How long can osmosis continue in a boat's fibreglass hull?
My older fibreglass yacht has a few underwater blisters, but the hull otherwise seems sound. Can osmosis carry on for years without causing serious damage, and does winter storage ashore stop it?
Answer
Osmosis can continue for years or decades in a fibreglass hull, with no fixed point at which it stops or makes the boat unserviceable. Check blister depth and extent first: shallow gelcoat blistering calls for different decisions from deterioration within the structural laminate.
Why osmosis continues
Water molecules slowly pass through polyester gelcoat and resin. In small voids, the water dissolves soluble substances associated with the laminate. The resulting concentrated solution attracts more water. Pressure can then lift the gelcoat or separate material within the laminate, forming blisters.
The process can continue as long as moisture and soluble material remain available. Its speed depends on resin chemistry, build quality, water temperature, immersion time and previous repairs. Fresh or brackish water can encourage osmotic water uptake because its dissolved salt concentration is lower than seawater's.
There is no reliable rule such as "five years after the first blister, the hull needs replacing". Some boats carry scattered shallow blisters for many seasons; others develop extensive damage that needs earlier treatment. The condition beneath the gelcoat tells you more than the years since discovery.
Checks at the next haul-out
Inspect promptly after lifting, before weeks of drying allow the blisters to subside. Photograph affected areas with a ruler alongside them, and record their positions against recognisable fittings.
- Location: establish whether bubbles are in the antifouling, between coating layers, beneath the gelcoat or deeper within the laminate.
- Depth: a shallow cavity beneath the gelcoat is generally less concerning than a pocket separating layers of glass reinforcement.
- Laminate condition: look for loose fibres, soft resin, weeping cavities and areas that sound noticeably different during professional percussion testing.
- Progression: compare blister size, distribution and exposed laminate condition with previous haul-out records.
A vinegar-like smell from fluid in an opened blister supports a diagnosis of osmotic blistering. Neither the smell nor blister diameter establishes structural severity. Wear eye protection and gloves when opening blisters: the fluid can be irritating and under pressure.
Moisture readings support an assessment but cannot diagnose osmosis alone. Antifouling, surface moisture, temperature and metal behind the laminate can affect readings. A meter's displayed number is not automatically the percentage of water in the hull; its meaning depends on the instrument and scale.
What winter storage changes
Time ashore removes the continuous supply of external water and may reduce moisture, but an ordinary winter lay-up does not reliably cure osmosis. Blisters can flatten as moisture redistributes or escapes, leaving soluble residues inside the cavities.