The Core of the Matter


The principles of fiberglass construction, what to look for in buying an older boat and how to assess and repair any damage  (published May 2015)

There are many types of materials used to build boats these days and have been for a long time—wood, steel, alloy and for a while, ferro cement. However, the vast majority of production boats are now made of fiberglass, either glass reinforced plastic or GRP. The “miracle material” of fiberglass was widely hailed as a maintenance free boat building opportunity in the 50s and 60s and has progressed—with some marked improvements and significant failures—ever since.

Today, production yachts are being seen more and more frequently in remote and popular cruising grounds that are a long way from their homeports. Yachts built by Beneteau, Jeanneau, Catalina, Bavaria and Hunter, to name a few of the larger builders, are crossing oceans and providing long term homes for an increasing number of people who opt for the sailing life. Not too long ago many of these fiberglass boats were routinely scorned and rejected as utterly unsuitable for cruising, never mind an ocean crossing or circumnavigation. But based on the number of old plastic tubs  (term of endearment, as I’ve lived on one or another for 15 years) that I see arriving daily from all over the globe, this is not the case anymore.

Logically it would seem that older boats represent an attractive price point and allow more people to make the move aboard and go cruising. The long shelf life of GRP construction means that vast numbers of older production boats are available; coupled with the many thousands of annual new yacht launches and the competitive nature of the production boat industry keeping new prices down means real bargains in used boats abound. This encourages those considering an early retirement or extended sabbatical to go cruising now.

Even though we can get a GRP production boat that is largely equipped for our needs, we need to ask and answer a few questions: Are all production boats equal? And is all GRP construction equal? The short answer to both is no. So to understand our production boat better and to make a more informed decision, we can examine a few mainstream build methods and materials.

Deck chips and voids are common and will let  water into the glass fibers over time.
Deck chips and voids are common and will let water into the glass fibers over time.

No, Keanu Reeves does not build boats to my knowledge; this matrix refers to the construction of the hull. Usually glass fibers and resin, a core (more on that later) may be used but typically glass fiber in various weights and numerous weaves are impregnated with resin within a gel-coated mold to form the basic hull. It is the nature of the fibers—combined with factors such as thickness and reinforcing—that determine the structural characteristics of the hull.

Essentially the resin does nothing but protect the fibers from water, sun, chemicals and other potentially degrading sources. Resin is not our friend as it is heavy and brittle, with little inherent strength. The best lay-ups have a low resin to fiber ratio; 40/60 percent respectively is regarded as good but many sources believe 60/40 is more like the typical reality. This ratio will largely depend on the skill, dedication and care factor of the people who do the work. Design and management plays a big part of course, but all the engineers and managers in the world will not make a badly hung over and sleepless Fritz, Francois or Frank do the best job on this particular morning.

Unless infusion technology is used (fibers are laid up, sealed, a vacuum is applied and the optimum quantity of mixed resin is then sucked in and dispersed through the fibers), then we are dependent on our chosen company’s work force to achieve the designer’s objectives. Too much resin means more weight and lower strength, too little resin means dry fibers and the likelihood that these fibers will absorb water over the years and encourage blisters and/or delamination. So it is a fine line the production builder walks, although according to the brochures and websites each has it down pat.

Of the common fibers used in a layup, chopped strand mat is arguably the most popular. Easy to wet out and work with, it is cheap and is therefore frequently used to provide thickness to a layup and prevent visible “print through” from other fibers such as rovings—larger strands or bundles of fiber running in various directions. This thickness is required to produce stiffness in the hull. Chopped strand mat consists of short fibers randomly oriented and held together with a binder. This creates a layer that has equal strength in all directions but is less strong than larger fibers oriented inline with the anticipated load, which is where the engineers make a difference.

In essence, the fibers are the strength and the resin just holds them together. Too thin and flex is introduced, which is not particularly desirable in a hull. This is why core products have been developed.

This long crack in new deck paint warrants a closer look.
This long crack in new deck paint warrants a closer look.

Core technology has been around for quite some time, which makes it highly unusual not to find a production boat without a cored deck. If you do, it will probably be quite apparent because you will feel the deck flex underfoot as you walk around. Plywood (hopefully marine ply) is frequently used to build a deck. The wood provides immediate thickness that is required to impart stiffness to a structure. This is a tried and true procedure that if properly done will provide a very long service life. Properly done is the key point.

Ideally where fittings will penetrate the deck—winch bolts, travelers, jib tracks, cleats and windlasses—the ply will be replaced at the factory with a solid glass layup or aluminum alloy insert. The alloy insert can then be tapped to allow fittings to be secured with machine screws and not through-bolted. This is popular with jib tracks and travelers. If the ply remains as it sometimes does, then when or if the sealant around the bolts fails, water will begin to find a way into the timber core. Good marine ply will resist this but will eventually begin to break down and cause the damage to spread. Spongy sections are the result, with a real risk of the fitting eventually pulling out in exciting circumstances—imagine that big genoa winch flying out of the deck when you are sheeting it on and you’ll know what I mean.

