When fibreglass boats stay permanently submerged, water can permeate the laminates and work through voids in the gel coat layer. The subsequent pressure from chemical reactions in the material is what causes the blisters that we recognise as a visible sign of osmosis in the hull.
The warmer the water, the higher the rate of osmosis will be. Fresh water also accentuates the problem because of its comparative low density to salt water
The biggest factor affecting the boat’s susceptibility to osmosis is the original build quality. The more voids in the laminate, the quicker the process will happen.
Boats built between the 1980’s and mid 1990’s are more likely to have problems, as the industry and materials evolved massively around this time.
Builds from the mid-90’s onwards are more likely to be from line-production manufacturers using moulds and vacuum bagging, with tighter regulations. Keep this in mind for your initial purchase.
If you already have your yacht then various preventative suggestions include installing a dehumidifier in a sealed area under the boat, as well as regular antifouling treatments. It goes without saying that it’s best to have the boat out of the water if it’s not being used in the winter.
Spotting the problem early is key. There are visual signs of the earliest stages of osmosis, and a strong magnifier can reveal pinholes, micro-cracks and prominent fibres.
Once you start to notice blistering on the hull, there are several options you can take. Some owners choose to grind out the blisters, going back to the clean fibreglass, leaving to dry, and filling with epoxy resin. Another option is to strip back the entire hull and recoat – inevitably a more expensive process.
The best choice for your boat will depend on its age, resale value, and whether you plan to keep or sell the yacht in the near future.
If left untreated then de-lamination can weaken the hull considerably, so this is not a problem to be swept under the rug. Whilst boats rarely sink from osmosis, a weakened laminate can result in fittings pulling out of the fibreglass. Although serious structural compromise would take time, the presence of osmosis is a cosmetic issue, which will undoubtedly reduce the boat’s resale value.
A surveyor can check how far the osmosis has developed, and identify how deeply the laminate has been affected. This is best done once the boat has been out of the water for several days, as recently lifted boats will all produce high moisture readings. By finding out how much water is trapped beneath the gelcoat, we can then advise the next course of action to take.
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