Using Moisture Meters Steve Zimmerman
Someone once said that whether a glass is half-full or halfempty depends on whether you are serving or drinking. Similarly, whether or not you believe moisture meters work might depend on whether you’re buying or selling. We refer to them as moisture meters, but “deal killers” might be a more appropriate name. Mysterious, misused, and misunderstood, these devices can sink a boat sale faster than any other tool in a surveyor’s arsenal. In many cases, withdrawing from the purchase makes sense, and in other cases, it can lead to hasty decisions.
Let’s take a brief look at how these instruments work. Moisture meters came onto the scene in the 1960s as a tool for the lumber industry. There are two types—with a pin and without a pin. For non-destructive testing on yachts, the pin-less type works best. I use a Tramex Skipper meter. Its two soft pads allow the user to quickly cover large areas without poking any holes into the surface. Many surveyors have other brands and each has a good reason for their preference.
Ironically, moisture meters do not measure moisture but instead infer moisture content based upon the electrical conductivity of the material. The meter sends out a signal and the more conductive the structure, the more of this signal returns. Wet wood is more conductive than dry wood and higher meter readings, therefore, point to elevated moisture. Other factors can affect conductivity, including metal content and density (solid blocking installed instead of core, for example).
Many people wonder how deeply into the material the meter can probe. The answer depends on a number of variables, but in practical terms, it doesn’t matter much on boats. If the deck core has absorbed water, that elevated moisture content will make contact with the outer fiberglass skin, and the meter will show high readings.
Due to all of these variables, meter readings can be misleading. Most meters have two scales: percentage and comparative. For boats, we only use the relative scale. Any conversation about percentage of moisture will lead you astray. Only destructive testing (core samples or probes into the laminate) will determine the percentage of moisture.
Given all that can go wrong, should we rely on moisture meters at all?
Absolutely. In fact, inspecting a boat without using one would be irresponsible. It’s the first tool out of my bag when I begin an inspection. Think of the meter as a comparative tool, not an absolute one. I always begin by finding a fiberglass surface that shows a low reading. If the interior has a fiberglass liner, we have a good reference point. Protected from the elements and above the waterline, this surface will read very low on the scale. Now I know the meter is working and has established a baseline. If I find a surface that reads significantly higher, then it differs in some way from this known dry laminate. I might not know why, but I know something is different. Perhaps a metal backing plate has been laminated into the structure, changing the signal propagation. Maybe salt crystals on the surface hold moisture.
At this point we use the meter to look for a pattern. If the elevated readings form a clear pattern, such as a twofoot by two-foot square, then we might suspect a backing plate or some other engineered difference in the structure that affects the reading. If the high readings emanate from a peak reading and then taper off to lower readings, we may reasonably conclude that moisture has found its way into the structure.
If the side decks read low on the relative scale, let’s say 20, and a large area of the foredeck reads high (80), and if those readings reach their highest levels around windlass foot switches, gradu- ally declining as the meter moves farther away, then we may conclude that water has found its way into the core from leaking deck hardware.
The meter tells us that the deck con-
tains elevated moisture, but it does not tell us how much moisture. Even if the meter sounds its alarm and pegs the needle at 100, this does not mean that if you cut out a section, water will drain out. In most cases, the core would appear to be dry. We tend to think of moisture in the core as puddles of water, but humidity would be a better image. Sometimes we find water, but usually the structure looks dry to the eye. To determine actual moisture content, the core and laminate plug would have to be sent to a lab. For now, the only conclusion we can draw is that a certain area of the deck has higher moisture content than other areas, and the core material most likely contains the moisture.
Is moisture in the core really a problem?
Let’s answer that by starting with a buy-sell situation. Anything that creates risk or doubt in a buyer’s mind qualifies as a problem. At this point many sellers will say, “But you can’t prove that there is water in there!” That statement has some merit, but as the saying goes, “Absence of proof is not proof of absence.” The meter tells us something is amiss and that unknown presents a risk to the buyer. At the very least, if the buyer proceeds with the purchase, this issue will resurface when the time comes to sell the boat.
Putting aside any buy-sell implications, moisture in the core doesn’t always mean you have a problem. Core materials fall into the three basic categories of closed-cell foam, honeycomb, and balsa wood. For closed-cell foam and honeycomb, moderate amounts of moisture do not present a structural problem, because moisture will not cause them to deteriorate. In some cases, the core loses its bond to the fiberglass skin and moisture collects between the two materials. Water in this boundary area can exert pressure
and detach more skin from the core, and weakening the structure significantly. This condition might be suspected from moisture meter results, but its detection results from sounding the structure with a plastic mallet, or by removing bottom paint and gelcoat for visual inspection. In addition, high amounts of moisture in the core can freeze. Unlike almost any other liquid on the planet, water expands when it freezes, and this action can detach the core from the fiberglass skin.
Unlike closed-cell foams and honeycombs, balsa cores will rot when exposed to elevated moisture levels. Unlike the synthetic cores, balsa is a type of wood, and a soft, absorbent wood at that. A fair amount of controversy and research has gone into this subject. Builders can (and some do) take steps to minimize the risk by sealing and protecting the balsa. Regardless, the fact remains–balsa will rot in the presence of water and oxygen.
What can be done once moisture gets into the core?