So decks have been cored for a long time and increasingly hull cores are also being used. Some builders have used cored topsides for quite some time; Bavaria has been using core materials above the waterline and solid GRP below for years. The benefits of core material in boats is plain, lighter weight, increased stiffness, better insulation and one would imagine less expensive in material, labor or both. Otherwise price point-conscious builders would not be using it. A number of high volume manufacturers who previously made solid GRP boats now produce cored hulls. Marlow Hunter is one example.

The downsides are equally apparent, less compressive strength and the potential for nasty problems like shearing—when the internal or external GRP skins part from the core—or water ingress that may compromise the hull to a greater or lesser extent. The strength of the core is directly related to the bond of the two skins to the core material, if this fails then there are two or three independent structures, not a single sandwich to bear the loads. All core materials are not equal and there are many products around that claim to be impervious to water ingress or of closed cell structure that will not allow water to spread to other areas.

Balsa has long been used as a core material and typically is supplied in small blocks that are about the size of a matchbox attached to a web like material—very similar to a sheet of ceramic tiles and all the blocks have a gap between them just like the tiles. The reasoning here is that GRP is applied between every block, which essentially encapsulates the individual blocks. If damage occurs that allows water into a block, it is trapped by the surrounding GRP and theoretically cannot migrate further. If poorly done, water will travel relatively freely and the core will rot; if damage is extensive, repair costs may exceed the value of the boat.

Balsa has good compressive strength and makes a good stiffening medium as demonstrated by Catalina with the 460 model in the mid 2000s. This hull had the solid GRP layup schedule of the 400 model but had balsa core added inboard on the topsides for additional stiffness. Many of the PVC and foam cores available also use the “tile” principle, which also assists in shaping the layup to curves but some do not and rely on the ability of the material to prevent water migration if it does find a way in.

More high tech boats, usually of the go fast variety, will use a lightweight core throughout the hull, both above and below the waterline. Impact resistance is lessened and exotic fibers such as Kevlar may be added as reinforcement. Kevlar has very good impact resistance and is difficult to penetrate. Jeanneau has been marketing “Kevlar energized” in their hulls for years and Hunter for quite a while also. Catamaran performance depends a lot on limiting the amount of weight, hence they perhaps benefit more from cored construction than any other type of cruising boat. Core material in boat furniture can also significantly reduce weight.

Out of the water is the time to check the hull
Out of the water is the time to check the hull

So you are considering a boat with a cored hull but there seems to be damage—what to do? Well the good news is that it can be fixed, but can you afford it? The biggest concern is the extent of the condition. Naturally a small amount of damage, identified early will not be an insurmountable problem. Essentially the process is to remove the damaged area, dry it if required and replace the material. This is easy if it is six inches of accessible deck, but is a massive undertaking if the damage is half of the hull.

For vessels with a large area of compromised core, a common owner’s fix has been to drill hundreds of holes in the affected area and dry it out as much as possible, which depends entirely on the people affecting the repair. When dry, the holes are then filled with a very thin epoxy so it can travel as the water had in the expectation that it will fill the voids in the laminate.

You are then left with these holes to fill, which will always be visible unless a full respray of the hull or deck is undertaken. The wisdom that “new paint hides many sins” sums this up, which is why many people are afraid of fresh paint jobs on boats that have just been put up for sale. A known cored hull can be closely inspected for damage or indications of compromise but can be hard to spot especially if the damage is manifest internally and may be concealed by furniture. Percussion testing and moisture meters are an imperfect science. Of course there is a whole lot more to building a decent hull than this and a million variables for the builder to consider. Curious souls could get a copy of Fiberglass Boats by Hugo Du Plessis.

    John currently floats around in Langkawi, Malaysia and offers appraisals and surveys for production boats in the region. If you are thinking of buying in Asia he can be contacted at,

A good practice for installing or maintaining fittings on cores or even solid GRP (this will also degrade over the years if water gets in) is to drill oversized holes for the fasteners and fill them with marine epoxy such as West System with adhesive suitable micro fibers. Once this has gone off properly, the correct sized holes for the fasteners can be drilled and the bolts sealed with the appropriate sealant. This means the precious core is isolated from the holes and will remain dry even if the sealant does leak down the track. Many core materials (particularly foam types) are not at the best under compression and can crush under the force of the tightened bolts; backing plates or the largest washers practical are preferred to spread the compressive load.

Author: John Champion