In this photo you can see the meter the copper faced calibration block and the acrylic spacer. These two items are used to calibrate the device. Though you will likely never need to calibrate it, the check takes just a few seconds to confirm the calibration is within spec.

Next the acrylic spacer is placed over the calibration block and it's ready for you to set the meter on. The meter can't see beyond the copper so it does not really matter where you set it to check the calibration. You can even set it on a saturated deck if you wish..

In order to display how the meter works I chose to use a known dry chunk of deck skin I had lying around. The deck skin is made of sandwiched balsa and is quite typical of most hull and or deck construction. This piece actually came off of a Maine built Downeast style boat but sandwiched balsa is fairly straight forward stuff.

In this picture I am holding a piece of metal up against the underside of the simulated deck/hull. As you can see the reading has gone from 6% up to 7.5%. This cored deck piece is a little over 3/4" thick and still the meter picked up the metal. If the deck were thinner the reading would have been higher.

It is very, very important to know what is UNDER the deck before taking a reading and the approximate thickness. Some new boats, from builders like Catalina, have aluminum plates glassed into the deck under winches, stanchions and other hardware. These aluminum insertions will cause "wet" looking readings as they are only about 1/4" deep at best. Large backing plates can also throw off readings so please do your best to look inside and to be sure your not reading metals through the deck.

This meter is designed to read approximately .5" deep but as you can see it will still, just barely, see a piece of metal at 3/4" depth. This is a good thing especially on cored hulls. Why? Well for one meters will read the copper in bottom paint as moisture so you can't use one over bottom paint.


Secondarily any hull that has recently been hauled will have rather high moisture readings. If the hull is cored, and close to 3/4 of an inch thick, the exterior paint won't throw off the meter by much and you'll be able to take moisture readings from inside. This is why it is best to go around thru-hulls from the inside not outside. On solid glass hulls I would not bother metering and would suggest focusing your efforts on the deck and bulkheads. If everything else checks out, namely deck and bulkheads, bring in your surveyor to do a full bottom examination.


One other area for potential high misreading could be a gelcoat with a high metals content. Some gelcoat manufacturers use titanium dioxide as a coloring/pigment agent and in some cases it can throw the meter off. If you get consistently high readings on a gelcoat find a known dry area and take a baseline reading to compare against.

I wanted to illustrate just how sensitive these meter are and to impress upon potential meter owners why the deck surface MUST be BONE dry before using a moisture meter.

I simply used a misting bottle of water and sprayed the surface of the cored deck chunk to simulate dew or rain. I then used a terry cloth towel in an attempt to dry the surface before taking a reading.

This is the scale for translating meter data. It's is fairly straight forward. The scale on the left is for reading wood, or a wood cored laminate and the scale on the right is for reading solid fiberglass. As you'll notice a 30% moisture saturation in wood or a cored section is only equal to 3% total absorbed moisture on a solid fiberglass with no core.

Core rot, degradation and the physical failure of the core can begin at about a 20% reading on the CT-33, sometimes sooner, which is in the high moist range. A pegged meter is indicating a moisture content in the core of about 35% or more!! If boats were made out of water this would be one thing but a core, with even close to 35% water content is not a good sign.

*Some other important things to remember:

Moisture meters read metals and display it as moisture. If you want to take reading bellow the waterline you'll need to scrape away bottom paint in order to get an accurate reading. This is a bad idea and best left to your surveyor. If you want to check for core moisture bellow the waterline do it from inside the hull on areas around seacocks etc.. I do not suggest that you use your meter for bellow water applications, with a solid un-cored hull, as these readings can be very misleading and are best left to a pro to interpret.



Moisture meters can not read frozen water. There is no benefit in even bringing your meter to a boat in the dead of winter. Along those same lines if a surveyor says he can survey a boat for you at 20 degrees F and get moisture readings walk away! Temps should be well above freezing, for a while, before even considering moisture readings.


Always find the lowest reading you can on the boat to use as a baseline. Many times on sailboats you'll find some dry core right above the wind speed & depth gauges if they are mounted on the cabin.


Look for differences in deck moisture readings. Diminishing changes in readings as you move away from a deck fitting are good indicators that the deck is wet around the fitting and that it is leaking or has leaked.


Most every boat has some deck moisture so don;'t be too alarmed at some "moist" readings. Readings in the moist range are certainly areas of concern but I would suggest bringing in a surveyor if you like the boat. If on the other hand the meter is pegged in multiple areas sch as mast bases, chain plates, genny tracks, bulkheads and other high load areas this is a boat that is probably safe to walk away from and continue looking.


Remember this is not for you to become an expert just to prevent you from spending $600.00 needlessly on a basket case boat. A few years ago I knew of a boat that had been surveyed almost ten times in two years on the market. She had wet decks, a saturated rudder and hull core moisture around the thru-hull penetrations. Each time the potential buyer either paid a discounted rate to "buy" the previous survey usually half price but it varies or to have a new one performed. A simple CT-33 purchase for under $200.00 would have saved these buyers some serious money. Each buyer could have ruled this boat out rather quickly with a little knowledge.


If you don't feel comfortable buying and using a moisture meter by all means don't. A little common sense goes a long way and is all that's necessary. Buying a meter when buying a boat will get you well on your way to saving money on unnecessary surveys. If the $179.00 price tag (including delivery) is still killing you it's very easy to sell it when your done on any of the sailing forums.


How many times do you need to fork over $600.00+ when looking for a boat before you invest in a moisture meter. Don't laugh I know some who have coughed up close to 2k in the search for a new vessel and the surveyors loved every penny of it.


The use of a moisture meter is only to determine if the boat is worthy of spending the money to continue on with a full survey. This is not written, nor intended, as a means for you to replace a professional survey. This article is intended to help you uncover boats which are clear lemons and which ones are worthy of pursuing further.





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This is an example of a "high moist" range reading on an O'Day. This deck is not destroyed but any further leaking needs to be curtailed asap or it will be ruined shortly. Because of the location, next to a chain plate, however this should be attended to and the core removed and repaired. There are some ares of a boat that you certainly don't want wet or even "high moist".

This is a very bad example of exactly what you don't want to see, a totally saturated core. This boat lives on a mooring and this is one of the primary mooring cleats. Perhaps there is likely some serious delamination going on and degrading or degraded core as one would assume. This core however was routed out around the bolt holes and found to be dryer than would have been expected. Meters can read metals and perhaps a backing plate so this reading was not as accurate as one would like it to be.

Remember the degradation and physical rotting of the core begins around 20% on the CT-33. The longer it has been above 20% the more likely are the odds for a total failure and delamination of the skins but this can still take a long while. I have seen decks with 25% readings going back 10+ years be taken apart only to find that they were damp, but still very, very structurally sound. ALWAYS incorporate spot soundings and core samples BEFORE digging into a deck.

This is an example of what a saturated core looks like during the rotting process. If you look ahead of the chisel you can see how the balsa just turns to mush when you touch it with the chisel. This was a deck penetration under a teak hand rail.

This is the adjoining deck section to the photo above with the chisel. As you can see the color of the balsa is telling you how wet it is. If you scrape out this core and compress it between your fingers it drips water like a sponge would.

It should be noted that at this stage of rot (early) the core is still tightly bonded to the skins. If this moisture were allowed to continue the color of the balsa would continue to darken and literally rot away leaving no strength.