Gelcoat Cracks In GRP Boats

It seems as if the more I look into this, the more convinced I am that I’ll end up with a GRP boat – even though I am also convinced that the prettiest and most diverse range of boats available are probably wood. I also feel that a compromise on my notion of the ideal cruising boat will be required. As John Almberg (The Unlikely boatbuilder blog) commented here recently – a list of shallow draft, bilged keeled GRP boats with gunter rigs may actually have no entries on it! Still, ever optimistic I’ve been reading more about GRP and the issues I am likely to encounter with an old second (or third) hand one. 

Compared with metal, GRP is a relatively brittle material. Whereas steel, might bend or dent under an impact, GRP is most likely to crack.

In themselves cracks in a GRP gelcoat are relatively easy to repair. If it is a simple matter of damage caused by a collision with the pontoon, grinding out and refilling is probably all that is required. But cracks can appear for several reasons and unless you know what caused them, you may be only dealing with the symptoms – not the cause.

Cracks often occur due to stress in the material, and boats can be exposed to many stresses. 'Panting', is a term used to describe the movement in a hull when it moves in and out like bellows. The cause is usually changes in the water pressure on the hull as a boat is moving through water in sever weather. At times, she’ll be low in the water as a wave rolls past, and then moments later a large portion of her hull may be out of the water as she breaks through the top of a wave. Pounding, can create huge local stresses on the bottom of a hull as it leaps out of the water and slaps down hard. Boats also suffer from twisting forces as they take waves on the quarter, and sailing boats can be subject to great stress where the mast meets the hull or on decks where shrouds take enormous strain.

It would not be cost effective or practical to construct a GRP craft with a hull thick enough over its entire length and breadth to resist every force applied to it. Generally therefore, a designer will draw plans for a boat with a hull skin strong enough to cope with the external pressure of water expected and then add strengtheners to cope with all the additional pressures that will occur in a seaway. Strengtheners are both longitudinal and transverse and designers have applied a great deal of ingenuity in building them in to the structure, in some cases as cabin furniture and bulkheads. The purpose is to break up the hull skin into panels so that the load on any particular part of the hull is ‘transferred’ into the stiffeners and then diffused and shared by the rest of the hull structure. If the panel is too small it will make the hull stiff and brittle in that area. If it is too large the panel may be too flexible, in which case, cracking may occur.

The essential message is that, you can fill cracks, but unless you know what caused them, and unless you address the root cause of the problem, you may have to fill them again fairly soon.

Seaward