered appearance) in this part, about to be vacuum-bagged at Sabre Yachts. Photo by Sabre Yachts.

Balsa Core: When first used as a hull stiffener, boat builders laid long planks of balsa into the hulls but this method led to rot and structural failure when water "wicked" through the entire plank. It's taken balsa a long time to live down this beginning, but modern balsa is now the most widely used coring material in boats. The solution came from slicing through the grain, turning it on edge, and producing a checkerboard pattern of end-grain pieces that do not transmit water. The result is a stiff, light and inexpensive core with good impact quality and high compressive strength. An added feature is the insulation quality of balsa against sound, thermal change, and vibration. One negative factor is that balsa can absorb resin, making the hull heavier, but quality workmanship can keep that from happening.

PVC Foams: Airex and Klegecell (pronounced kledge-a-cell) are the most popular commercially produced foam cores that are used today. Both are closed cell foams made from polyvinyl chloride, but each has different characteristics. Airex is a non cross-linked PVC, making it more flexible and resistant to damage. Klegecell is a cross-linked foam that is extremely rigid. Foreign-built boats often use Divinycell, a Scandinavian version of Klegecell.

Honeycomb: Honeycomb is just what it sounds like: a waffle-pattern of material to give the highest stiffness of any core of equal weight. The compression and shear strengths are second to none, which might be expected from a material originally used in aircraft for flooring and bulkheads. Nomex honeycomb is the most commonly found on yachts, although it is definitely a high-ticket extravagance for owners in search of the last ounce of weight savings. Surprisingly enough, some honeycombs are made of paper. A kraft paper is impregnated with resins and then formed into a honeycomb, making it water resistant as well as sturdy, but the paper honeycomb is heavier than Nomex. "Skinned panels" are ready-made sheets of honeycomb resembling a piece of plywood, and are available with teak veneer or various other overlays that can be cut into ready-made honeycomb bulkheads.

Get some insight into how core works inside a fiberglass part by watching our Boating Tips: Understanding Foam Cored Boat Construction video.

Editor's note: This article was updated and expanded in March of 2016.