Osmosis is about as welcome in a yacht as flatulence in a space suit, but both problems have to be dealt with sooner or later.
If, when you run your hand fondly across your boat’s underwater hull you find some blisters, you have every reason to inwardly groan. It probably means that small amounts of water have found their way through the glossy gel coat and have sunk into voids in the substrate beneath. This water has gradually reacted with leftover chemicals in the curing process, and begun to create by-products. These have swelled to force up the blisters that are the sign of advanced osmosis.
While it takes a really bad case of osmosis – with blisters the size of saucers – to put you in any danger of leaking, osmosis causes the value of your boat to fall by the cost of a professional repair, which will usually run into thousands of pounds.
In another article we tried some DIY temporary repairs, which involved grinding out the blisters, letting them dry, washing the voids, and then filling them with epoxy. This is followed by a primer, and then a standard antifouling (putting on an epoxy barrier coat like Gelshield will only seal the problem in, and even cause it to accelerate).
A temporary repair is cost-effective on a boat of low value, because there’s no point spending five grand for a proper job on two grand’s-worth of yacht – unfortunately, the boat will still only be worth about two grand afterwards.

As good as new
But what if your boat carries a reasonable value? If you want to sell it, you could simply allow the price to be knocked down by the amount of an estimated repair. Or, if you intend to hang on to it, you could opt for the full work. The result will be a virtually new underwater hull, and a resale value that reflects the boat’s true worth. Guarantees for treatments tend to be rare, but new coatings should have a service life of around 10 years
But why are these treatments so expensive? It’s mainly because they consume large amounts of skilled labour, and pricey materials, which all adds up to an average price of around £150-£200 per foot of waterline length.
We paid a visit to Southampton-based specialists Bare Marine to watch a Westerly Seahawk get the full osmosis treatment. The boat had spent nearly 16 years afloat in the warm waters of the Med, and the combination of warm saltwater and only brief periods ashore had grown a complete tea service of blisters below the waterline. She was in a bad way.

The Surveyor’s View: Tony Staton-Bevan

s Our osmosis expert takes a closer look at the stripped Westerly hull

‘When you find blisters on the underwater area of a boat, there are several things to take into consideration:
‘Firstly, how old is the boat? If less than 15 years old and from a reputable manufacturer, the condition is unlikely to be serious, although there is always the chance of a ‘Friday afternoon’ hull. If the boat is over 15 years old, and in most cases more than 17 years old, the hull is more likely to have been moulded using a polyester resin based on orthophthalic acid. From the late 1980s, most manufacturers changed to using isophthalic polyester or other resins with improved water-resistance. Used in conjunction with an outer layer of powder-bound chopped-strand glassfibre mat (not emulsion), these resins have proved very effective at reducing the amount of water ingress into the hull. This, in turn, has reduced the development of blisters. In fact, this layer has proved to be even more important than the type of gel coat used.
‘Next, the history of the boat is important. Is she wintered ashore regularly, or is she afloat for 12 months of the year, happily soaking up water? Has she been moored in fresh or warm water? Blisters develop much faster in these conditions than in cold saltwater.
‘All these factors will have a bearing on the degree of deterioration of the hull laminate, as the blisters on the surface may indicate deeper defects in the hull.
‘Where there are only a few blisters visible, my usual advice is: “Don’t panic.” The rate of any deterioration is normally very slow and any form of remedial work can usually be planned for the following winter.
‘Apart from the surveyor’s eyes and experience, the most effective piece of equipment we have is our moisture meter. Most of us use at least two types: one which will detect moisture in the outer layers of the hull, and one which will read more deeply – particularly important on cored hulls. The moisture readings, together with the blisters, the age and history of the vessel, all begin to form a picture from which a diagnosis can be made.
‘With more advanced blistering, significant deterioration is often found in the outer layers of glassfibre reinforcement. As this will have weakened the hull, it will need to be removed back to sound laminate using a gel coat planer. This will allow the eventual replacement with a new layer of laminate, before the application of epoxy paint.’
 Stage 1: the survey

How bad is it?
A surveyor will take a good look at the hull with the help of a moisture meter. His job is to see how far the osmosis has eaten into the laminate. He is particularly concerned about possible delamination, where the voids are so large that the layers of chopped-strand mat start to separate out as the resin loses cohesion. This weakens the hull considerably.

