Boating Advice: Osmosis

Over 27 years in the industry, there's one question that pops up on every boat buyer's or boat owner's lips and that is: "Is osmosis an issue on my boat?" Or "Should I buy a boat that has blisters?"

Let’s burst the first bubble, most forms of what is commonly termed 'osmosis' is in fact hydrolysis, where the hull becomes saturated, dissolves and causes blisters.

But I hear you say: "My boat is made of fibreglass, resin and gelcoat, surely it can’t absorb water?" Sorry to shatter you, but all these components are porous to a degree and will absorb water or retain moisture.

Osmosis is the blistering process, which concentrates solvents within the blister void that dissolve the plastics. The amount of fluid contained in these blisters is often so small it doesn’t threaten the laminate. If you have been around when a marine surveyor or boat builder pops one, you will notice the pungent smell and acidic nature of the fluid.

So now we have a basic handle on hydrolysis and osmosis, but let’s keep it simple and refer to it all as osmosis as this is not a science lesson.

Do I buy or walk away?
If you’re buying a boat that is more than 10 years old, the percentage is high that it will have some sort of blistering or osmosis.

Let’s face the truth, boats live in water and, as we’ve mentioned, the materials used are porous, so science will come into play.

The majority of cases that I have seen are related to the gelcoat and tie layer only. The tie layer is the first layer of chopped strand mat (CSM), usually 225gms/sqm or 450gms/sqm used to tie the gelcoat to the main hull laminate eliminating shrinkage and print from the heavier structural laminates to follow. More often than not the voids are there from birth and the immersion process causes these to form on the surface.

It is quite common for these to be about 5mm to 10mm in diameter and, in many cases I have witnessed, these are only on one side of the hull, or restricted to isolated areas. This leads me to believe the process started in the manufacturing stage. Perhaps it's a case of one laminating team taking more care than the other to ensure the laminate was saturated properly?

When these blisters are popped, they contain no fluid that we have previously mentioned. They look dry and more often than not the blisters have occurred because of poor laminating.

When we get to larger blisters, say 20mm and beyond, it would be fair to say the osmosis process is in play. When these are popped they contain the pungent acidic fluid that identifies the osmosis process.

Boats over 10 years of age are the ones that might have this affliction in some form or another.

Over the years I have found the older the boat, the less likely this will affect the sale of the vessel. The biggest issue is that the internet has made everyone an expert, so everyone knows boats have osmosis and it's all bad news! Well it's not...

Beneteau bought out the BWS (Biosphenolic Water Shield) to combat its bad reputation for osmosis in the Eighties. This was just a layer of Vinylester resin used with the CSM in the tie layer. It was believed the vinylester resin was less porous, hence the issue was resolved. Although it minimised and reduced the cases, the issue was still there and in my opinion it is back to the original hand-laminating faults in the tie layer.

In the modern era, many boat builders have moved to cored boat constructions, high-density foams and balsa-cored constructions. In the early days, boats were just solid glass construction and as the capabilities of the products where largely unknown, these were over-built, hence why in my opinion we don’t have many cases of total destruction from osmosis.

On these new-age constructions, however, the osmosis or blistering is a concern because the laminates are reduced. Any blistering is not only isolated to the tie layer, it is very close to the structural laminate and core, therefore playing with the integrity of the laminate. Once the laminate is penetrated, the core becomes saturated and the integrity has been seriously diminished.

So should you stay or should you go?
My answer is what I say to all potential buyers and owners alike. I don’t know of and nore have I been able to find a case where the structural integrity of a hull is jeopardised because of osmosis on a solid0laminate constructed hull. As mentioned, it’s isolated to the tie layer not the structural laminate.

Size and locality matters if we have multiple 50mm or larger blisters located close to each other, so close that they are almost overlapping. This is when you need to consider walking away. If you are a serious yacht racer, then the uneven surface could affect performance more than the integrity of the hull.

The modern cored-constructed boat with blistering or osmosis needs careful consideration and, as this is still relatively early days in construction compared to solid glass laminates, it is something I would consider walking from if I was buying the boat.

Should I have it repaired or not?
Repair yards have made a huge amount of money worldwide on the word osmosis, so this will be touchy ground.

My advice is simple: all cases should be dealt on a case-by-case basis. Get an unbiased opinion, not just from the repairer, most just see the big dollars.

Remember how the issues develop and bear in mind that nobody can honestly guarantee that the process won't return. If you can get it in writing from the repairer please send it in and we'll frame it!

If the boat is over 10 years old, you need to really think about the value of the repair compared to the value of the boat. Is the boat worth enough to warrant the repairs? In most cases it’s not, but it does become more attractive at the time of sale.

As mentioned, the yacht-racing fraternity will want it fixed if you are serious, as the uneven surface will cause drag.

If you have a cored-constructed boat, the landscape changes dramatically when you have blistering or osmosis evident. Seek professional advice and further investigations are required to determine how deep the issue is. Core samples maybe taken to determine the real extent.

Keep in mind the majority of boats over 10 years of age will have some blistering of sorts evident. More than likely, in fact in the majority of cases, the blistering has already occurred and is unlikely to worsen. Good news indeed.

*Gavin Clark is a contributor at boatsales.com.au and BoatPoint.com.au who writes under the pseudonym 'The Surveyor'. He is a shipwright and marine surveyor with a business called Slipstream Marine based in Sydney. He has 27 years experience in the boating industry, is an avid boater and owner of a 33ft Sharpie (trawler). Gavin can be contacted at http://boatsurveying.com.au