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Eliminate Osmotic Blisters In Your Hull

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Illustrations by Steve Karp

Hotvac Illustration by Steve Karp

Adore A Vacuum?

You just might. Hotvac claims this one can cure hull blistering once and for all.

The Scots, as it turns out, have a knack for innovation. After all, what would a Sunday morning be without golf? Or a Friday evening cocktail party without whisky (no “e”). And where would the world be without Dolly, the world’s first cloned sheep? Scotch inventions all. In fact, arguably the most important discovery of the 20th century was made by a Scotsman, Sir Alexander Fleming, when he stumbled across the mold that would become known as penicillin. Into that illustrious mix, comes a new contender from the blustery Highlands, one that is alleged to cure hull blistering, an ailment potentially no less dire to your boat’s health than any penicillin-treatable ailment would be to your own.

Hull blistering is a fairly common sight in boatyards and, as its name implies, consists of small, unsightly bumps on the boat’s hull below the waterline. Blistering can adversely affect a boat’s fuel efficiency, speed, strength, aesthetics, insurability, and in some cases, safety. The phenomenon occurs because of a nasty little chemical compound called polypropylene glycol, which is a high-molecular-weight alcohol that acts as a “water scavenger,” sopping up excess water in polyester resin during the laminate’s curing process. However problems occur because that glycol has nowhere to go after the hull is cured and when the boat has already been launched.

When a boat sits in water, osmosis naturally occurs. You remember osmosis from your tenth-grade biology class I’m sure. But if you need a refresher, it can be defined as the tendency of a solvent to pass through a semipermeable membrane in order to achieve stasis, which is the balance between both sides of the membrane. In a boat’s case, the solvent is sea water and the semipermeable membrane is your gelcoat. Yes, that’s right: All gelcoats are essentially plastic and thus to some extent permeable. Once the water infiltrates the gelcoat via osmosis, it is slurped up by those thirsty glycols. This process in turn creates hydraulic pressure that forces the gelcoat outward, and voilà you’ve got yourself a hull blister.

Historically speaking, there wasn’t much you could do once your boat started developing blisters. The common “cure” was to take her out of the water, scrape away the gelcoat, grind out the offending blisters, and let the fiberglass air-dry. It was, and by some accounts remains, a cross-your-fingers situation. If in fact the hull did dry out to acceptable levels as registered by a moisture reader—commonly put at about five to six percent moisture—the blisters would be filled with an epoxy and the gelcoat reapplied. But oftentimes it didn’t work that way, and the boat owner was up the creek without a paddle, not to mention a damaged boat.

Enter those resourceful Scots. In 1995, engineer and ship surveyor Terry Davey decided he had seen enough hull blisters in his line of work to last a lifetime. So he decided to do something about it. The result was Hotvac, a system designed to quickly and effectively dry out a wet hull for good. The components are fairly simple: a silicone-rubber electric blanket, sealing tape, and a vacuum machine. Once the gelcoat has been peeled off by a technician, the blanket is placed over the affected area like a patch and then sealed to the hull with tape. Then the vacuum, which is connected to the blanket via hoses, is cranked up until the atmosphere beneath the blanket is five millibars absolute, which is almost a complete vacuum. The blanket is then heated. Because glycol is an alcohol, it normally has an extremely high boiling point—392°F—a temperature that would severely damage a boat’s laminate. However, since the area is under a vacuum, the boiling point of any glycol under the blanket decreases drastically, down to about 190°F, which is safe for the laminate. Once they reach this temperature, the glycols evaporate and are sucked out through the tubes. When the process is finished, the blanket is removed, the blistered areas are coated with thickened epoxy and regelcoated, and the boat is good as new. It’s a great invention in theory. But does it actually work?

To find out the answer to this question, I interviewed Chris Bubin of Anchors Away Boatyard in Hampstead, North Carolina, and Arthur Swygert of Ross Marine in Johns Island, South Carolina, both of whom have used the Hotvac system extensively.

Illustrations by Steve Karp

Both men agree that Hotvac does in fact work. Bubin has had his machine for about eight years and completed dozens of jobs with it. “It works great,” he says. “We’ve never seen any Hotvaced boats come back for blistering after we used the process.” But he does admit that it works more quickly on some boats than it does on others. “We did one racing boat that had a lot of carbon fiber in it and a foam-cored hull. It was wet to the core,” Bubin says. He told me that job took longer than the two to four weeks it usually takes to complete the process, but that no boat has ever taken more than six weeks. This is opposed to the four to six months (or even more) a boat can take to air-dry.

For his part, Swygert also likes Hotvac, though he doesn’t always recommend it. “I only tell people they need Hotvac if their hull pegs my moisture reader, otherwise it may be more cost-effective to air-dry,” he says. “But even if the reader does peg, Hotvac can still get the moisture readings down to about two or three percent. Which is pretty dry. So yeah, it works.”

He raises an all important concern however: cost. Drying out and repairing a blistered hull ain’t cheap. Prices vary depending on the size of your boat, but all in—including labor—jobs typically run about $14,000 to $16,000—compared to the roughly $10,000 it costs to air-dry and repair a boat. And so the choice, as it so often does, becomes a personal one. Spend around $15,000 to have a totally dry boat in about a month, or spend ten grand and have your boat out of commission for five months with no way of knowing if her hull will actually be fully dry. Now if you’re possessed of proverbial Scottish frugality, you may go the air-dry route, but otherwise, Hotvac seems to be worth a look.

Ross Marine
(843) 559-0379.
Click here to visit Ross Marine's website

Anchors Away Boatyard
(910) 270-4741. 
Click here to visit Anchors Away Boatyard's website

Hotvac Hull Cure
(+44) 1796-482104.
Click here to visit Hotvac's website

This article originally appeared in the April 2012 issue of Power & Motoryacht magazine.