Boat Osmosis

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Information about Boat Osmosis

What is it boat osmosis  ?

Fibreglass osmosis  is the process of a lower density liquid moving into a higher density liquid through a semi permeable membrane.

In lay mans terms it is a natural process where two dissimilar liquids try to mix. the weaker trying to mix with the stronger. This effect results in a chemically induced pressure rise in the area of the high density liquid.

A Glass Reinforced Plastic( GRP) boat hull is a made up of a mixture of polyester resin reinforced with glass fibres, formed in layers which make up a laminate. This laminate has small voids or  air pockets and micro cracks within the resin. These occur at the interface between the resin and the glass fibres. Water diffuses into and then passes through the gel coat and the laminate in the form of water molecules, not a liquid. Water may pass slowly through a Glass Reinforced Plastic hull in this way or the water molecules can collect and condense within these voids. These are the blisters and you you burst one it will smell of strong vinegar. Within the voids/laminate are various water soluble components. These are solvents and unreacted constituents from the manufacturing process.

The water within the voids dissolves and reacts with these components. (Hydrolysis). The ongoing (Hydrolysis) will continue within the voids enlarging the cavities and forming a solution. The solution is water absorbing (“hydroscopic”) and once the solution is released in the voids it accelerates the rate of water absorption into the laminate. This process cannot be reversed simply by taking the boat out of the water. As this continues the voids are increased in size and the pressure within increases. At some point the pressure may become too high for the surrounding material to support and a blister is formed. As this process continues, moisture continues to be absorbed, the laminate break down accelerates and multiple blisters form. In time some larger blisters may develop within the laminate as well as those more commonly occurring between the gel coat and laminate.

Eventually at this stage, treatment will be required as the structural integrity of the hull can be effected.

If you inspect a hull yourself you can detect them. Look closely at the example pictures I have put on the page. However it is better that a skilled person does this for you. Any  professional boat surveyor will recognise the early signs of boat osmosis.


In the Mediterranean Sea a boat is usually safe from osmosis if it has been anti fouled correctly and not left in the water for more than three years(rule of thumb only)

No yacht has sunk from boat osmosis blisters,  not that I am aware of anyway, but big boat osmosis blisters weaken the laminate and ultimately affect the seaworthiness of the boat.

Boat osmosis treatment cost

The cost of boat osmosis repair can be high 6000 pounds for a 35 footer and five months out the water.

Boat osmosis repair

The water and the contaminants in the laminate have to be removed and whilst drying will remove the water it leaves many of the contaminants behind so for this reason steam cleaning and washing the hull surface is important but not sufficient. Depending on how big and how many blisters you have, the decision on how much work is down to you. A few isolated blisters can be carefully cut off with a chisel, washed out and filled with epoxy resin. Even large blisters can be treated this way.


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Boat osmosis blisters


Peeling the gel coat is common but extreme and on its own often adds to problems. Blisters will return, even after peeling, if there are still voids in the laminates and uncured gel coat,  more blisters will occur. So if you are the owner and don’t intend to sell, my personal view is deal with them individually as they occur. If you are planning to sell, yes the fact is, boats with untreated osmosis are harder to sell, so you will need to get a professional, warranted job done.

The gel coat is a very effective water barrier and does not allow the passage of glycols so it has to be removed usually by the use of a “Gel Peeler”. This removes a controlled thickness of gel coat/laminate leaving an even surface.

To promote drying and to abrade the surface left by the “Gel Peeler” the peeled surface should be “grit blasted” after peeling, for maximum effect this work should be done immediately after the end of the season. Thereafter the hull should be steam cleaned and washed regularly for a period of time to ensure that the solutions are washed out. At this stage, I would recommend HOTVAC treatment. This cures the uncured resin and reduces he chance of re-occurrence. When the surface is found to be neutral, the drying process can commence. Initially this can be air dried but eventually the hull will need to be heated to reduce moisture content to a very low level. Once the hull is dry a new coating can be applied.