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Osmotic blisters
In the boating world the word osmosis has come to mean a particular condition, whereas it more accurately describes a process that, in other contexts, is absolutely essential to life itself. Animals (including us) rely on it to control the fluids in our bodies, and plants to extract water and nutrients from the soil.
More specifically the word describes a situation where a solvent passes through a semipermeable membrane from a less concentrated solution into a more concentrated one. A solution, incidentally, is a solvent containing dissolved substances known as 'solutes'. Yes, I know. Brain-ache time again.
So, let's put it into context. In our case the solvent is water and the semipermeable membrane is the gelcoat.When a boat is first launched, water passes slowly through the gelcoat and starts to dissolve loose chemicals – mainly glycols which are strongly hygroscopic, readily attracting the water, and also the polyvinyl acetate binders used to make some glass reinforcements easy to handle. The combination of the water and chemical solutes form a high concentration solution, and an osmotic cell is born. For now we have the classic scenario. Driven by the osmotic process, the struggle to equalise the concentrations will continue inexorably, drawing yet more water from outside in an attempt to dilute the solution within. The osmotic pressure created is quite considerable – quite enough to bulge the gelcoat outwards into those wonderful blisters we all so love and admire.
Not all polyester resins are equal
At this point it’s worth making some distinctions between polyester resins. For day-to-day constructional purposes the normal laminating resins are known as ‘orthophthalic’. These were developed first. They have good strength characteristics but are relatively permeable. Up until the late 1980s these were used for both gelcoats and laminating. Then along came ‘isophthalic’ resins which are just as strong but more resistant to water absorption, and –you’ve guessed it – more expensive.
Get extra keen discounts and beat expensive shipping costs by buying books through this site! The free delivery service applies to most established destinations worldwide. The upshot of this development is that hulls moulded from the late 1980s onwards are likely to be inherently better protected against the onset of osmosis than those built before.
But don’t despair. Unwelcome though it is, osmotic blistering isn’t the end of the world. The process advances very slowly and only rarely reaches the point where it becomes structurally detrimental. However, the effect on a boat’s value can be dramatic, possibly slashing thousands of pounds off its worth on the open market.
Prevention and cure are subjects for other articles.
This article is adapted from the RYA Boat Maintenance Handbook due to be published in 2012
Related reading: Waterline Stains
Boat maintenance Osmotic blisters
