That seems like a hefty title, but apparently its needed. Start a conversation about osmosis, google it, read about it, most of the time no one knows the real deal. Trust the surveyor, who’s happy to look like an expert with a moisture meter in his hand, of trust the boatyard, who is happy to do an extensive repair, or trust the neighbor in the marina, whose boat has had osmosis for years and doesn’t like the idea of it losing value because of it. And most of all, why would anyone trust me?
Well, except from traffic to my site, I don’t have a reason to sell you anything.
Yes, I do surveys, and yes, I do repairs, but I am never ever doing osmosis repairs again. Not because I think they aren’t necessary, but because I have better things to do than grind gelcoat of an old boat. So here we are, a story about osmosis from someone who has done the repairs, who knows about composites, and doesn’t have anything to sell to you. And of course, If you need a reputable yard to do an osmosis treatment, you can always contact us, I know several personally I can recommend, and even more I won’t!
So, Osmosis, what is it?
Actually, this is the least interesting part of this article. Who cares, if it’s there, it’s there…
But for the sake of completeness, lets indulge in our curiosity for a bit. We are here anyway, but we’re staying at the surface. I am not a chemist, I am a boatbuilder. If we take a look at Wikipedia, it states: ‘Osmosis (/ɒzˈmoʊsɪs/, US also /ɒs-/)[1] is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of higher solute concentration),[2] in the direction that tends to equalize the solute concentrations on the two sides’
applied to the bottom of our boat, the forming of blisters… The not so watertight laminate is a permeable membrane, and due to the incomplete mixing of the polyester molecule strings, there will be some substances like glycol, which are hygroscopic, attract water. Slowly the water will get sucked through the skin, and there it might form a blister. It might not however…
Why does a boat blister?
Not every grp boat will form blisters. Sometimes a manufacturer is known for boats that blister, sometimes its series, but most of the time, its just individual cases. To find out more, we need to dig a bit to get in to the construction of a grp boat. And then especially in to the Hand layup technique.
To build a boat, first of all a gelcoat is applied in a mould. A gelcoat is a polyester resin, which has been thickened a bit with fillers such as chalk, pigments for the color, possibly some extra chemicals to make the surface harder, more UV resistant, etc. This is the shiny outer part of your boat.
Then a skin layer is applied. This is usually a thin mat, 125cm/4 ft. wide, made of chopped glassfibre strands, held together with a binder. Using a thin mat will prevent thicker glass strands printing through, and is easier to conform to the shape of the hull. The builder will wet out a portion of the gelcoat, put a piece of mat down, wet it out till the white glassfibres become see through.
After that it will become consolidated, all the small air bubbles will get popped by rolling the mat down with a special bubble bursting roller. It can have grooves, spice, a bristle, there are many forms. Following layers will be layed up and consolidated as well, up to the desired thickness. In this bubble bursting lies the secret of a good laminate.
Every percent of air will decrease stiffness by a lot! Not only that, earlier was stated the polyester itself is a permeable membrane. This means water molecules can slowly but surely get in to the laminate (more on this later). In a well consolidated laminate there are barely any voids for the water to hang around, react to the chemicals not a part of the polyester matrix, and attract more water. But what if there is a tiny void, where the water can sit, get acidic, attract more water, build up some pressure, attract more water, etc? Well, that is the start of a nice blister in the bottom of your well taken care of boat. How do those voids end up when the builder consolidates the laminate carefully and makes sure there isn’t any air in there?
There might be lots of reasons. Poorly trained ‘craftsmen’ , monday mornings of Friday afternoons, bad or no quality control, too much time pressure, etc.
End even then, some resins and binders are just simply more prone to the chemical processes that create the substances that increase the risk for osmosis.
‘wet’ hulls, surveyors, and their moisture meters.
You fell in love with a beautiful boat. You’ve called the number under the ‘for sale sign’, met the owner, and the boat is in a great form, clean, shiny, fair . To be completely sure, you’ve hired a surveyor before finalizing the sale. The surveyor turns the boat inside out, looks at everything carefully, knocks on the whole hull and rudder to check for delamination, and gets out the moisture meter. Bad news, the boat is in a great condition, but, the whole bottom is wet…
‘The problem is not the problem, the problem is your attitude about the problem’
No shit sherlock, the boat just came out of the water, of course it’s wet! The permeable polyester had months to collect moisture! If there are no blisters, there is no problem with a wet bottom.
