Osmosis in Sailing Boats

Have been reading up more about osmosis in GRP boats. I need to be able to diagnose it if I am to avoid buying a boat that has it. The more I read the more reassured I am.

Blisters? yes one or two but is it Osmosis?

Essentially, (let’s not get too scientific here), Osmosis is like a piece of string with one end in a saucer of water, the string acts as a wick and although most of the string is out of the water, it still gets wet along its whole length. As far as GRP boats are concerned the standard gelcoat is not 100% waterproof, and unless a great deal of care and attention goes into the manufacture of the hull, there may be hollows (voids) within the layup. So, water can get in. But that’s not all. There are certain chemicals binding agents and release agents used in GRP hull manufacture which can also end up within the layup, filling some of these voids. The result is that if you have fluids either side of a membrane, i.e. water on one side of a gelcoat, and other less dense fluids on the other side, there is a tendency for the denser water to pass through the membrane and mix with the lighter, less dense fluid on the other side. Over time, the water inside the hull reacts with the chemicals and causes blisters. If the blisters break, more water is admitted into the GRP layup, causing more blisters, admitting more water … well you get the picture.

So, over time GRP can soak up a considerable amount of water making the hull heavier and weaker. The cure, depending on how badly the boat is affected, can be to peal off the gelcoat, dry the core and then reapply a new gelcoat. A small repair might be within the skill set of an amateur. A bigger job might require professional expertise. A preventative measure might be to coat the existing underwater aspects of the hull with a more water resistant material such as epoxy.

Some GRP boats never suffer from Osmosis

The good news seems to be that despite the panic which ensued when it was realised that GRP was not the ‘no-maintenance’ material it was originally thought to be, there have been few (if any) reports of boats coming to grief because of it.

The other good news is that, although it is wise to inspect any blistering on a GRP boat hull, not all blisters are caused by Osmosis and some blistering is relatively easy to cure. So how to tell the difference?

Osmotic Blistering

Blisters caused by Osmosis will be found around or under the waterline. They may be small pin pricks or large swellings as big as the palm of your hand. It would be unusual to find osmotic blisters on a boat until she had been immersed for several seasons. Puncture an osmotic blister and there will be fluid behind it. Fluid in itself is cause enough for concern but if the fluid has a pungent vinegary smell, the indications are towards osmosis. The smell comes from acetic acid, due to the hydrolysation of the emulsion used as a binding agent in the manufacture of chopped strand mat.

You can confirm your diagnosis by using PH paper. A pH value of less than 7 indicates acid. Readings of between 5.5 and 3.0 are typical when water and binding emulsion have joined forces. This would be indication enough to call in the experts or consider whether you really want to pursue this purchase.

Non Osmotic Blistering

This can be found in the same areas as osmotic blisters, in clusters along or under the waterline. They tend to be small pin pricks – rarely large. They tend also to be much tougher than osmosis blisters. They can be very hard and difficult to break. Most importantly though, when you do break into them they are dry and there is no smell.

They can occur for two reasons, either:

air was trapped between the gelcoat and the inner mat during manufacture – and the voids have enlarged due to temperature changes;

there is a fault in the gelcoat due to the absorption of  water into the pigment or extenders used in the gelcoat when it was manufactured.

Unlike Osmosis, this problem is more likely to surface within a few months of launching. The cure? Strip off the antifouling, degrease and sand the gelcoat, fill the holes with epoxy and then apply an epoxy paint and re-antifoul. A serious job – but not a disaster. – something a DIYer could probably cope with

Seaward