Female sailboat mold

In my last missive on this subject I introduced the concept of building fiberglass boats in female molds, just like the one pictured here. Now we need to talk about the business of building up a glass laminate within a mold in more detail. To understand fiberglass lamination, it is best to focus first on simple solid laminates in which multiple layers of fiberglass fabric are built up to the thickness necessary to make a part strong enough to do its job.

Solid hulls were the rule in the early days of fiberglass boatbuilding and many are still found in both older boats and new boats. There is a popular myth that early glass hulls were built as thick as wood hulls because builders didn’t know how strong glass was and wanted to play it safe. This is not true, and you’ll often find solid laminates in older boats are a bit thinner than their owners like to believe. Still, early solid hulls were built very robustly and many show little sign of deterioration even 40 or 50 years after they were first created.


Types of fiberglass fabric

As I mentioned last time, the major ingredients in any solid laminate are resin and fiberglass fabric. Various types of fabric have different properties and different types are usually used together in the same laminate. The crudest fabric is chopped-strand mat, or mat, which consists of fibers chopped into strands up to 2 inches long that are laid down in a random pattern and pressed into a spongy, felt-like material. The fibers are held together by a light binding adhesive, usually a polyester powder or polyvinyl acetate emulsion, that dissolves when exposed to resin.

Fiberglass fabric, chopped mat
Chopped mat, the crudest fiberglass fabric

Mat is easy to work with because it wets out quickly and is bulky. This makes it possible to build up thickness in a laminate with minimal effort. Mat also bonds well with other layers in a laminate, particularly other layers of mat that are still wet with resin, as the fibers from each layer can then intermesh with each other. Because it becomes quite malleable after its binding agent dissolves, mat is particularly good for working into crevices and corners in a mold. Because the fibers in mat are randomly oriented it is equally strong in all directions, but because the fibers are also very short, it is not as strong as fabrics with longer continuous fibers.

Chopped fiber can also be laid down in (or on) a mold with a chopper gun. This device cuts continuous bundles of fiber called rovings into short strands and spits them out into the air while simultaneously expectorating streams of resin and catalyst. Pull a trigger and a gooey mass of catalyzed resin mixed with chopped fiber spews forth from the gun. Though messy to work with, chopper guns make it easy to build up laminate quickly.

Fiberglass chopper gun at work
Once used to build whole hulls, chopper guns are still used by many builders to quickly lay up small parts

In the early days it was common to see entire boats built of mat, and after the advent of the chopper gun in the mid-1960s it became even easier to build hulls of nothing but chopped fiber. Such hulls are perfectly sound if built thick enough to compensate for the short fibers they contain. However, because they are thick and contain a lot of resin, they  are quite heavy and have poor strength-to-weight ratios.

The next coarsest type of fabric is woven roving. Here bundles of fiber–those rovings just mentioned above–are woven together at right angles into a loose, bulky cloth. Though the fibers are crimped by the weave of the cloth and so lose some unidirectional strength, they are long and continuous and thus much stronger than the short fibers found in chopped-strand mat if they are oriented in more-or-less the same direction as the load being imposed on them. If the load path, however, runs at an angle to the fibers, their strength decreases proportionally. Any woven fabric with fibers oriented at 0 and 90 degrees is weakest when resisting loads imposed at a 45 degree angle. In such instances, woven roving is in fact weaker than chopped-strand mat. Because of the thick, bulky weave of the cloth, woven roving is also harder to wet out with resin than is mat. Its knubbly surface, once the resin has set, also bonds poorly with other layers in a laminate.

Fiberglass fabric, woven roving
A bolt of woven roving

One very good way to build up a laminate is to alternate layers of woven roving and mat, as the two fabrics complement each other very well. This was the best practice in the early days of fiberglass boatbuilding and is still viable today. Because they are often used together, there is also a popular composite fabric, known as combi-mat, which consists of a layer of mat pre-stitched to a layer of woven roving. Like mat, woven roving is bulky; it quickly builds thickness in a laminate, but also takes a lot of resin to wet out. A traditional woven-roving/mat laminate, though it has many virtues, is therefore still heavy compared to other more sophisticated laminates.

The finest sort of fiberglass fabric is cloth, in which individual fibers, rather than bundles of fibers, are tightly woven together. A wide range of weights and weave patterns are available, including numerous sophisticated satin weaves and knitted cloths that minimize the crimping of the fibers, thus enhancing their strength. Because it is a finer fabric than mat or woven roving, cloth takes less resin to wet out, which reduces laminate weight and increases strength-to-weight ratios. Because it is woven, however, its strength still varies depending on the angle of the load path imposed upon it.

Fiberglass fabric, cloth
Fiberglass cloth

Fiberglass cloth is both expensive and quite thin, thus is not a cost-effective material for building up bulk in a laminate. Normally it is used in the body of a laminate only in small boats or in race boats and high-end performance cruisers where saving weight is a priority. It is sometimes used as an outer finish layer in laminates in larger general-purpose boats, as it does not “print through” a surface coating of gelcoat as easily as woven roving. Most builders, however, prefer to put chopped-strand mat under gelcoat because it is much cheaper.

Some quality builders not only use cloth under gelcoat, but also to sheath a hull’s inner surface to further improve finish quality and increase overall strength. Cloth also may be used to reinforce heavily loaded areas of a hull and is very commonly used to sheath wooden hulls and/or decks.

