No matter how hard you try, a completely dry boat is a rarity. The older a boat gets the more likely seals will start shrinking, sealant around deck gear will start to dry out and recede, and stress cracks will open up in bad weather. Then, before you know it there’s water in the bilge. Even without leaks there’s always condensation to consider. But the most common thing that owners and buyers concern themselves with when considering the purchase of an elderly GRP yacht is osmosis. Although osmosis can be progressively damaging if left untreated for too long, it is usually fixed quite easily when dealt with in good time. A few ugly blisters are not going to sink your boat overnight, but it makes sense to sort them out before it turns into something more damaging.
WHAT CAUSES OSMOSIS?
Osmosis is caused by the migration of water and solvent molecules through a semi-permeable membrane such as a GRP hull laminate. This moisture will often form blisters as the resulting hydraulic pressure increases in the laminate. The visible blisters are commonly formed between the outer layers of the laminate and the thinner gelcoat outer surface, but the process can also easily form voids in the laminate itself as the GRP fibres are forced apart by osmotic pressure.
Although water ingress