Except: When the boat has had a previous treatment, then it might show that its failing.
Can a moisture meter detect problems? Yes, if handled well, like possible water ingress in a cored boat. Can a moisture meter detect osmosis? NO!
One more thing about wet hulls: There have been tests and it is known for the polyester matrix in your glassfibre boat to weaken when its wet. A wet boat is also kilos heavier! This, for me personally, is not a problem however. Decreased strength of the resin will make it easier for osmosis to blister the laminate and delaminate the fibres. It won’t affect the strength of the hull itself by much though, the strength of a composite will come from its fibres. The overall strength of a wet hull is therefore minimally affected, and well within the limits of the design.
Back to the moisture meter, is it completely useless in regard to osmosis? No, in contrary, it is actually a really useful tool for the boatyard. It can check if the boat is dry enough to start sealing it up. When the boat enters the facility, after removing the gelcoat a reference is taken. The yard will measure in grids of +/- 25cm/1ft to 50cm/1.5ft, and note the values on those spots. Every 1 to two weeks another measurement is taken, written down under the original value, etc. This gives a good idea how fast the boat is drying, and when its dry (value stops dropping) This also eliminates the mistakes which are often made, when there are ‘wet spots’ in the hull, which turn out to be metal inserts, tanks, etc. (here the value stays roughly the same while the rest of the boat is drying nicely)
Do I actually have osmosis?
Walking around a marina, or sailing club, I’m often asked ‘to take a quick look’.
somehow when people find out there is an actual boatbuilder nearby they’d like to hear an opinion, to either confirm what they are thinking or to disagree (I find it, even as a professional, nearly impossible to changes people’s minds whenever they made up their own mind first) Although I rarely work with grp anymore, osmosis is one of those ever-recurring things to have a look at. (Along with rudders, keel to hull joints, ‘leaking’ daggerboard cases, etcetera, etcetera.) Is it osmosis? Well, usually its just badly applied primer or antifouling. The blisters are not even under the gelcoat in most cases, that would actually make it osmosis of the expensive type.
For it to be osmosis you might have to pop the blister to find out. If you pop it and you find it to be in between the glassfibres, or the gelcoat and the glassfibres, then it is not a paint problem, but then it is the dreaded problem. If it is still in the paint, or when you encounter the relatively thick gelcoat layer, its no GRP problem. Usually the acidic moisture is still in it, especially when its just out of the water, and it has that peculiar smell. A bit acidic of course, but it may smell a bit like the resin (the styrene part of it) too.
My boats full of osmosis blisters, is that a big problem?
Hard to say. A few blisters in the outside skin of the boat wont ‘sink’ it. Nor will it do a lot to a cruising boat which is quite over engineered and built. It might make the boat less worth though. The problem becomes more severe, and might affect the strength when it causes significant delamination. I wouldn’t be very happy with blisters big as my hand near the keel, rudder or even chainplates, would you be? Even in such a case the boat is probably a lot stronger than its needed, but as always with such things, you’ll find out at a time you don’t want to. This kind of delamination due to blistering is very rare however, and usually takes a long time to develop. In case of doubt, get some qualified assistance!
Big blisters in thin skins do affect the strength and stiffness as well, so with a cored boat the need to remedy osmosis is higher and sooner than a full glass boat.
Repairs and their pros and con’s
If you made it this far in the story you’re very interested, or still very worried.
Can osmosis be repaired? Yes and no:
If there is a chemical reaction taking place in the laminate, it will stay there. We can’t remove the chemicals responsible for the blistering. If the laminate is of poor quality, we cant replace the whole hull either, well, not in a cost effective way.
What can we do?
An owner might decide to just repair the blisters every winter, taking it as just another job on the list. I get it, there is more to do, the boat can handle it. Grind them out, fill them with an underwater epocy filler, prime and antifoul. Easy, cheap, fast. Next year, few more blisters, few more repairs, who cares?
Personally, if my boat would blister that slow, only a few per year, I’d do it that way.
A more permanent way, and advisable if its more than a few: Repair the delamination caused by the blistering, and seal the bottom so it’ll stay dry (No moisture, no more blisters!)
As described before, the gelcoat has to be removed first, possibly a layer or 2 (1-2 mm) of the laminate as well and the hull needs to be dried. Dry hulls have better bonding with the resins used to repair the damage, so it is usually required by the yard to give a full guarantee. Blisters might reappear just by attracting water what is already in the laminate if a wet hull is sealed, it will be limited however, but the boatyard can’t give any guarantee on the work carried out is the hull is not dry.
