Fiberglass Yacht Building

The composition of fiberglass.

To get the most out of this article, it will help to have a basic understanding of the materials and methods used in the construction of fiberglass boats, as well as their specific strengths and weaknesses and, most importantly, the visible signs that manifest themselves to the owner, such as stress cracks, de-lamination, osmosis, and so on.

How does it get better?

Styrene is one of the reactive diluents found in polyester resins, along with glycol and organic acid. When the catalyst is introduced, [MEKP typically] a series of reactions begins. Through a series of “cross-linking” reactions, styrene can generate “bridges” that connect all the mixture’s chemicals. Glycol/acid chains gel forms a solid mass as the chain reactions increase.

These “cross-linked” bridges eventually solidify into a plastic mass that firmly anchors the fiberglass fabric (or matrix). Heat is released as the chemicals combine and react (an Exothermic Reaction). Isn’t that incredible?

Typical Building Methods

Many layers of glass fiber fabric impregnated with polyester, vinyl ester, or epoxy resin make up the hull of a fiberglass boat. The fiberglass hull is often constructed in phases, beginning with creating a “female” mold.

The desired hull form is first cast into the “female” mold.

The surface of the mold is treated with a waxy-releasing agent.

Third, a “gel coat” (10-25 mils thick) of polyester resin containing pigment (color) is applied to the mold. This provides the hull’s sleek, colored finish.

The “gel coat” is supported by a thinner fiberglass cloth, and then the primary hull is constructed by adding many layers of thicker material.

Additional glass and resin layers are often applied to stressed parts of the hull, and the entire hull is sealed with a final layer of transparent resin. When the hull is complete, it is removed from the mold, and the remaining interior components, including the roof, decks, bulkheads, and keel, are installed. (There are exceptions! This varies from one constructor to the next.

Parts made of wood

Wooden components are commonly employed to reinforce places such as galleys and so on in fiberglass hulls. Whenever wood is exposed to water, it expands and eventually rots and decays.


Internal cores and resins have been used to build many contemporary boats. Many modern racing hulls use lightweight “honeycomb pattern” materials, which can be anything from polyurethane foam to end grain balsa cores.

These materials reduce the hull’s overall weight, typically incurring little strength sacrifices. Many of these “composite constructions” have been protected from premature failures by the use of “closed cell” foam cores mixed with epoxy resins; nonetheless, due to repeated high loading, all must be subject to high quality and standards, particularly where deck installations and fittings are concerned.


Submerging the hull does not automatically hasten its deterioration, but low maintenance standards could be a contributing factor. Marine growths flourish when anti-fouling measures are not taken. If you leave barnacles to continue working on your gel coat, they will destroy it.

Underwater metal fittings are vulnerable to galvanic electrical corrosion, and a hull covered in weeds will disguise the dreaded “osmosis blisters.” It’s common for rudders, propellers, and shafts to be neglected during a hasty sliding, typically for a rapid anti-foul.

The terrifying beasts

The feared “teredo” worm completely decimated the rudder’s fragile inner core during a recent repair job I completed. Fiberglass protected the rudder, but a worm entered via a tiny hole and destroyed the inner core. Never assume anything!

Caution is advised!

Use a professional marine surveyor to buy a used fiberglass boat. Unlike you, they have the education and experience to identify problem areas before they become significant issues immediately.

You have only yourself to blame if you waste these funds.

Glassfibre cloth comes in various forms, from the straightforward “chopped strand mat” to the more complex (and costly) Kevlar Aramids and Carbon fiber. These fibers can be mixed, and each contributes unique properties, such as stiffness and strength. Weaved fabric, chopped strand mats (CSM), and bi- and tri-axially stitched fabrics are all examples of such fabrics. Regarding general repairs, E-glass is by far the most popular option.

How does it function, exactly?

Most of us understand the fundamentals of how glass fiber and resins function. The glass fabric, by itself, is very malleable and adaptable. When combined with the catalyst (a peroxide catalyst, typically MEKP), the polyester resin (or any other) forms a transparent, sticky liquid that generates heat (an exothermic reaction) and hardens. While each of these two parts has limitations, they create a fiber re-enforced plastic (FRP).