Topsides (left) gave a low reading on the meter

but the underwater hull (right) read much higher

The moisture meter is a major weapon in the diagnosis of osmosis, and will show how much water is trapped beneath the gel coat. The meters can be fooled, though. A boat freshly out of the water after a long time afloat will give high readings to start with, but these should fall away quite quickly. Water sitting in the bilges, or stored in tanks alongside the hull can also give false readings, as do wet clothes left in lockers – so bear this in mind before the surveyor calls.

 Stage 2: the work

Step 1. Prepping the boat
If the surveyor is convinced that the osmosis really needs treating, he is likely to recommend a hull peel. While a DIY repair is within the realms of an accomplished hobbyist, the strip and rebuilding is best done by the professionals, who have the facilities and the expertise.
The boat is fully prepared by removing all the internal furnishings, drying and vacuuming the bilges, emptying all tanks and installing a boat dehumidifier. Every nook or cranny that could harbour moisture is vented and dried out. Even the anchor chain is removed. The idea is to make the boat as dry as humanly possible, both inside and out.

Step 2. Peeling the gel coat
You can’t suck the water out through the gel coat, so it has to come off. Gel coat can be removed in several ways, including grit blasting, but the best method for a uniform depth is to use a gel coat peeler. Rather like a mechanical plane, rotating blades strip off material to a set depth. It’s a noisy, messy job, but will reveal a perfectly smooth substrate that can then be explored more closely.
After the planing, the waterline is ‘dressed’ with an angle grinder, so the new bottom will butt up seamlessly against it. The skin fittings are also dressed, although they are often also removed and later renewed.

Step 3. Slurry blasting and steam cleaning
The exposed substrate is then slurry blasted, as the planer tends to give a smooth finish which can seal up some of the smaller voids. The slurry knocks these open again, so the acids and salts can be freed.

The hull is subjected to rigorous steam cleaning every other day for the next two weeks. It took about four hours each time to do our 9.45m (31ft) Seahawk. The steam penetrates the voids and washes out the acids that have been reacting with the seawater. It also gets rid of salt, which will suck moisture back in.

The culprit – acids exposed by the gel peel need steam cleaning

Step 4. Check the readings
Using a Sovereign moisture meter, Bare Marine’s Sean Singleton then monitored the moisture readings after the hull had cooled from the cleaning. A ‘high’ reading is anything over 15 on the ‘A’ scale, and our boat was peaking at 25.
Normally, the drying process could now begin, but our surveyor, Tony Staton-Bevan, found that the damp had penetrated far deeper than usual and caused delamination, so a second peel was organised to cut back to sound substrate. This only happens in the worst cases, and can add considerably to the cost.

Step 5. Drying out
The planing and steam cleaning can be done outside, but to dry the boat it needs to be moved into the controlled environment of a shed. Some larger boats will have their own temporary shelter built around them.

Hot-vac pads remove trapped moisture by injecting steam (at 150C°) into the hull, then sucking it back out

The hot vac method is widely used by professionals, and involves attaching hot rubber vacuum pads to the hull. They heat up the trapped moisture by injecting steam at 150°C, turn it to vapour, and then suck it out so hard that it carries the undesirable solids with it.
The pads are moved around the hull in a carefully recorded order based on a T pattern, as no two pads should lie directly alongside each other. During this rotation, readings are taken and the moisture content written on the hull. In areas where the vacuum pads can’t be attached, infrared heaters are used. Only when all the figures have fallen to an acceptable figure will the rebuilding of the hull begin. This drying process usually takes about two weeks.
‘A popular misconception is that osmotic hulls become heavy with all the water they have absorbed,’ Tony Staton-Bevan told PBO. ‘In reality, even a really “wet” 9m (30ft) hull will only have soaked up about 14.5kg (32 lbs) of water – not enough to get excited about.’