There are many ways to remove the damaged skin, with special planers, or just grinding it with sanding discs for example. No fun job either way! But, the problems with the repairs start there and then. Earlier the right way to measure a hulls moisture content was described. Some yards don’t, just guess. Or take readings from the meter once, compare it with the deck for example, and try to determine if the boat is dry. Many boats have been left to dry for years that way, all the while collecting storage fees for the owner without getting any drier. Hotvac (vacuum and heat) methods could be used to speed it up, but a winter outside or even inside a heated building should be enough. Here comes the ; el-cheapo, crap yards who have no clue’ method: Slap some epoxy primer on it! I am not a fan. Most primers contain solvents, which evaporate during the cure, leaving tiny channels. Not watertight at all. Mathematically, if you apply enough (5 layers) every channel is closed off by another layer. That’s why most brands advise 5 to 7 layers. Yep, great, still not a fan. It’s a very thin primer layer, easily damaged, and you probably have to fair out the 1 to 2 mil you’ve just shaven off it.
In my eyes, there is only one way that is worth doing: new laminate, either with vinylester, or epoxy, both about as watertight as possible. One could just coat the bottom with the resin, a layer or two, but that’s cheap and dirty, works great, not pretty. Better is to re-laminate with a layer or 2 of glass. Either woven or stitched materials for epoxy, and preferably chopped strand mat (csm) for vinylester. This gives a nice protective new skin on your boat, which is strong enough to survive some less than pretty situations involving scratching, and can be sanded without jeopardizing the watertight qualities. It’s just so much more durable, and will protect your boat for the rest of its lifetime. Most boatyards only give 5 years of guarantees on the ‘primer’ method (Guess why!). I know boatyards which give a lifetime guarantee on the ‘new skin’ method, not worried at all.
How to avoid blistering?
In modern boats blistering is almost completely eliminated. Resins have been changed, craftsmanship and quality control are implemented, and even building methods have changed.
Vacuum infusion is being implemented by more and more yards, eliminating air in the laminate, and therefore minimizing the risk of blisters. Vacuum infusion also minimises emissions of styrene and other chemicals, and improves the workshop to a cleaner and healthier place. Vinylester skincoats are used in some boats, minimising the water intrusion in to the laminate, again minimizing the risk of osmosis. Styrene free gelcoats have recently entered the market, also providing barriers.
If your boat has been in the water for a long time, there is a good chance there wont be coming any blisters if there aren’t. Winters on shore where the boat dries out a bit, sometimes reduces the chance of getting osmosis, depending on your bottom paints. After spotting a blister or two, a full repair is usually unnecessary, but prevention would be in order(most of all to prevent further ongoing work, and depreciation). This is the place for epoxy coatings, if you’d ask me. There are several on the market without solvents, being the coatings of choice, but plenty of layers (minimum 5) of a comercially high build epoxy coating should suffice.
Buying a boat which has or had osmosis.
Would I buy one? I wouldn’t have any problems with it. As a boatbuilder, I’d prefer to have an unrepaired one, just because all the messing around with it thats going on. Depending on the damage I would spot repair, and epoxy coat it, or have a full repair done by one of my partner boatyards. And never worry about it again.
If a repair had been done, its important to ask where it has been done, an by whom. Are there pictures available? Info on the materials used? After reading the article you should have a good idea on the pro’s and cons of the different repair methods, and where they should, and should not have been applied. Surveyors only check the current state of the boat, it might have just had a bodgy repair job, looking great, but prone to a lot more blistering. A lot of yards only give 5 years of guarantee, with winters on the hard, and a lot of extra requirements. About the time needed for new blisters to appear… Ask for a lifetime guarantee, or at least over 10 years. That way you know the yard has confidence in their ability to do a good repair and keep blisters away, and have knowledge and understanding about the subject.
Here at the end a little disclaimer. Most people I know in the marine industry try to do their best for their clients. This might involve choosing a less good, but more cost effective method, or , at the other end, Overkill. Try to have an open conversation with your boatyard about the solutions they will provide to you, and what you expect of the end result! Inform yourself, not only about the subject, but about the people you are doing business with as well. A reputable business is no guarantee for a good result, but its more certain then expecting a good result from a business with a bad track record. I consider myself knowledgeable on the subject trough study and experience, but others may have different views and experiences.
English 
Nederlands