What is hull osmosis? and how to repair it!

If you own an old boat, then you are probably familiar with what “hull osmosis” is.

And if you are interested in buying old boats, then you surely should know generally what is “hull osmosis”  or in other words “blisters”.

Generally, when you lift your boat up on a shipyard, you see small bubbles forming on the bottom of the hull (sometimes even above the waterline), it is usually water absorption under the gelcoat of the boat and sometimes into the fibreglass. Gelcoat is a upper layer of the fibreglass, which is made to be shiny and glossy to give it the white look.

So basically, Fibreglass is not the ultimate water proof material as it was thought to be. There is no such thing as water proof fibreglass. But it is water resistant. Through out a period of 10 or 15 years, the boat’s fibreglass absorbs moisture and water into it. This process is called as Osmosis. As the water enters the hull. This water makes the outer layer of the boat’s hull to swell and form bubbles, Which are called as Blisters. When you poke a knife into these bubbles, An acidic liquid oozes out, which smells like vinegar.

So, when the water penetrates the gelcoat and goes into the fibreglass, it becomes acidic when it reacts with the chemicals in the fibreglass, and the acidity of this fluid causes the hull to absorb more water, Thus creating larger chain of blisters or “osmosis bubbles”.

So the question is why and how the water can get into fibreglass which we thought was water proof.

Well, as mentioned earlier the fibreglass not water proof. It is a mixture of glass strands or mats, and polyester resin which has tiny microscopic openings that let the water in and cause reaction with the chemicals with in.

This happens more to the older boats. Because in older days, they used polyester resin which was less water resistant compared to the later version of polyester resin. Of course, the later version was more expensive.

Nowadays boat manufacturers use vynlester resin, which is said to have very high water resistant properties. Some manufacturers boast that usage of vynlester resin eliminates all chances of osmosis.

Vynlester is however, the most expensive of them all.

Therefore, most manufacturers (almost all) use vynlester resin as top layer of the fibreglass and the inside layers are made of polyester resin. This protects the hull from getting blisters, and does not cost a lot more than having the whole hull made with vynlester resin.

So the fact remains, If you should panic when you see Osmosis on the boat you are buying.

The answer is, If the boat is only few years old, then you should be calling the deal off unless you are getting a very good price. If the osmosis appears on a boat which is over 10 or 15 years of age, then its a common issue and no reason to panic.

So what is the real damage that boat osmosis can do. surprisingly, the main issue is that it is ugly.

And if your boat is severely damaged with osmosis, then it has only absorbed 2% to 3% water of the boat’s own weight. Therefore, it mainly slows the boat down a bit.

99% of the time, even severe osmosis does not cause any structural damage to the boat. Therefore it is safe to say, that the boat will not sink or breakdown solely because it has osmosis.

What you should be concerned about is that, if the boat is in such shape due to negligence from its owner or defects in manufacturing, and therefore you should be surveying your boat carefully to find any other serious negligence or manufacturing issues.


So how to cure osmosis ?

the cure is simple, but not cheap. Basically, you have to lift the boat up, and keep it in a dry place until all water that was absorbed has dried out. But that will take a long time.

So usually a shipyard in Hong Kong, will sand down the gelcoat and expose the boat’s fibreglass, cover the bottom area with plastic, as if its curtained “see the picture” and have blower fans faced to hull inside this plastic curtain to remove moisture. This is done continously for about a month. After the hull is relatively dry, they further sand the area which is affected by osmosis, and put epoxy, or in some cases fibreglass with epoxy on the affected areas, and re-gelcoat and repaint the bottom of the hull.


This is by far the most effective way of fixing the problem. Of course some shipyards will use heater fans, and some will use new technology of hotvac covers to dry up the hull. These methods of course sound very good but are costlier. They also do not guarentee total cure and protection in the future. You can get into never ending details on how to fix these problems, but they will cost you a lot more in labor, time and shipyard storage costs and in some cases may not be worth it.


There are usually 2 options, Either you can sand down and repair the fibreglass of the entire bottom hull of the boat or you could just repair the infected parts. The price difference for a 50 footer boat is usually 60,000 HKD (8,000 USD) for repairing infected parts only. Or  about 200,000 HKD (25,000 USD) to fix the entire hull. So there is quite a bit of difference in the price.

I usually recommend to go for a cheaper option, if the boat has limited patches of osmosis. Eg. Few big patches. This is because, there is no guarentee that osmosis will not show up again even if you cure the entire hull. Usually according to our experience, the boat will remain in a good shape, if you cure just the bits that are infected and take good care of the hull by lifting it, cleaning it and antifouling it regularly.

If the boat has many many small patches of osmosis spread through out the hull. Then ofcourse the repair of entire hull is recommended. Because it will cost you almost the same amount in labour and materials if you start repairing very small bits on the entire hull.

The most dangerous types of boats to have osmosis are the ones which are built for speed and have thinner hulls, basically because there is not much room to repair a thin hull and it may weaken the boat’s hull tremendously. Also, if you know that your boat’s hull was built using vaccum lamination technology, then you should know that your hull will be thinner than other boats of its size which are built using the traditional handlaid fibreglass procedure. This is mainly beacuse the new technologies are trying to make thinner hulls to reduce the boat’s over all weight for achieving high speeds. But overall, if you do see osmosis damage when you lift your boat. Then there is no reason to panic.

 Thank you for visiting www.asia-boating.com

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