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Osmosis on boats and yachts: a few facts and fables

GRP, Glass Reinforced Plastic, or rather better FRP (Fibre Reinforced Plastic) is probably the most perfect material to build boats with: it’s light, fairly strong, relatively cheap to fabricate, we can make the most fantastic shapes with it and it’s low maintenance. But it’s not all roses and moonshine.


A number of moisture measurements can cause a panicky reaction among boat owners when osmosis is detected.

Moreover, even after 50 years since this material has been used in shipbuilding, the most diverse misconceptions about osmosis in boats and yachts are still circulating, even among ‘specialists’.


Good video on osmosis and its destruction!

What is osmosis?

Osmosis is, simply put, the transition of a liquid through a membrane that is slightly permeable to the other side of the membrane where there is another liquid.

This phenomenon occurs in a certain direction: from the one with the least density to the one with the highest density. Trees use this method to ‘suck up’ water.

How osmosis occurs on hulls of boats and yachts

However, we often find that small parts of polyester are not perfectly cured (this can happen by applying the ‘mix’ too quickly, for example) and that there are also small air bubbles in the polyester (these are normally pushed out with a roll during construction). If these products come in contact with water, they will bind with the water and an acidic solution will form, which is denser (more concentrated) than the original product and which in turn will ‘suck up’ much more water.

Great DIY job on a budget!

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