To what end does this function?

This remarkable physical collaboration allows for the transfer of enormous stresses and loads through the “cured” plastic, allowing for the construction of shells with huge load-bearing capacity, such as boat hulls.

Although “fiberglass boats” represented a revolutionary shift toward more durable boat-building methods, experience has revealed that they require some upkeep. Boat hulls experience significant damage from bending, flexing, fatigue, unexpected collisions, etc., during their service lives. Weakness, cracking, and de-lamination of the glass-impregnated cloths from the internal components can result from the fatigue cycle breaking down the cross-linked “constructional chains” of the hull.

Chemical Formula

Intriguing chemical events contribute to the disintegration of the once-solid “chains of strength” in the glass/resin link, in addition to the aforementioned physical causes of deterioration. Water in the fiberglass cloth can react with the polyester resin and glass to form a third, undesirable “partner” when a hull is constructed under unfavorable conditions, such as high humidity.

The result is a yellow, very acidic liquid that attacks its surroundings and significantly degrades the chemical “building blocks” of the resin and glass. This sets off a domino effect of devastation that will eventually compromise the hull’s integrity.

So, how do you know?

There are a wide variety of outward signs of chemical and physical degradation. Fine hairline cracks appear at the base of high-load stress locations prone to high rapid impact loads, such as handrails, stanchions, cleats, etc. These, in turn, permit the seepage of outside water. The destructive cycle is then allowed to progress gradually.

Tiny bubbles or “pimples” are another form that gel coat blisters can take. There may be a few, a lot, or even many. Pricking them often exposes a foul-smelling yellow acid material. The term “osmosis” is also used to describe this phenomenon. Take care: keep the chemical away from your eyes! Do not forget your eye protection!

A “hard spot” in the hull, manifested as a complex “line” in the hull, can result from a stressed bulkhead or improper furniture placement. The gel coat around the location may have fine cracks (star cracks).


This is the worst-case situation, in my opinion. The region has been damaged because water has been easily absorbed by one or more of the techniques above, and the deterioration has progressed to the point where the glass cloth has fully split from the resin. The low resin was used during construction, or the resin was squashed due to excessive tightening of through-deck bolts, leading to this problem. Swelling from the inside out causes these places to seem soft to the touch or to bend visibly when pushed.

Other potential trouble spots:

The loads on deck hatches tend to be sudden and cyclical. It’s possible for stress cracking to lead to total failure.

Cracks, warping, and discoloration of gel-coat near MAST/DECK FITTINGS (keep an eye on chain plate areas).

Winches and heave pipes should be inspected for micro cracks.

Inspect riveted areas for leaks and compression by popping them.

FADING – Unfortunately, our region has some of the highest levels of Ultraviolet activity in the world, and the pigmentation on fiberglass is particularly vulnerable to this type of radiation. Even if polishing can assist, a complete repainting with a two-pack or polyurethane paint system is typically the only cure for fading, especially darker hues.

Closing Statement

After being nearly frightened to death in the preceding chapters, rational thought must now prevail. It’s possible that none of this will happen or that it will happen only in part. The boat’s age, location, and quality of construction and maintenance all have significant roles. To expect your boat to function perfectly with zero prior maintenance is foolish. Despite all the other technological marvels we’ve created, we haven’t invented the self-repairing boat.

Maintaining routine maintenance is essential, and the average handyman, armed with the proper knowledge, can easily handle most repairs. You may get valuable data from places like glass and resin manufacturers. Stop watching TV and give me a call instead. If you’re unsure about the state of your boat or the condition of one you’re thinking of buying, don’t take any chances; instead, hire a marine surveyor to do the legwork for you.

Check out the webpage in the “Further Reading” section below if you want to learn more after reading this article.

Terry Buddell is a marine surveyor, yacht designer, shipwright, and freelance journalist.

He built his yacht, “The Nicky J Miller,” and sailed along Australia’s East Coast to the breathtaking Whitsunday Islands, where he now makes his home. He lives in Gladstone, Queensland, and is adding to his online store of boating blueprints by constructing yet another boat. Terry maintains a comprehensive, exciting website about sailing and boat building in his spare time (what spare time?).

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