How to Use a Moisture Meter

Not just for the surveyor anymore, find out how a $250 tool can save your boat.

— Story and photos by Nick Bailey

During a routine pre-launch moisture inspection, the author discovers a water soaked bow on his ‘60s vintage glass-over-ply Thunderbird.

The story is always the same. A fiberglass boat is for sale. “She’s mint!” says the owner or broker. Sure enough, it looks to be in good shape but the buyer wisely insists on a survey before inking the deal. Or perhaps your own boat requires a survey before you can get the insurance policy renewed. The surveyor’s report comes back with a long list of minor problems and a few ominous paragraphs stating: “High moisture readings with indications of delaminated core at the following locations.”

Armed with the survey report, you contact the local repair yard for an estimate and are shocked to discover the cost to repair the wet core is a significant fraction of the boat’s value, and maybe exceeds it! Why? Unfortunately, wet and delaminated core can only be properly repaired by major surgery. The outer laminates must be cut away and the wet core in the affected areas replaced or at least dried out if it’s not too badly deteriorated. This is a time-consuming and expensive process.

“I had no idea the boat had these problems,” is the owner’s lament. “Everything seemed fine.” All of which is probably true, but a leaking fitting may not show any visible signs as water seeps ever so slowly year after year, deep into the core. Once moisture gets into the core it can’t escape. In northern climes in winter, the moisture freezes and expands. This frost heave can damage the balsa or PVC foam core structure and force the fiberglass skins and core apart. In turn, this frost-induced delamination makes it easier for water to spread throughout the core. Damaged areas continue to expand as long as water can gain entry. By the time you notice the deck has a spongy feel under foot or you spot the dreaded brown ooze seeping out from under a fastener on the deck head it’s already too late. At those locations, deterioration has already reached an advanced state.

Examine deck fittings regularly with a moisture meter and rebed any that give a high moisture level to prevent further damage.

How can the insidious problem be detected before significant damage occurs? The only simple, non-destructive detection method is a moisture meter. This electronic tool is usually considered to be strictly for use by the boat surveyor or repair shop. The alternative is to purchase a moisture meter and do your own diagnostic check a few times a year. For simple diagnostic work you don’t require an expensive unit.

Strange as it may seem moisture meters were not developed for marine use and most are not even designed for use on fiberglass. This tool is primarily used by lumber and paper industries to gauge the dryness of wood and paper. It also has a variety of uses in agriculture and the construction industry. This means you don’t necessarily have to pay “marine” prices to get your hands on one. Meters that are specifically adapted for marine use cost more and will give you a more quantified reading of relative moisture content. The more general-purpose meter can still give a useful reading but it probably is calibrated for lumber.

Readings on fiberglass and core materials just under the outer skin will be relative readings or qualitative, which means they cover a range of wet here, dry over there, for example. Once you’ve tested the unit for sample readings on a variety of materials with known moisture levels, you’ll begin to understand how to interpret meter readings. Regardless, you’ll find places where moisture is sneaking into the core around leaky hardware on deck in time to rebed the fittings. If you find very high moisture over more than a few square inches, I recommend you call a surveyor or marine repair shop to assess the extent of the situation. Most older boats with cored decks have some moisture problems. Cored hulls are less prone to trouble simply because there are not as many holes drilled in them, but when trouble occurs in a cored hull it can be severe.

high moisture reading signifies that water has entered the deck core.

Moisture meters are also useful on wood boats. This spring I thought I had a minor paint repair to do on the stem of my 40-year-old glass-over-ply Thunderbird. While casually checking the stem with the shop moisture meter (a rather battered GRP 33 model purchased from J.R. Overseas) prior to doing what I thought would be a touch-up, I discovered the whole bow above the waterline pegged the moisture meter. Apparently the forestay fitting had been leaking for some time. I ground back the paint, fiberglass and epoxy fairing filler to expose and dry the wet plywood. Then, with fear and loathing, I removed the dubious looking false stem and found to my relief that the massive stem timber was still fresh, though wet. If I had not checked with the moisture meter I would have been none the wiser, while the soaked stem slowly turned to rot. In this case, I was happy to do a fiberglass and epoxy and paint repair instead of replacing the stem timber.

About the author: Nick Bailey is DIY Magazine's repair specialist and has spent 26 years in the boat repair business.