Osmosis, the pimple of the sea! Put simply, Osmosis occurs when moisture makes its way through the gelcoat and becomes trapped within voids in the fibreglass laminate. Commonly found below the waterline on fibreglass boats and can also occur under outdoor cushions such as sun pads on the bow where moisture can not evaporate and ultimately penetrates the gelcoat.
What is osmosis and what does it look like?
When moisture passes through the gelcoat of your boats hull, it reacts with solvents and uncured resins that are trapped within voids in the fibreglass laminate. In time, the continued moisture absorption and chemical reactions cause increased pressure in these voids. As a result, a blister forms on the surface of the hull. These blisters are referred to as osmosis.
Blisters are much easier to see on an antifouled boat when the hull is wet. All boat hulls contain some amount of moisture however visual blisters are a sign of advanced osmosis.
Extensive osmosis on a boat that has been in the water for years with little maintenance.
Early detection of osmosis
Early detection and treatment of osmosis is essential. Blisters start small and may not even form a lump on the surface but with a keen eye, you can catch it early and make isolated repairs.
The yellow arrows show small bits of antifoul missing for no apparent reason. Whilst these areas did not look or feel raised, the blister is small and has raised just enough to pop the antifouling off the boat yet not raised enough to see or feel the blister.
Take note of the red arrows. Stress cracks in the antifouling due to a blister forming below. Very minor but less water in the laminate means less drying time and quicker repairs.
Whilst some owners may choose not to do repairs at this early stage, these areas will only absorb more moisture, creating bigger blisters and larger repair bills in the years to come as the boat will need to sit on the hardstand to dry the laminate prior to repairing.
Grinding out osmosis
Osmosis blisters need to be cleaned out to solid laminate so that they can dry and repairs can be undertaken. This can be a little daunting as the water trapped within the hull can cause delamination of the initial layer / layers of fibreglass laminate. Making the repair much larger than the original blister.
The delaminated layer of laminate is ground away to ensure repairs can be made on solid, dry laminate. A small blister can easily become the size of a 50c piece or larger.
While we aren't going to touch on osmosis repairs in this blog, finding osmosis on your boat isn't the end of the world. Repairs can be undertaken successfully so that you can enjoy many more years of fun on the water. But remember, If in doubt... grind it out!
Boats live in a harsh environment so be vigilant, and haul your boat out of the water every 12 months (minimum) as part of a preventative maintenance program.
If you have a boat on the Gold Coast and want your antifouling, Propspeed and Anodes done the right way, why not give us a try? You won't be disappointed.
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