In the early days of fiberglass boat construction the durability and strength of the material was underestimated. Builders formed thick hulls with integrated tubular ribs and stringers.
Since this was the time before computer aided design tools, builders in the North Western United States built using the old default method of more is better. In 1956, when the first fiberglass boat was built, the material was very new but already found acceptance in aviation and automotive industries.
The only way to build at that time used layers of fiberglass impregnated with acrylic resin that hardened when cured. Large molds allowed entire hulls to be made as one piece with no seams. Some wooden structure was added inside the hull for rigidity and it was bonded in with more fiberglass material. No precautions were taken to compress the curing hull or eliminate air bubbles in the structure as is done today. We know this method as solid core construction.
Fiberglass materials remained expensive, and as demand for these new boats increased, manufactures began cutting costs to compete in the marketplace. Soon a layer of wood was added to lighten and strengthen the hulls and decks. The fiberglass and wood sandwich was a great combination until one of the outer surfaces of the fiberglass was breached. This is called wood core construction.
It didn't take a crash on the rocks to let water into the wood layer. Small cracks allowed the wood to become soaked and it swelled, and then rotted. Soon the inner and outer fiberglass layers couldn't do their job and broke down from repeated flexing.
This was the first type of fiberglass delamination and the failures damaged the boat building industry badly since many manufacturers had transitioned to all fiberglass construction, leaving more traditional materials behind. Fiberglass construction was quickly becoming known as poor quality because of delamination issues.
Two Types of Delamination
The first type of delamination, where a wood core either separates or disintegrates, is very difficult to repair. One of the fiberglass surfaces needs to be removed to access the core. It's usually the inner skin that's removed because it's less visible so finish quality is not as important.
The process is expensive and requires skilled labor; many boats were scrapped because of the cost of repair. Even with today's modern materials and processes this kind of repair is difficult.
Another type of delamination is similar but without the wooden layer. In these cases tiny flaws in the fiberglass itself allow air to be trapped. If the hull is cared for badly, water can enter through microscopic channels and enter these voids filled with air. Expansion and contraction of these tiny bits of water will make the voids grow horizontally along the layers of fiberglass cloth and resin binder.
Temperature fluctuation causes the expansion and contraction of the water and if freezing and thawing are encountered the voids will grow quickly.
Small bumps soon become visible in the smooth finish. These bumps are called blisters and it's a serious condition.
The only way to repair this damage is to remove the outer gel coat and underlying fiberglass material to access the damage. It is then filled with new resin and the gel coat is patched.
It sounds easy, but unless you have considerable experience working with composites it's easy to make the situation worse. If the boat is going to get a new coat of paint the problem of color matching isn't an issue. Blending a patch into existing paint is an art form and lighter colors are much easier to match than bright or dark paints.
Mechanical bonding is the larger issue since the new patch is only connected to the hull through adhesive properties. The same vibrations that formed tiny cracks will cause the boundary of the patch to loosen. Some blister repair involves drilling a few very small holes and injecting an epoxy compound. The blister is then compressed while the epoxy cures. This allows the patch to become a more integrated part of the hull.
Causes of Blisters
Marine growth can penetrate the gel coat and allow water into the structural area. Keeping a clean bottom and using an anti-fouling paint is the most important step.
Abuse is another way tiny cracks form and allow the entry of water. Some boats are exposed to these conditions as a normal course of wear. Other boats are needlessly used in a careless way and this causes hull problems. Never allow someone to load heavy objects on the cabin top or jump onto the deck from the dock. Not only is it dangerous, but it can lead to delamination in these areas which will grow with further vibration from normal use.
Poor storage practices like leaving water in the bilge can lead to severe delamination. Even in tropical climates the expansion and contraction of water trapped between layers of fiberglass can raise blisters. In climates which freeze and thaw often it's possible a small blister can turn into a “pop” where the outer surface is torn away by the pressure of internal ice. Pops can be fixed with the same processes as a blister but the extent of the damage is unknown and the hull is permanently compromised. Sonic survey can reveal some of the damage but prevention is far easier.