Measuring moisture levels in boats is often done as part of a marine survey to assess the condition of the hull and deck of the vessel.  Depending up on many factors moisture ingress in to fiber glass (FRP) structure could result in problems that are simply an unseen nuisance with no long term degradations, all the way to completely destroying major structures of the vessel via rot or delamination rendering them unsafe.  Delamination of the skins from the core, core rot, or osmotic blisters are serious problems and may cost boat owners in potentially expensive repairs or may kill a sale when the potential buyers discover the issues during a survey.

So what are moisture meters?

Moisture meters come in three basic technology types: capacitance, probe, and free field effect, most meters are capacitance type.

Capacitance meters have two electrodes or probes, one that sends an AC voltage signal and another that receives it. Because water has a much higher dielectric constant or permittivity than air or fiberglass, the strength of the transmitted signal and the return received signal can be used to determine moisture content. This drop in the signal is then measured and displayed on the meter, either by an analog needle or digitally in an LCD window. Many meters give a saturation percentage or some other indication of how much water has ingression has occured.

Other meters work on resistance. These also have two pin probes for measuring moisture in wood or other materials a probe will penetrate; these work by resistance and use a DC voltage. Using the same methods above the DC signal is measured and the meter displays some type of indication of water content.

A third method, that does not use probes is called free-field effect, is slightly different in the fact that energy does not travel through the part. A favorite of mine is the REED 3808 unit pictured below. Instead of two electrodes (maybe we could call them an antenna) are next to each other in the meter, one is generally in the head of the instrument or in an external ball on a rod, this directs a signal into the laminate where it is redirected back by any moisture present. The second electrode (antenna) in the handle senses the energy return and makes calculations based upon the signal and then displays moisture content. In effect, the water acts as an media to re-radiate or reflect the RF signal. If you have ever seen wet sails on a sail boat effect the radar on the boat, this is the same principle.

So now we know a bit about how the technology of the meters work, how effective are they and how accurate are the results they give?

How Accurate Are Moisture Meters?

From my own personal experience using all three types of these meters over the years… the only thing I can say with confidence is take all moisture meter results with a grain of salt. I have found that multiple things can induce errors in the meters, these include temperature, bottom paint, glycols left behind during the boat building process, changes in core types in the vessel, among a laundry list of other issues. Experience using meters across many vessels is really the only way to have the ability to understand and use the results you encounter from these devices.

Although I just discounted moisture meters and their results I do believe they are necessary and a very good tool for evaluating a boat or yacht. When I use a moisture meter I do not rely upon the meters output level to determine if there is moisture ingress in the structure, I try to develop a baseline for the area I am working within and then determine if there are areas that read differentially to the overall area baseline. I have seen too many cases where bottom paint or fairing materials induce errors into the meter which could be misconstrued.

Percussive Testing

Most believe that the moisture percentage of a fully saturated solid (no core) fiberglass hull will have no more than 2.5% to 3.5% moisture. Most all meters are designed for wood, and thus sound an alarm at 20-25% moisture, which is a typical saturation point for most woods. Some meter manufactures provide a conversion table for the actual readings conversion to fiberglass (eg., 25% wood = 3% glass) hence there is no correlation between the percentage of moisture indicated by the meter and the actual amount of moisture in the laminate unless it is converted with a table, or via changing settings on the meter.

Moisture meters are great tools, but please DO NOT buy a moisture meter and think that you will understand the results and detect potential problems. To master the use of a moisture meter takes time and multiple comparable experiences on various vessels. Second any results should be used with percussive soundings, something many boaters don’t have the ear or experience for and again requires time and multiple comparable experiences to understand.

In closing, I believe moisture meters are a valuable tool that can be use to help determine the overall quality and health of a hull or deck laminate. Moisture meter data alone I believe is non-conclusive at best, the operator of the meter needs to both have experience with various laminate schedules and also have a lasting knowledge base of how the meter responds to various conditions and core materials. Moisture meter results should be used along with percussive testing, visual inspection, and other non destructive test methods to determine any trusted results. If results indicate more invasive testing such as drilling or core sampling should be accomplished.

Enjoy those moisture meters!


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