Osmotic blisters

If there is one thing that most owners don't want to find when they haul their boat for an inspection it has got to be the dreaded hull blisters. Osmosis is the most often used term for this malady and is caused by water migrating through the outer layer of gel-coat and into, usually, the first layer of laminate which is often the chopped strand mat. Because water is required for the blisters to form blisters above the waterline although possible are rare and as was the case  with this particular boat the topsides looked great and it was only when the boat came out on the hoist that the extent of the damage became evident. The blisters in this particular case ranged from a half inch in diameter up to about one and a half inches, not the worst i have ever seen but bad enough to warrant a major repair.

In a nutshell water migrates through the surface of the hull where it is trapped in small voids that it finds in the laminate. The water then reacts with the styrene and forms an acidic fluid. When the blisters are pictured a smelly brown fluid is released which is the result of the water sitting in the hull for some time. If left unchecked the blisters will grow progressively bigger and permeate deeper into the laminate until the structural integrity of the boat is compromised. We are not at this stage with the boat shown in the picture above but the boat will have to be peeled and the bottom made good, a new barrier coating applied before finally being anti fouled. The cost to repair the damage to this 70 foot motor yacht was estimated at $25,000, not a small sum. If you have a glass fiber boat then be on the look out for blisters each time the boat is hauled and if you spot a problem occurring look into fixing it early. Small blisters can often be filled fairly easily but waiting until things get to this stage is not a good idea. If ever there was a time to use the old adage ' a stitch in time saves nine' then this is it.

Comments

[roberts]

Also, it is good to mark the blisters the moment you find them during the haul out. Blisters can seemily "dissappear" while the boat dries on the hard. Unless fixed they will simply return when back in the water.

[Mark Corke]

I fully agree and a smart plan indeed and one which works well for isolated blisters. But this boat was completely covered so there was no part that did not have a blister. Had I used a red crayon I could have just drawn a circle around the waterline as everything below that will need the peel treatment.

[DIY Boats]

It is a real pain in the ass (sorry for my expression) when you find osmotic blisters on your boat's bottom. It's a lot of work fixing it. And I mean a lot!