Do you have water in your hull?

11/7/2026

This week we will discuss some of the work a Marine Surveyor will do when they are inspecting a vessel. Most surveys involve checking the hull for deformities or other anomalies. One of the things a surveyor will look for is moisture in the hull. There are a few ways to help ascertain moisture in the hulls without destructive testing such as drilling holes or opening up the fiberglass.

Moisture Meter

The most common way is utilizing a “moisture meter”. I put moisture meter in quotations because these devices do not detect moisture. The Tramex website details how these meters work. “The Tramex Skipper Plus moisture meter detects moisture in wood or GRP (Glass Reinforced Plastic) by measuring the resistance between two low frequency AC signals transmitted from the soft rubber electrodes on the base of the instrument.” As you can see these results can be left up to interpretation.

FLIR Camera

Another way we can help identify moisture in a hull is with an infrared camera such as a handheld FLIR device. These devices also do not detect actual moisture but instead they detect subtle changes in temperature. These variations in temperature can help identify moisture in a hull due to the rate in which different materials change temperature. For example, if you have a section of dry cored hull it will have a different temperature than a section of wet cored hull. This is due to the difference of the water changing temperature compared to the section of hull with no water.

Phenolic Hammer

A third way of determining if there is moisture in the hull is with a phenolic hammer. This hammer is not hard enough to damage a sound fiberglass hull but it will reveal areas of possible moisture or delaminating via the sound the hull makes. This is used similar to looking for a stud in a wall made with drywall. The hammer will sound noticeably different when you hit the unsupported drywall than when you hit a section of drywall with a stud supporting it. When fiberglass delaminates it makes a similar sound when hit with a phenolic hammer.

Many marine surveyors will use only a moisture meter and a hammer. Some will only use a hammer to determine moisture in the hull. Very few will use all three devices together. This can lead to erroneous high moisture determinations. This may cause undue work to prove there is moisture in the hull resulting in unnecessary costs to the seller or buyer or in most cases cancellation of a purchase contract. The buyer may not end up with a boat he wants because he is incorrectly turned away from a “wet boat”. Marine Surveyors can only provide the results they witness. How they choose to verify their findings can vary widely. The only way to be 100% sure there is moisture in a cored hull is to open up the hull and physically check. Destructive testing such as this is almost always denied by a seller due to the costs of opening up the boat.

While all three of the above methods have their limitations, when all three are used together there is a greater chance of accurate results. When we survey vessels for our Factory Certified Program we use all three methods to determine defects in the hull. This along with our intimate knowledge of Antares and our Factory Certified Program partners vessels allows us to make accurate determinations more often.

Check out our latest video to see how we resolve “high moisture” readings on a boat that was surveyed by another marine surveyor. Unfortunately for the potential buyer he did not move forward with purchasing the boat he wanted due to an error of the marine surveyor.

Frequently Asked Questions

Do moisture meters actually detect water in a boat hull?

No. Moisture meters measure electrical resistance, not water directly. Many factors can influence the readings, so a high reading does not always mean the hull is wet.

Can a thermal camera find moisture in fiberglass?

Thermal cameras detect temperature differences, not moisture itself. Wet areas often appear cooler, but temperature variations can also be caused by sunlight, shade, or recent rain.

Is a phenolic hammer test reliable for finding hull problems?

It can be useful when performed by an experienced surveyor. A dull or hollow sound may indicate delamination or moisture, but results are subjective and should be confirmed with other methods.

Why do some surveyors give different moisture readings on the same boat?

Because they often rely on different tools and interpretation methods. Using only a moisture meter can lead to false positives, while combining multiple tools (moisture meter, thermal camera, and hammer) provides more reliable results.

What should I do if a survey shows high moisture in the hull?

Don’t panic. A single high reading is not definitive. We recommend getting a second opinion using multiple testing methods before making any major decisions about repairs or walking away from a purchase.