Us­ing Mois­ture Me­ters Steve Zim­mer­man

Passage Maker - - Contents -

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.


Through bot­tom paint: In most cases, if the an­tifoulant has dried out, the me­ter will pro­vide re­li­able read­ings through the paint.

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-

Re­mov­ing the wind­lass foot switch in or­der to be rebed­ded re­veals the ex­posed core ma­te­rial which must be pro­tected from mois­ture.

Above: In this test, a dry piece of solid ¾-inch fiber­glass has been placed into a shal­low tray of wa­ter. The test piece read 20 be­fore be­ing placed in the wa­ter, and 60 af­ter. A thin­ner lam­i­nate would read be­tween 80 and 100.

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 has been re­moved around these fas­ten­ers and re­placed with an epoxy mix­ture. This im­prove­ment will pre­vent wa­ter from en­ter­ing the core.

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