The benefit of GRP hulls is that they are cheap to produce, long lasting, relatively easy to repair, and easy to maintain. It is not unusual to see a GRP hull constructed in the early 60s still in near perfect condition today, and this is will be largely down to a quality construction process and a heavy lay up (thick laminate). Of course other factors such as climate, vessel usage, maintenance and skipper experience all play a part in the longevity of any vessel.
There are myriad causes for GRP hull failures which have become more complex in recent years with the advent of new construction methods. The heavily built laminates of yesteryear may fail due to an initially poor lay up, known as a Friday afternoon hull. This could be due to poorly mixed resin, old resin, laminating in poor conditions, or an unskilled laminator leaving numerous voids in the laminate which, through the process of osmosis and aeration, encourages propagation of the void, which is the start of a delamination process. However, these poor layups tend to show their true colours within a couple years of being built, therefore, you can hedge your bets that a 1960s GRP hull afloat today, having stood the test of time, was not a Friday afternoon hull, and that if a new vessel leaving the factory today exhibits the above defects within a couple years of service, you can bet it has been constructed to a poor standard, and which is unacceptable.
It should stand to reason that if a GRP vessel built 50 years ago is still afloat and in service today with relatively little maintenance, then GRP vessels built today, with their technological advances such as computer modelling, and improved resins and coating systems should remain in service for twice as long. Sadly, this is not the case for modern day GRP vessels, which follow the trend in modern construction (new builds constructed today in England are only designed to stand for 60 years). Alas, it is due largely to economical reasons why modern GRP vessels suffer failures.
Polyester and epoxy resin comes from oil which continues to increase in cost as the finite resource is sold to the highest bidder and horded. This has lead to yet thinner and thinner laminates. One failure as a result of this can be a resin starved laminate, which is unduly flexible and porous. It is no surprise to find resin starved laminates on a daily basis, especially inboard, underneath cockpit lockers and on stiffening members such as stringers and frames. To get around cost constraints and to save weight, laminates are stiffened with a core between two thin fibreglass laminations constructed usually of end grain balsa wood or closed cell foam. This creates a thick I-beam structure that is stiff and light. However, by adding more layers, faces, and materials to a composite, it becomes more complex it the way it fails and requires repair. The great majority of GRP hulls built since the 70s will have decks at least partially constructed of a cored laminate deck. Delamination/debonding, water ingress, rotting of the core and the crushing of the core are all typical problems with cored laminates which are encountered on a daily basis and are costly to repair.
Take a look below at a few ways GRP laminates and cored laminates can fail.
The photo opposite shows what delamination of a cored deck can look like. In this case 2 factors were at play. Firstly, the adhesive used to bond the outer laminate (deck side laminate) to the core was faulty (this can be due to the resin/adhesive being stored below an adequate temperature, being past its best, not properly mixed prior to application, or the application surface not being properly prepared, amongst other things). Secondly, this multihull vessel was based in the med, and the wide flat deck becomes very hot during the day which expands tiny air voids between the core and the outer laminate. The poor quality adhesive is not strong enough to stop the voids (blisters) propagating.
In this case the fault was clearly built into the vessel, and it is the responsibility of the builder to repair. Core delamination is very costly to repair if the delamination has been left to spread extensively. If it is not too severe, there are cheap and effective ways to effectively repair it. There are also foam spray repair products which are cheap and easy to use, but act as more of a sticking plaster than remedial surgery!
The dreaded boat pox.
The thing that still concerns many owners and to be owners of GRP hulled vessels is the nightmarish idea that their boat will suffer from huge, uncontrolled blisters, which will lead to the disintegration of the hull leaving it with as much integrity as a ham sandwich.
The truth is, if a vessel is going to fall apart due to osmosis, its going to happen within the first 10-15 years of its life. This is because severe osmosis is a direct result of a poor build with few, if any, quality control measures.
Old GRP hulls were more greatly affected by osmosis due to the less water resistant resins and coatings, however, they were heavily built meaning the inbuilt stiffness within the laminate was/is too much for the pressure of a blister to separate, alas, blisters form on the outside, or on the inside, such as in internal fibreglass water tanks, but rarely deep within the laminate. The good news is since the arrival of isophthaltic resins, a resin used beneath the gelcoat, osmosis is rarely a problem. These resins have been in wide use since the 90s.
The difficulty is when a vessel was built in a cross over period, where the hulls were becoming thinner, but the quality of resins had not improved. Add to this a lack of quality control one Friday afternoon, and you have a vessel which is covered in boat pox almost from the word go.
There are 3 ways to deal with osmosis, 1: full gelcoat peel and reapplication (very expensive). 2: pop, epoxy fill and fair the blisters every couple of years, as long as the blisters are not too deep! (not costly, or particularly time consuming). 3: Do nothing. It is best to have the osmosis inspected, because a surveyor should be able to explain the extent and necessary course of action.
This slightly grainy photo was taken on a 23' sports fishing vessel (a fishing boat with a powerful outboard and deep V hull). What can be seen is a GRP frame which has split transversely right down to the hull. Note hate crack propagating into the hull laminate.
This failure not only weakens the hull structure, but it increases the stresses adjacent to the tear, as the hull acts like a hinge, and hull laminate failure is not far away.
What is more concerning is the fact that the vessel was built like this. The top of the frame had been cut away to fit the inner moulding, and no effort was made to bond the inner and outer laminate, or strengthen the laminate.
This vessel had been constructed 19 years previously and this was never seen or repaired.