Yet another even more sophisticated sort of material is known as unidirectional fabric. In a “uni-di” fabric the glass fibers are laid out parallel to each other in bundles that are lightly stitched together or held in some binding or seizing. Because the fibers are not kinked or bent by weaving and all run in the same direction unidirectional strength is maximized. Because they are packed close together and are neatly aligned, much less resin is needed to wet them out. By carefully aligning uni-di fabric along anticipated load paths, builders can thus greatly increase a laminate’s strength-to-weight ratio. By orienting layers of uni-di at specific opposing angles, multidirectional loads can be supported as efficiently as possible. As with combi-mat, multiple layers of uni-di can be pre-stitched together to create a biaxial fabric (two layers of uni-di oriented in two different directions), or even or a tri- or quadra-axial fabric. Uni-di or biaxial fabric is also often pre-stitched to chopped-strand mat (this is called a “stitch-mat” fabric) as the mat, again, improves the bond between layers in a laminate.

Fiberglass fabric, stitch mat
Stitch mat with a biaxial fabric married to a layer of chopped mat

Like regular fiberglass cloth, these directional fabrics are quite expensive. Mass-production builders thereforeuse them sparingly, if at all, and only in specific high-load areas, such as around frames and stiffeners, chainplates, mast steps and partners, and keel stubs. Those building race boats or high-end performance cruisers are much more likely to use these fabrics to reduce weight and maximize strength.


Exotic Fabrics

Not all fabric used these days to build laminate boats is made of fiberglass. Over the past 15 years both Kevlar and carbon fiber, which are much stiffer and lighter than glass, have appeared in more and more race boats and high-end performance cruisers. Kevlar is extremely impact resistant, thus is often used as a reinforcing material, particularly around the bow, which is most likely to be involved in collisions. It can also, however, be difficult to work with in a laminate because it does not like to bend and is hard to wet out. As far as I know, no one has ever built a boat of any size entirely out of Kevlar. It is common, however, to see large boat hulls reinforced with Twaron, an aramid fiber very similar to Kevlar, or with glass-Twaron hybrid fabrics.

Kevlar fabric
Kevlar fabric

Carbon fiber, meanwhile, has become the most popular material for building the lightest, fastest, most cutting edge race boats. Not only are entire hulls now built of carbon fiber, but also masts, booms, rudders, spinnaker poles, steering wheels, and all manner of small components. Carbon fiber, in a word, is trendy. Its sleek, black finish personifies all that is cool and hip in modern-day yachting, and there is a tendency now to assume that anything must be better if it’s made of carbon fiber.

But carbon does have an Achilles heel. It is very stiff and light, but it is also very brittle, has low impact resistance, and is not resilient. Unlike a fiberglass laminate, which bends and flexes quite a bit before breaking, a carbon-fiber laminate hardly flexes at all when subjected to severe loads. Up to a point this is good, but when it does reach its breaking point, carbon fails suddenly and catastrophically. It also fares poorly in collisions and other sudden point-loading situations.

The fragility of carbon fiber has been amply demonstrated. In the past 10 years or more, three different all-carbon America’s Cup boats have sunk after experiencing critical structural failures. At least two large carbon racing cats have had their bows suddenly break away. And the list goes on.

For a cutting-edge race boat, where small advantages are very important, building in carbon is a no-brainer. For a cruising boat, however, even a serious performance cruiser, it makes little sense. Other sophisticated materials–most notably S-glass (that is, structural grade glass, as opposed to the more commonly used electrical grade glass)–work much better. An S-glass laminate is just 2% heavier than an equivalent carbon laminate and is three times as resilient.

Carbon fiber, like Kevlar and Twaron, is also sometimes used as a reinforcing material within a fiberglass laminate. This makes good sense in theory, because carbon is very good at resisting compressive loads, but it must be done very carefully. Because carbon is so much stiffer than glass, a local carbon reinforcement must be properly engineered and installed or it can actually increase stress under certain loading conditions.

BoaterMouth link: here

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8 Responses
  1. Tom Foley

    Agree. Learned a lot as a beginner. I would like to have seen a little info, though, maybe as a conclusion, that explains what fiberglass type _most_ DIY boatbuilders end up using. In other words, for basic, budget boatbuilding, what is the consensus about which type makes the most sense?

  2. Nate

    Tom, it usually depends on the layup schedule called our by the designer. Older designs use glass for waterproofing and abrasion resistance and rely on the wood stature for carrying the actual loads. Theses usually call for plain glass cloth. At the other end of the spectrum the wood or foam is only there as a core and the fiberglass skin carries the load. Generally those designs use heavier biaxial cloth with multiple layers in higher load areas. Mat is rarely used as it adds a lot of weight and soaks up a ton of resin with little added strength. Carbon is rare due to cost and the higher level of skill needed to ensure good wet out while maintaining the correct reins to fiber ratio.

  3. Ogo

    Thank you for condensing books and years of experience to bring us this synopsis that can be inferred to any fiberglass project of size.

  4. Bart

    “As far as I know, no one has ever built a boat of any size entirely out of Kevlar.”

    I have an Orca 36 whose hull is kevlar with vinyl ester resin. She got tossed around and bounced off other boats in the boatyard during a hurricane and only suffered cosmetic damage.

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