Boat Osmosis Explained

Osmosis quite simply is the physical process by which a liquid from a weaker medium is drawn through a semi-permeable membrane into a stronger liquid medium. So what does this mean and how does this relate to boat hull damage?

Osmosis blisters are not only a cosmetic problem. They are the visible sign that the hydrolysis of the polyester has affected at least one laminate layer. By the time the first blister shows, hydrolysed alkali products will have reduced the bond between fibre-glass and polyester deep in the laminate below the water line!

It is known that some brand new GRP hulls will start to absorb moisture through the gelcoat almost as soon as the boat is launched and this will gradually increase the longer she is afloat. Initially this moisture causes very little damage as it passes slowly through the laminate.

A GRP boat hull is usually a mixture of polyester resin reinforced with glass fibres, created in layers which forms a laminate. This laminate will have small voids, air pockets and microcracks within the resin and at the interface between the resin and the glass fibres.


Water may pass slowly through a GRP hull in this way, or the water molecules can collect and condense within these voids. 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


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 breaks down and accelerates and more blisters are formed. In time some larger blisters may develop within the laminate as well as those more commonly occurring between the gel coat and laminate.

A simple explanation is that the process can be thought of as a similar process to acne which is caused by a pore becoming blocked.


Hayling Yacht have always been supportive of all of our projects. They have always, at every level, been great to have in your corner.

Alan Priddy

Round the world yachtsman

Excellent service, they fully understood the problems with my boat. Gave me a quote and a timescale and met both.

Graham Smith

Osmosis Client