Once moisture finds its way in, removal can be extremely difficult. In all but the most extreme cases, very little standing water can be found. You can use a pump to empty water out of swimming pools, but removing fog continues to be a better analogy. On several occasions, we have used a vacuum pump to reduce moisture levels. Even with readings in the 80-100 range on over half of the surface on a 50- foot boat, the pump will usually collect a quart of water, and removing it will not change the readings because the humidity level in the core remains high. Some combination of extraction and pumping in very dry air, with perhaps some heat, can slowly produce results.
In severe cases, such as decomposed balsa, the only solution is to cut off the outer fiberglass skin, remove the damaged core, bond in new core, and then attaching a new section of outer skin. This process usually costs tens of thousands of dollars.
Sometimes you hear about drilling holes and injecting resin, which makes sense only when you have sound core material that has come loose from the skin. If the surfaces are dry, injecting epoxy into the voids can successfully reattach the core to the fiberglass.
Will a moisture meter work below the waterline?
Moisture meters work very well on a boat’s bottom, provided it has been out of the water for roughly 24 hours and it hasn’t rained. This fact complicates the pre-purchase survey process. For boats in the water, survey day usually includes a quick haul-out with the boat left hanging in the slings for an hour or two before relaunching. This tight window rarely provides enough time for the bottom to dry, rendering the meter useless below the waterline. If the boat can be hauled and blocked a day or two ahead of time, the readings will be much more reliable.
I often hear that metal in bottom paints makes the readings below the waterline meaningless, but that has not been my experience. Bottom paint holding moisture after a haul-out can give false readings, but dry bottom paint usu-
ally produces reliable results. One clue: If the entire surface gives you consistently elevated readings, then you might be getting false readings. Usually, however, once the paint dries a bit, the readings will vary from 20 or less on a relative scale up to 100, depending on the condition. If you find good readings somewhere, you can rule out the bottom paint as the cause of high readings.
If the meter sings and you have reason to suspect moisture in the laminate, patch tests will be the best way to verify the condition (see March 2015, Patch Tests). When you suspect moisture in the hull core, access to the inside skin of the hull can be helpful, but test areas are limited because of interior joinery and fiberglass liners.
Are moisture meters only used for pre-purchase surveys?
For many boat owners, survey day provides the first indication that moisture has entered the hull laminate or deck core. The bad news comes all at once, but leaks from windlass foot switches or flybridge ladder bases have been allow- ing water to migrate to much wider areas for months, if not years. Had the leaks been identified early on, simple rebedding would have prevented or contained the moisture, possibly saving tens of thousands in repair or devaluation costs. A moisture meter can be of great value when used this way. Checking around all deck hardware once a year with a moisture meter can pay off handsomely. The entire deck and cabin of a typical 40foot powerboat can be done in an hour. Whether you engage a marine surveyor or your boat yard, this would be money well spent.
What should be done if the hardware is allowing moisture into the core?
If the meter shows elevated readings around a piece of deck hardware, then it’s time to remove and re-bed. Before reinstalling, you have an opportunity to take care of some important preventative work. Look closely into the hardware holes. Poke around with an ice pick or awl to make sure the core is not exposed. You should feel solid, hard mate-
rial around the perimeter of the fastener hole. If you see or feel core, now is the time to take action. Remove the core to a distance of three times the fastener’s diameter. In other words, if you are rebedding a stanchion base with ¼-inch bolts, remove the core in a diameter of ¾ inch around the center of the bolt.
You may remove the core using a screwdriver, a bent icepick, or with an allen wrench set into the chuck of a drill so that the short end digs into the core as it spins. The cleanout does not need to be pretty–in fact, a rough, uneven edge helps.
With the core removed, tape over the bottom of the hole from inside the boat. Use two-inch tape on the deck or cabin surface to seal the area around the hole. For the next step you will need a large plastic syringe. Mix epoxy resin with reinforcing additive, fill the syringe, and slowly fill the hole until it comes flush with the surface. Check inside the boat and make sure no resin drips through. Allow the epoxy to harden and then redrill the ¼-inch fastener hole.
You have created a solid area around the fastener and now any high loads will not crush the core around the hardware. When the core compresses from a load, the fasteners become slightly loose, allowing water in. Surrounding the fas- tener with epoxy will block water from entering the core if a leak develops.
This preventative moisture check should be done in conjunction with hauling the boat. Give the bottom a few days to dry out and check below the waterline. If you have isolated areas of elevated moisture, you might want to mark them with chalk, take some photos, and log the readings. At the next haul-out you can compare areas and readings and if either one increases you should start thinking about removing bottom paint and applying a barrier coat.
A moisture meter can be a powerful tool and its readings should never be dismissed without careful evaluation. These meters work best when used as a comparative tool providing relative readings. Find an area with low readings, even if you have to go inside the boat to do it. Then compare other readings to your baseline.
Most important, the day you sell your boat should not be the first day a moisture meter comes into use. Every season, or every other season, hire a professional to check your deck, cabin, and hull above and below the waterline. Using a meter preventatively can save you tens of thousands of dollars and will help avoid bad surprises on survey day when you decide to sell.
Below: Foredeck hardware often allows water into the core. The number of deck penetrations combined with a wet location often leads to problems and the high reading indicates trouble.
The core around this windlass foot switch has never been sealed and water has found its way into the deck structure, creating elevated readings over a wide area. Since the core is balsa, the ongo-
Through bottom paint: In most cases, if the antifoulant has dried out, the meter will provide reliable readings through the paint.
The core has been removed around these fasteners and replaced with an epoxy mixture. This improvement will prevent water from entering the core.