After a $600 survey found moisture in the cored hull and deck of an otherwise immaculate boat, I'm wondering whether I should avoid cored hulls altogether?
Leo R. Vigi
Jon Shattuck responds:
Thank goodness you had the boat surveyed and your surveyor was up to snuff! (Or maybe the boat was fine and your surveyor didn't know how to use his newfangled moisture detector.) While cored hulls are stout, strong and dependable—when laid up by a reputable builder—moisture will ruin the integrity of plywood and balsa cored decks and hulls. Bad decks are more common because of all those deck fittings. Decks may be cored with plywood or balsa, and the former is far more susceptible to water damage. Coring (almost always end grain balsa) problems in hulls are rare and are usually caused by voids (air pockets) which occur during the construction lay-up, inferior construction methods (very rare), or localized problems which occur when through-hull fittings were not bedded properly.
Both cored and solid filberglass hulls will absorb moisture by sitting in the water, and will give a moisture meter a positive reading (as will any metal, carbon fiber, and Kevlar). Truly high moisture in a fiberglass hull can lead to blister problems. But more than once I have seen a surveyor place a moisture meter on a hull which was just hauled out that day. They slap their shiny new moisture meter on the wet, slimy underbody and gasp, "This hull is full of water!" Moisture meters are great tools, but dangerous in the hands of novices.
Here's a pragmatic bit of advice for owners of all fiberglass boats. When the hull is good and dry, apply a barrier coat on the bare hull, and then keep your bottom paint up to snuff.
Back to your personal situation—burning $600 on a survey is a lot cheaper than spending $5,000 to $10,000 on a deck repair or a blister job.
If you're shopping for a boat 'down south' where most boats stay in the water 100 percent of the time, odds are you will have some level of blister activity, unless the boat has a proper barrier coat under the bottom paint.
If you are shopping for a boat 'up north' where boats 'dry out' when hauled out of the water for the winter, odds are you won't have any blister concerns. But put on a barrier coat at some point, just to be safe.
And to ensure you don't have 'wet decks', buy the highest quality boat you can afford, and then re-bed your deck fittings and hardware.
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