Osmotic blistering of the hull, or what used to be known as the dreaded boat pox, affected about one in four boats, although fewer modern boats are affected now. It’s caused by the migration of water vapor through the gelcoat into the laminate of glass fibers and polyester resin that we call GRP, or glass-reinforced plastic. The blisters usually manifest themselves in the outer one-tenth of the GRP.
Blisters come in all sizes, but luckily most of them are fairly small. Consequently, most cures involve nothing more than drilling out the blister site with a conical bit, letting the hole dry out, and then filling it with epoxy and filler.
If you’re really unlucky and have a more severe case of blistering, the gelcoat will have to be removed — peeled off by a professional, usually. The hull must be dried thoroughly, often for several months, and the gelcoat must be replaced by coatings of epoxy or vinyl resin.
Bad cases of osmosis are comparatively rare but they are expensive to fix — often amounting to 50 percent of the boat’s value or more — so the resale value of an affected boat is low.
Although we now take a more casual approach to osmosis, it’s best not to let things get out of hand. Examine the underwater hull every year for osmosis. Catch it quickly, while the blisters are still small and fix it.
Today’s ThoughtBuild me straight O worthy Master!
Staunch and strong, a goodly vessel
That shall laugh at all disaster,
And with wave and whirlwind wrestle.
— Longfellow, The Building of the Ship
TailpieceA grateful importer wanted to show his appreciation to a Customs officer who had smoothed the passage of a large consignment of exclusive French perfume.
“Here, I’d like you to accept a large bottle of our most expensive scent,” the importer said.
“Sorry, sir, but the regulations don’t allow us to accept gifts.”
“No problem,” said the importer. “Tell you what. I’ll sell you this bottle for 25 cents.”
The Customs man looked at him thoughtfully. “In that case,” he said, “I’ll take a dozen.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)