Step 6. Relaminating
(if necessary)

Relaminating with biaxial cloth is a team effort

Wetted with epoxy, the laminate must be purged of air bubbles

Because part of the hull lay-up had been removed on our Westerly, it had to be relaminated back to its original thickness. This involves using 450g/m2 biaxial glassfibre cloth and epoxy resin.
First, the hull is rolled with epoxy resin. Then a cloth panel is carefully pushed into place and rolled flat, followed by another alongside and so on.
Finally, another coat of epoxy is rolled over the top, with the laminators careful to keep the shape of the boat. To make adhesion of subsequent coats easier, a coat of nylon peel-ply is rolled on. This can literally be peeled off once the epoxy is dry, leaving an abraded surface with no need for hours of laborious sanding.

On a less badly affected hull, the exposed substrate would still be treated with epoxy, to seal up all the voids, however small (some will be no bigger than pinholes).

Step 7. Fairing
This is the really skilled bit, which requires a good eye and a degree of stamina. The laminator has to mix up workable quantities of epoxy filler, and build it up over the relaminated hull to match the original shape of the boat and to roughly the same thickness.

Neil Surtees uses a flat trowel to lay on epoxy filler

Bare Marine’s Neil Surtees is a master at this, and uses the Veneziani system of epoxies and fillers, which is mixed at a ratio of 1:1. Once activated, it is applied with a large flat trowel.
‘The trick is to keep the filler layer quite thin,’ Neil explained. ‘After a while, you can tell how well it’s going on. You can feel the bumps and troughs.’
Once dry, the filler is then sanded back with a circular sanding machine – a long and careful process to ensure that the hull retains its shape. Neil prefers to use a large disc sander for the best results.

Once dry, the filler is sanded back…

The final touch is to run a screening board along the waterline so that the filler merges seamlessly with the rest of the topsides.

Step 8. Epoxy barrier coats

…before the epoxy paint is applied…

With the filler as fair as is humanly possible, now the epoxy outer layers go on. Epoxy resin is far more resistant to water than the old polyester gel coat it is replacing.
Just as with any paint system, the hull has to be completely clear of dust, using a vacuum and tack-rags. The epoxy is activated and then rolled on in a series of three coats. The coats are best applied wet-on-tacky for the best adhesion. Neil uses a roller followed by a tipping brush, and to ensure that brush marks are eliminated, the first coat is brushed out vertically, and the second coat horizontally. The third and final coat is finished vertically. The result is a smooth layer of waterproof epoxy 400 microns deep.

Step 9. Antifouling

The final step sees antifouling and a primer applied

Once the epoxy has cured, the skin fittings are replaced and all that remains now is to add two primer coats to act as an interface between the epoxy and the antifouling. Examining the Westerly as she waited for the crane, we were very impressed with the standard of the finish. The underwater hull had been very skilfully rebuilt, without a detectable ridge or depression anywhere
Although the repair had been more expensive than usual due to the need for relamination, the resale value of the boat has risen sharply. But the owners know that they have a good solid hull beneath them, perhaps a full five-year ‘goodbye osmosis’ guarantee, and no chance of a blister for the foreseeable future.

Prevention is best
‘To keep osmosis away, we suggest that the boat is wintered ashore, preferably with a dehumidifier or a heater running in the accommodation,’ Sean Singleton explained. ‘Thenosmosis should be gone – for good.’

When buying second-hand, it pays to get the boat surveyed and checked to see if it’s had osmosis treatment.

What does it cost?

With epoxy an expensive material, and many man-hours of skilled labour required, the prices for a full professional cure (without relaminating) work out at around £492 per metre / £150 per foot of LOA. For a more accurate quote, the treatment centre will apply a formula to work out the wetted area of the hull, and this can vary quite considerably between hull types.
There is always a risk with bad cases of osmosis that the costs could escalate if the problem is found to be far worse than was initially thought – the situation being that, having exposed deeper problems, it’s false economy to seal them in again. Once started, it has to be a proper job, but really bad cases are fortunately quite rare.

About the expert
Tony Staton-Bevan, a surveyor for over 30 years, is the author of Osmosis & Glassfibre Yacht Construction, which is currently out of print. Try second-hand bookshops or your local library.
Details of his company can be found at: www.abbeyyachts.co.uk