Lauren  Marine
Group Holdings

Woolston Southampton

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The following is a brief explanation of the condition usually referred to as “Osmosis” or the pox and a general outline of how I see the subject.

The original definition of osmosis is the biological process where by a liquid (usually water) will pass through a semipermeable membrane ie in this case the gelcoat. The gelcoat is permeable to the water molecules but not the various compounds dissolved within the moisture of the hull.

The term “osmosis” was adopted by the boat industry  in the early 70’s to describe the blistering found on many GRP boat hulls and is now in common use. The osmotic process probably does occur within the blisters but is not the only process involved and is not the full story.

A GRP (glass reinforced plastic) boat hull is a mixture of (usually) polyester resin reinforced with glass fibre matting, built up in layers. The final hull laminate will have an approximate ratio of 30% glass to 70% resin. This laminate is not homogeneous, even within a well built GRP hull there will be small voids, air pockets and microcracks within the Hull lay up and at the interface between the resin and the glass fibre.

Water can enter into, and through the gel coat and into the laminate (the polyester resin and the glass fibre mats) as water molecules, not the liquid form. So a boat hull can absorb a maximum of approx. 25% water in this way on a sovereign moisture meter . Water may pass slowly through a GRP hull in this way and disperse in the bilges as water vapour. The moisture content of a new hull will slowly increase during the first few seasons that she is afloat, the moisture content will similarly reduce slowly when she is out of the water.

As mentioned earlier there are various small voids within the laminate. The water molecules can collect and condense within these. Within the GRP laminate and the micro-voids various water soluble components will be found. These are solvents, locked in during the boats build process and manufactures constituents from the resin manufacturing process.
The moisture within the hull matting voids, is able to dissolve and chemically react with these components. This process is known as “hydrolysis”. “Hydrolysis” will continue with the voids enlarging, a dissolved solution is formed, the main ingredients being, acetic and hydrochloric acid and glycol. These products give “osmotic fluid” its characteristic are a vinegary smell  black or grey in colour and it has a greasy texture.

The glycol in particular is “hydroscopic” (water absorbing). Once this is released into the voids it will accelerate the rate of water absorption into the laminate. This process will now continue and will not be reversed by simply taking the boat out of the water. Moisture content will drop slowly if left ashore but will rise again fairly rapidly when immersed again. The various hydrolysed products cannot pass back out through the polyester gel coat / laminate but the water molecules can.
As this process continues, at some point, the concentration of the vinegary substance within the voids will become greater than the concentration of the water the vessel is floating in (sea water). At this point the “osmotic” process occurs and more water is drawn in and the apparent blistering will occur .

The interface between the glass fibres and the resin can also be broken down. The binder used on the glass fibres (particularly emulsion bound mats with polyvinylacetate binder) are water soluble. This can allow liquid water to pass along the fibre bundles, producing some swelling at the fibre ends and the characteristic “wicking” or “fibre aligned blisters”.
As this continues the voids are increased in size by “Hydrolysis” and the pressure within is increased by “Osmosis”. At some point the pressure may become too high for the surrounding material to support and a blister will be formed.

This all sounds fairly alarming, however it is important not to over react. Many boats are used for years in this condition, at this stage the processes are chemical with very little loss of mechanical strength of the laminate / or hull structure. Many boat hulls may take 10 - 20 years or longer to reach this stage.

As this process continues, moisture continues to be absorbed, the laminate break down 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. Eventually at this stage, treatment will be required

What to do next If you Think your Boat has Osmosis