When you’re looking at a used boat, water intrusion into the hull, deck, or stringers is definitely not something you want to see. It’s a telltale sign of repairs to come. But how would you know about it, anyway, unless water was obviously seeping out of the structure somewhere? Well, that’s where moisture meters come in handy. But first, some background.
Strands of fiberglass, whether in chopped or mat form, don’t absorb moisture. Neither does resin. If water intrudes into a panel that’s just a gelcoated fiberglass laminate, not much really happens — provided the resin was fully catalyzed and cured to begin with. (If that didn’t happen during the manufacturing process — and it’s largely impossible to know — it can lead to blistering and delamination.)
The moisture-intrusion problem lies with wood and coring materials, be they synthetic or organic. Marine-grade plywood, foam, and balsa wood can all absorb water; then the absorption spreads by capillary action and wicking — and that’s when bad things happen. The water adds weight, reduces strength in the structure, and can lead to the failure of important fittings like cleats and stanchions.
Moisture meters can help detect where water intrusion has occurred. There are different types of meter: One type uses probes inserted into the wood, and passes an electrical current between them to measure resistance, which is then related to the amount of water present. The other type creates an electromagnetic field in the nearby structure and measures frequency differences to determine water content. That type doesn’t require the insertion of probes.
Both kinds of meter have their advantages and disadvantages, and neither is foolproof. The really good meters are expensive, about the price of a survey of a trailerable boat. There are cheaper models available for about the same money as an infrared thermometer, but it’s important to note that there’s no standard scale of moisture measurement. One meter’s midrange reading can be different from another’s, so it’s important to understand that when using one.
In fact, moisture meters are best used by professional marine surveyors, who combine their readings with other ways to detect and verify water intrusion, such as tapping the structure with a mallet and listening to the sound it makes. Talk about a black art. (Surveyors cannot do any destructive tests. They can’t disassemble, or cut or drill, but they are looking for anything that might signal trouble.)
Again, the meters are best used for detecting moisture penetration around deck hardware, such as cleats and other through-hull fittings. If you’re looking at a stern-drive boat, the most critical area is the transom, where the gimbal housing bolts on and connects to the bell housing on the inside. Even through-bolts and self-tapping screws in the hull and deck can let in moisture, and should be checked.
Small areas of water penetration in a used boat can usually be removed, replaced, and sealed easily — and should provide a bargaining chip for you when it comes to negotiating price. Larger areas are another story. They require money, or advanced repair skills — or a brisk walk in the other direction.
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