Guide to boat fiberglass

Fiberglass mat

As we consider buying a larger and more comfortable boat for long voyages, I find it interesting to examine the different qualities of fiberglass and its safety at sea. For fiberglass is not just fiberglass, it can consist of different components, is built up in different ways, and both price and quality are parameters we must relate to.

My background for writing this article is not as a professional. I am an architect and experienced sailor, but very curious and do my best when I research. If you have something to add, you are very welcome to write to me at or leave a comment at the bottom of the page.

We want the boat to have the greatest possible strength in the form of both compressive and tensile strength, lightweight and stiff. The boat will also last for many years. Therefore, it is important what it is built of and how it is built.

What is fiberglass?

What we call fiberglass is basically fiber-reinforced plastic. It can consist of different types of plastic resin, and the reinforcement can be made with different types of (free) fibers or different types of mats.

The advantages of fiberglass are, among other things, that it is cheap, light, durable, has great tensile strength, and can be shaped indefinitely.

Different types of fibers for fiberglass mat

The fibers in fiberglass do not necessarily have to be made of glass, but they often are. It can also be, for example, coal, logs, or even paper or wood fibers, although the latter is rare.

Different types of mats for fiberglass

There are many types of mats that have quite different qualities: Eg fiberglass cloth and mats, carbon fiber, kevlar, woven fiberglass cloth, biaxial, triaxial, quadrixal, unidirectional, and CSM / chopped mats.

The last CSM, or chopped mat, is the most used. It is cheap, and the fibers lie helter-skelter. It is easy to work with, and you can easily get the binder to fill it.
The woven mats, however, are far stronger but harder to work with and more expensive. At the most expensive end is the carbon fiber and log mat. Carbon fiber is largely used only for masts and for very ambitious racing boats, precisely because of the price.

All types of mats add more strength to the fiberglass than the free fibers, but some are stronger than others, weigh less, and so on.

Different types of resin in fiberglass

The binder (the liquid part) in fiberglass is typically either polyester or epoxy. Other types of glue may also be used, and it can be problematic if there are water-soluble ingredients in it. So it is good to know which plastic the boat is molded from before buying it.

The difference between epoxy and polyester resin

Basically, epoxy resin is stronger and more resistant than polyester resin, but polyester resin is cheaper. Older boats are always made of polyester because epoxy is a newer product.
Fiberglass made with polyester resin gets osmosis more easily than ones made with epoxy resin and polyester resin is more difficult to work with.

What type of resin to choose when repairing fiberglass

When repairing a fiberglass boat, it is important to know what it is made of. If you want to repair it with polyester resin, the boat MUST be made of polyester resin in the first place. Polyester adheres poorly to epoxy even though it may look fine and hardened. It is therefore always recommended to use epoxy resin on top of existing fiberglass.

Different casting methods for fiberglass boats

Hand-laid fiberglass

In hand-laid fiberglass work, the boat is cast directly into a mold by manual work. You can read more in detail about how to do this at

The disadvantage of this method is that there is a risk of moisture coming from the air into the fiberglass, so it must be done in a protected atmosphere with temperature control. If there is moisture in the construction, it will more easily delaminate and develop osmosis, which is seen in the form of vinegar-smelling blisters.

Vacuum-infused fiberglass molding

With this method, a vacuum is used to get the plastic completely into the fibrous material and mold the shape of the hull. As it is completely airtight, the risk of moisture in the construction is minimal.

Injection molded fiberglass

Injection-molded fiberglass by blowing mixed fiberglass into a mold. The fibers are therefore added in the form of free fibers, and the construction does not become as strong as if it had been constructed with a mat. However, I think you can only get injection-molded small boats, such as kayaks and dinghies.

What is a sandwich construction?

A sandwich construction is a construction of several materials, which together have greater strength than the individual materials alone have. Sandwich constructions in boats consist of several layers of fiberglass and a core of eg balsa wood or a form of foam. When you talk about a hull being built in a sandwich construction, it’s a bit like saying: “I have a cake”. We do not know what kind of cake it is and what ingredients it is baked from.

Foam for sandwich construction

The foam in sandwich constructions consists, for example, of the plastic types Divinycell (PET), PVC, or polyurethane (PU or PUV).

Advantages and disadvantages of divinycell

PET plastic (Polyethylene Terephthalate) and is a type of polyester often simply called Divinycell. It is easy to work with, is very strong, and has good properties against fire, smoke, and poison development. It can withstand high temperatures and can be recycled. It can not be degraded by nature which is an advantage while in the construction of the boat, but of course a disadvantage in the long run.
Divinycell is an expensive material.

Advantages and disadvantages of balsa wood in boats

Balsa is a fast-growing tree that contains up to five times its own weight in water. It must therefore be dried for a long time to be able to be used as a building material. Dry balsa wood is so fragile that it can be broken by hand and burns easily. Used in a sandwich construction, the wood will absorb the wet component. The very big disadvantage of using balsa wood for boats is that organic material rots in a humid environment. If a hole is cut for a hull penetration, for example, or if there are cavities due to poor execution of the construction (where condensation can occur, for example), there is a risk of fungus and rot. We know that wooden boats must be maintained, but with balsa wood encapsulated in the construction, we do not have this option.
Balsa wood adds strength and rigidity to the construction and is found, for example, in catamarans because it is light and cheap. If the wood rots, it is, therefore, a big problem for safety.

Control of balsa wood in sandwich constructions

If you need to check the balsa wood in a construction, it may be necessary to cut a hole in the hull and measure with a moisture meter.
You can also hit the fiberglass with a small hammer. The sound must be fixed. A soft sip is a sign of moisture.
Other methods of determining whether the balsa wood may have begun to rot are to notice whether the material is soft – for example, whether the covered springs or that the plugs in a possible teak deck work up.

So what should we buy?

That is the big question and not so straightforward to answer. If you have a budget of several million, it is easier to be picky.
Buying the most powerful boat can be a daunting task. Nice security, but maybe not necessary at all. Even if you have been vaccinated, you can also get other diseases, so the safety probably lies most, or just as much inexperience, abilities, and knowledge.

I have met many sailors who are not so happy about newer boats because there is not as much hull thickness in them as there is in the older models. This is probably due to the fact that the material in the new boats has been optimized, but if you have that feeling, you have to follow it. I probably belong to that type.

Many boats are sailing around the world, with balsa wood and chopped mat, and they are still produced, and in most cases, it works flawlessly.
I often return to that the cheap boats to a greater extent than the expensive ones come out sailing. Their owners have time for them and may even enjoy making them ready to sail.
You can also save up for so long that you never start sailing the world.

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