Nightmare. Your boat has been lifted out and washed off and you spot blisters developing under the antifouling below the waterline.

That can frequently be the early warnings of elevated moisture developing within the GRP laminate.

But, as marine surveyor Ben Sutcliffe-Davies is keen to point out, there’s no need to panic… just yet.

The hull after the keel (pictured above) had fallen off. Credit: Ben Sutcliffe-Davies

“Nine times out of 10, the blisters on a modern boat are not osmosis; they are often just water blisters that have developed between the hull and coatings. This is often caused by a number of potential issues that include the coatings not being applied properly, that the boat has been epoxied when the vessel wasn’t dry enough, allowing the moisture to permeate from the inside out, or more frequently the internals are not kept dry and moisture is held within the laminate. It’s well worth employing a surveyor to look into the issues if you’re concerned, especially if it’s an older vessel, in case it is a much deeper problem,” he explained.

Why do blisters form?

Glycol is found inside boat laminate. This chemical is highly hygroscopic – it readily absorbs and retains water.

The gelcoat is porous which means tiny amounts of water pass into the boat’s hull.

The water is drawn into the glycol and the two liquids combine to form a new chemical, which can be acid or alkaline, and smells like vinegar.

Moisture will continue to be absorbed and laminate will break down, and eventually, the moisture won’t be able to escape, causing the blister.

Once you’ve identified unusual bumps in the hull, pierce them. If plenty of liquid comes out, then it is likely you have a serious problem with moisture. Credit: Graham Snook

Over time, as the laminate absorbs more moisture, the blister will become larger and there is a higher risk, especially on older craft if the main laminate is predominantly of chopped strand mat, that the laminate will soften and eventually could fail.

This is something Ben has experienced with a number of older craft, both fin and bilge keels, where the laminate has significantly softened.

Why are older boats more prone to osmosis?

Older fibreglass boats from the 1960s, through to the early 80s were commonly built using basic polyester resin – which is slightly porous – and chopped strand mat fibreglass.

Chopped strand mat has plenty of end filaments, which can soak up moisture for years and years.

From the late 1980s, the outer laminates of boats were built using isophthalic polyester or other resins which were more water resistant, but frequently they still had normal polyester resins used inside.

“The problem with blisters is that the glycol is squeezing out of the chopped strand mat and pushing the gel coat off and then the filaments of the chopped strand suck up all the water. Woven glass has very long filament strands so are less likely to suck up moisture at the same rate as the chopped strand,” explained Ben.

What other factors affect osmosis?

Environmental factors can play a part in whether your boat suffers from osmosis.

The less dense water becomes, the easier it is absorbed.

Warmer water is less dense than cold water, so boats sitting for long periods in warmer water can be more susceptible.

The difference in density between freshwater and saltwater (saltwater is more dense) means vessels kept in freshwater are also more likely to suffer from osmosis.

Even ashore, osmosis can develop as a result of rainwater becoming trapped in lockers and moulding returns within the hull or under decking.

A moisture meter will easily identify any areas where the laminate is wet. Credit: Graham Snook

It goes without saying that if your boat is kept permanently in the water year-round, then the risk of osmosis and laminate softening grows.

The way the boat is constructed can also be a factor, including the use of cheap materials, which can be more porous, pigmented resin or bad practice when building the hull, such as curing the resin for too long, or not enough resin being used in the wetting out stage of construction when the fibreglass is applied to the hull.

Other issues with laminate can be down to the types of core used.

Many boats are built with a balsa core. If moisture can penetrate this core it will, over time, rot, weakening the structure of the boat significantly.

What can I do to mitigate this problem if I have an older boat?

If you can, don’t leave your boat permanently in the water for years and years, especially if it’s an older vessel.

Keep the bilges dry and try to have the vessel out on the hard for at least three months a year.

When ashore, make sure the boat has good ventilation and that nothing wet is left in the bilges and lockers; Ben can often locate a cockpit locker from the outside of a boat due to the higher readings from his moisture meter where items are regularly stowed damp.

When the vessel is lifted, try to do a tip test on the keel/s and make sure the laminate isn’t flexing around the keel roots.

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To do a tip test, the boat needs to be suitably lifted in strops.

The tip of the keel needs to be carefully loaded side to side; there should be no opening of the hull-to-keel joint with minimal movement from side to side.

The hull should not flex excessively when tip testing and there should be no movement of the laminate when the boat is lifted or set down.

If there is movement it’s advisable to ask a qualified surveyor to take a closer look.

Will osmosis sink my boat?

Osmosis takes some time to develop and when you see the blisters it is often very well advanced.

Many people argue that boats do not sink from osmosis, but according to Ben, they are technically wrong.

“I’ve looked at several boats on behalf of concerned owners or insurance companies over the last few years that all have extremely high moisture within the laminate, causing significant loss of strength from the moisture ingress.”

He has also posted several videos on his YouTube channel, including examples of tip testing on softened hulls with keel movement.

“I looked at one 40-year-old motorboat where the blisters were over 5½in in diameter and the delamination was affecting three to four layers in a six-layer laminate hull.

“This is the point in time when osmosis is going to lead to the boat sinking. The boat was also so wet that when it was lifted out of the water, it slumped over the straps because it was so soggy,” he added.

Ben has also seen boats where the keels have fallen off, the hull torn around the keel roots due to soaking wet laminate.

As he advises, if in doubt, get a professional assessment from a surveyor with at least a proper tip test with the boat held in strops.

Enjoy reading Osmosis on a boat: will it cause my vessel to sink?

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