Fibreglass Boat Survey Glass Reinforced Plastic (GRP)

Fibreglass Boat Survey

Fibreglass (GRP) Hulls

For a fibreglass boat survey the exterior and interior surfaces are examined where accessible. Sample areas of antifouling are removed to expose the gelcoat, but protective epoxy paint schemes are not removed unless either they are already flaking off or if the Owner gives specific permission.

The gelcoat surface is examined for the presence of of osmosis-related deterioration such as blistering, fibre-end blistering, and wicking. Barcol Hardness may be measured using a special impresser if the state of cure is in doubt: that is usually only relevant in the case of repairs, but it can be a useful diagnostic tool when considering gelcoat defects at other times.

The moisture content is measured with both Sovereign and Tramex meters, because they read to different depths. A raised moisture content is not necessarily evidence of osmosis, and by itself is not a sufficient reason to recommend an osmosis repair.

If a hull has a protective epoxy scheme, it may not be possible to obtain representative moisture content readings until it has dried, typically taking about 7 to 14 days. That rules out holding in slings over lunch-time, the usual cost-effective way of surveying GRP hulls. Alternatively, the coatings can be scraped back to expose the original polyester gelcoat, but permission may not be forthcoming and such patches are difficult to repair effectively.

The hull is examined for stress cracking, and in the case of motor vessels with sharp-angled spray rails, antifouling may be removed to check for cracks along the rails and at other high-load points.

The hull is hammer-sounded below the waterline to identify any delamination.

The hull above the waterline is inspected for cracks and repairs, and is selectively sounded with a plastic mallet for delamination.

The interior of the hull is examined where exposed or through readily-removable traps. Apart from the internal surface of the hull itself, the reinforcement members such as bulkheads, frames, floors, stringers and engine beds are surveyed with particular attention to the state of the GRP webs that attach them to the hull.


GRP Decks

The exterior surface is inspected for stress cracks, the state of the gelcoat, repairs etc. It is lightly hammer-sounded and checked for movement or creaks to determine the state of the sandwich core.

The underside of the deck is visually examined where accessible, as is the joint to the hull.


Some Common Defects In GRP Vessels


The term ‘osmosis’ is loosely used to describe damage to a polyester hull by water vapour penetration. The damage is caused by the hydrolysis of the laminate materials in water. There are too many different types and symptoms to detail here. A full article will be posted on this site at a later date.

It is a relatively common problem, although less so now that the superior Isophthalic resins have been employed since the early 1990’s. Until then, some osmosis could be expected at five to ten years of age. In its early stages, some deterioration is very unlikely to weaken the hull to the point where it is no longer fit for the designed purpose. After 25 years in the industry, I have only come across three yachts that have been rendered unserviceable due to osmosis damage, and two of those were caused by defective repairs.

As the repair process is more or less the same regardless of the nature and extent of damage, it is in my view reasonable to defer repairs until they are essential.

A raised moisture content does not necessarily mean that a yacht has osmosis or will go on to develop it – it is only one of several factors which have to be taken into consideration.

In very bare outline, if a yacht has significant structural deterioration because of blistering etc, the industry-repair involves removing the gelcoat and any damaged laminate below the waterline and to about 4″ above, cleaning and drying the laminate, and then applying a protective coating scheme of solvent-free epoxy resin.

Partial or local repairs are seldom successful in the medium or long term. One of the leading manufacturers of the epoxies used recently stated that their predicted life was no more than 8 to 10 years.

I have therefore formed the opinion that the only long-term solution is to reinforce the prepared hull with one or more glass cloth laminates laid-up with epoxy laminating resin, before applying the protective coats. That will replace the strength lost both to the hydrolysis and the removal of the surface glass during preparation. It provides significant extra toughness and also thickness of epoxy resin.

While relamination adds to the cost, it seems reasonable to do the job once, properly, rather than twice – when the requirement for relamination will be beyond doubt.

Whatever you do, do not follow advice recently seen in one of the popular boating magazines, which is to grind off the surface blisters and then repaint. That does not address the fundamental problem, but only deals with the outward signs. The hull will deteriorate rapidly after such abuse.


Stress Cracking

Some hairline cracking may be expected in the gelcoat almost anywhere. Frequently it does not penetrate into the glass reinforcement: the gelcoat is more brittle than the glass laminate, and so cracks more easily. Some may date from mould release. Below the waterline, cracks should be sealed to protect the glass from water penetration, but above the waterline, and in the deck, repairs may be more obtrusive than the cracks. However, you need a surveyor to determine which are significant and which may safely be left unrepaired.

Stress cracks are particularly common along the chines and spray-rails of fast powerboats. That may well be a sign of excessive flexing when pounding in a seaway. The sharp corners necessary to produce good handling are difficult to back-up with the glass reinforcement during build, and cracks may develop in the voids. In some cases, additional reinforcement may even be needed inside the hull, perhaps by way of frames or stringers. These cracks always need careful inspection and are relatively costly to repair if extensive.


Failed Bonding Webs

It is quite common in older boats to find some failures in the GRP webs used to attach plywood bulkheads, frames or stringers to the hull shell. Unless these failures are very extensive, which may signal either gross misuse or some more extensive structural defect, they are not normally a significant problem and are usually quite straightforward to repair.


Deck Delamination

This is mainly a problem for older boats.

Most GRP vessels have a sandwich deck, having an end-grain balsa or foam core between the two laminates. That is intended to keep the weight down while maintaining stiffness. If the bond between the GRP and the core breaks down, the deck will become flexible and spongy under-foot, and will probably creak when walked over. Some such weakening is to be expected in older boats. Small areas may not be a problem, but larger areas will probably need repair. That can sometimes be achieved by injecting an epoxy resin, but if water has entered the sandwich that will not work.

Delaminated decks are in many ways more difficult to repair than osmosis, because the repair site is inaccessible. It is almost impossible to open up the deck surface and reinstate it without it showing in the non-slip pattern or other such detailing. Repairs can be done from the underside, but again there are problems of access and the difficulty of applying resins, core material and glass ‘upside down’. There is almost always a means of solving this.

A small amount of delamination is unlikely to make the yacht unseaworthy, but any damage around such things as mooring cleats, chainplates and so on does need careful consideration.


Call 01929 480064 or email Anthony Byrde to discuss your own requirements further