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From the Bilge


Category: General

I'm not sure how many fibreglass boats with osmosis that I have seen over my many years in the boat broking trade, but I can tell you that it would be several hundred.

The very word osmosis seems to strike terror, fear and loathing among boat buyers but it is, however, very unlikely in my experience, to be a serious enough problem that could put a vessel at risk. In other words, not a structural issue that could cause a boat to take on water into the hull and founder. Osmosis is usually only a minor problem that can be easily checked and handled when the boat is pulled out of the water for the annual antifouling job.

Most people would know roughly what osmosis is, but for those who don't, it 'shows' itself as blisters on the hull (normally under the waterline), these can be just a handful or perhaps hundreds.. The blisters are usually approximately the diameter of a fingernail and about one or two millimetres proud of the hull surface.

The next time you have your boat out of the water, clean off the marine growth with a high pressure cleaner, and while the hull is still wet, have a good look below the waterline. If you spot a slightly convex area, carefully prod it with a sharp spike (nail will do) and if a foul acidic smell is evident (it's a bit like the smell of a crushed ant), then your boat more than likely has an osmotic condition.

The generally accepted main cause of osmosis is that it originates during the 'laying up' process of the fibreglass when the boat is being built. Fibreglass boats are ideally built in controlled low humidity factories these days, to reduce the opportunity for moisture in the atmosphere to become trapped in the glass fibre strands.

Twenty or thirty years ago the risk of moisture was perhaps not as well known and boats with osmosis are often but not always, older boats.

So if small amounts of moisture does get trapped in the building process, eventually the result is a little blister.

Usually these can be easily identified and dealt with by carefully grinding a very shallow area around each blister to enlarge the area. This can then be carefully cleaned and reglassed, sanded, primed and painted. A competent trained shipwright ca do this easily or can show you how to do it yourself.

In very servere cases (rare), the upper level of gelcoat may need to be carefully shaved back one or two millimetres with a special planing power tool. The hull surface is then brought back to a finished standard with a new outer coating of glass and gelcoat. This can be expensive, but fortunately it's very unusual that this will be necessary and I have seen only a handful of cases in my many years in the trade.

So if you are looking at buying a boat, get a good marine surveyor to check the hull: there are also plenty of articles on the subject which can explain it better than I can, so if it's something that worries you, have a look on the 'net'.

If you have your heart set on buying a nice boat, don't let a little bit of osmosis spoil it for you, it may be quite safe and a simple matter to monitor it from time to time.

Happy Boating !!!

Chris and Lach