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Osmosis: blisters, moisture and the age of the hull
Short answer
Blisters in the gelcoat are the visible end of a very slow process, and slow is the word that decides everything else. What a schedule says about age, and what a surveyor can actually measure before the money moves.
Osmosis begins where water finds a way through the gelcoat into the laminate below. Soluble compounds left over from the build draw more water in, pressure rises, and the result appears as blisters on the underwater hull, often first noticed in the week after the boat comes ashore. Prick one and the fluid smells sour or vinegary, which is the field test every yard hand from the Hamble to Enkhuizen knows. A hull that has spent its life afloat in warm fresh or brackish water is the classic candidate, and the first proper season ashore after twenty years afloat is when it shows.
The insurance question is not really about blisters, it is about the difference between sudden and gradual. The hull wordings we reviewed are built around a sudden and unforeseen event, and wear, ageing and lack of maintenance sit outside them as a matter of principle rather than of small print. Some schedules make a narrow exception for osmosis on boats below a stated age, others say nothing about it at all and treat it purely as a matter of condition. Our Premium cover includes osmosis damage on boats up to 48 months old. Which of the two you are holding is worth establishing before a purchase rather than after a lift out, and the answer is in the schedule and the conditions, not in the brochure.
For a second hand boat the practical answer is a survey before the money moves. A surveyor from the YDSA or an equivalent register will sound the hull, read a moisture meter once the boat has dried for a sensible period rather than on the day of the lift, photograph what is found and put a figure on the work. Ask for the readings themselves and not only the conclusion, because a repeat reading two seasons later is what turns a vague worry into a trend. Record known defects in the contract of sale, keep the invoices for any peel and reschedule job with their dates, and put the whole folder somewhere that survives a change of laptop.
Worth remembering
Gradual
is what osmosis always is
Sudden
is what hull wordings are built around
The blisters are the easy part. The argument is about which side of that line the damage falls on.
Questions
What owners ask us about this
How do I recognise osmosis?
By blisters on the underwater hull that give off a sour, vinegary smell when pricked, and by moisture meter readings that stay high after the hull has had time to dry ashore. A single blister is not a verdict, a field of them is.
Is osmosis treated as a sudden event?
No. It develops over seasons, and hull wordings are built around sudden and unforeseen events, with wear and ageing outside them. Where a schedule makes an exception it usually ties it to the age of the boat, so that line is worth reading before buying.
Does a survey stop the problem?
It cannot stop it, but it dates it. A report with moisture readings, photographs and a cost estimate lets you negotiate the price, plan the work and show later what the hull looked like at a known point in time.
Is an older boat simply uninsurable?
Age on its own rarely decides anything. What counts is the documented condition of the hull, the deck fittings and the rig, which is why the owner of an older boat gains far more from a dated survey than the owner of a new one.
Related
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Whether it is “Osmosis: blisters, moisture and the age of the hull” or another question: send us your residence and the flag your boat flies. You will get a straight answer in one email; including a no, if that is the answer.
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