DIY's Buying & Using Moisture Meters

HAL · Nov 6, 2008

checking fittings

I was just about to ask if a moisture meter could be used to check for water intrusion around deck fittings. Apparently it can.
We didn't want our deck pressure washed for fear the pressure would damage the bedding of fittings on a relatively new boat. The yard went ahead and did it anyway. With a meter I can keep an eye on any water that gets in and stop it before rot starts.

dvideohd · Nov 6, 2008

Since you have had both meters...

can you talk about the two - and how they differ...???

I spent 6-8 months watching Ebay for a "used" moisture meter - and did not see one come up or sell for much less than the cost of a new one.... So come next payday...

I did get a Marine Planer - to work on my obvious bad fiberglass spots... I also want to give it a whirl at taking off bottom paint... The Vacuum was a big part of why I think it a good idea...

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My plan this fall ( :> ) and winter is to strip bottom paint with the Marine Planer and work on fiberglass spots as well. Since the boat is in my back yard, I DO NOT want bottom paint in the grass - and I want a MINIMUM of Fiberglass crud floating around... Got a ground cloth to go on the ground.. and plastic barrier for the top of that... that is for a better surface to vacuum.

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Going to need a moisture meter for deck work... that is my #4 item. Seems that the moisture meter gets used at each section I will be working on.

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--jerry

RobG · Nov 6, 2008

Hey Maine Sail check the link out.

Digital display, self cal at $120. I would assume you could use the same GRP cross reference that Electrophysics provides?
http://www.extech.com/instruments/product.asp?catid=11&prodid=75

dvideohd · Nov 6, 2008

I called Electrophysics...

nice, nice folks...

here is a link to a store - makes it easier...
stores.ebay.com/Electrophysics-Meters

CT33 or GRP200... still puzzling it out..

--jerry

--FWIW - the versions seemed to differ in one important way - the DEPTH below the surface... CT33 was rated 1" - GRP200 was rated at 1.5".... What I need for an old, thick H37C hull - I am not sure... I would think that the cross reference would vary depending on the "depth" reading...

sailingdog · Nov 6, 2008

Jerry-

If you've got the bucks for the GRP200, which doesn't require a conversion table for the readings IIRC... get it..

Bill Roosa · Nov 6, 2008

Soooooooo

Are you going to give a block of instruction MainSail???

When is "wet" really wet and what % moisture does wet begin at?
When is "dry" really dry and wha.........

i suspect wet for balsa is not the same as wet for fiberglass or metal

While it ain't rocket science it does require some background info

Where can I get the background info?

Don S/V ILLusion · Nov 6, 2008

Bill
You raise the issue basis to the need and utility of a moisture meter in untrained hands. Although it may seem straightforward to use, many people tend to interpret the reading simplistically. In addition to the variables in interpretation of the results, as Mainesail implied, there are a number of variables in how and where it is used that can make it's interpretation either sound or misleading. As you implied, the same reading in a cored hull and a solid hull are quite different in what they signify just as is the wide disparity along the same hull where bulkheads and backing plates are located. Although some of the variability's can be eliminated with common sense, it can just as often lead to misunderstandings.

Unless someone knows what these subtleties are and how to best either avoid or interpret them, it is often best to leave this to someone who can.

This applies as well to relative reading across a hull as an example unless one is aware of the underlying substrate, it's composition and abutting materials.

More directly responsive to your question, I'm not sure there is a consensus acceptable reading for example in an FRP hull - I have read so-called experts who have quite divergent views on what an acceptable range of readings should be.

ITMaster · Nov 6, 2008

Your Article That You Wrote

Is excellent Maine Sail. Like all your articles that you have written and documented with supporting photography, I find them most informative and an excellent reference source. Thanks for sharing with us; we owe you a debt of gratitude.

Based on your comment about the Catalina's having aluminum for support under some of the glass work of the deck (which I knew they did), I would presume these moisture meters you have cited here work on the prinicple of density?

Thanks again for another excellent contribution

to this fine site!

dvideohd · Nov 6, 2008

depth...

In an earlier call to Electrophysics, they indicated that the meter difference was related to "depth" of reading - with 1" for CT33 and 1.5" for GRP200. For a Mac26, probably 0.025" would work (it feels like a rigid inflatable in some spots...) - but the H37C has battleship armor for the hull... it is AMAZINGLY thick in some spots....

If I had to guess.... I would say a 1" reading is all that you need - since if it IS going to be moist/wet - it would likely be in the first 1/2 inch or so.... That may not be true around the Keel or Rudder areas... I don't have the experience to really tell - and have not seen any real discussion.... ( I have read Casey's and Vaitses's books...)...

My guess would be that if you are surveying or doing a quick once over - then there is just no issue about using the CT33.... On deck work - it was not so clear if you actually needed the "deeper" test...

--jerry

F.P. · Nov 6, 2008

Intrepreting the GRP33

I have a 1995 Catalina 36. I bought the GRP33 3 years ago and discovered some moisture in varing degrees near the chainplates and nearby stanchions. Some areas are in the moist reading of the meter. But tapping on the deck indicates no issues and the deck seems as solid as a rock. Stanchions are good too, no looseness whatsoever. I removed the chainplates, dug out the area around them, and filled it with epoxy and then resealed with caulk. My guess is the PO didn't take real good care of her although the chainplates were sealed well when I got her. Moisture readings still read the same after 3 years and all is still solid. I don't understand how, if there truely is wetness under the glass, the water travelled so far from the chainplates down to the stanchions and hasn't caused any other issues besides a moist or wet reading. What do you think? Will I eventually have other problems? How long will it take?

sailingdog · Nov 7, 2008

F.P. said:
I have a 1995 Catalina 36. I bought the GRP33 3 years ago and discovered some moisture in varing degrees near the chainplates and nearby stanchions. Some areas are in the moist reading of the meter. But tapping on the deck indicates no issues and the deck seems as solid as a rock. Stanchions are good too, no looseness whatsoever. I removed the chainplates, dug out the area around them, and filled it with epoxy and then resealed with caulk.

This probably saved you the hassles of needing to do a full re-core of the deck in those areas.

My guess is the PO didn't take real good care of her although the chainplates were sealed well when I got her. Moisture readings still read the same after 3 years and all is still solid. I don't understand how, if there truely is wetness under the glass, the water travelled so far from the chainplates down to the stanchions and hasn't caused any other issues besides a moist or wet reading. What do you think? Will I eventually have other problems? How long will it take?

If there was moisture in the core, and it had gotten in via the core being exposed around the chainplates, where would it go after you sealed the core with thickened epoxy. It would still be in the laminate... The moisture near the stanchions is probably more likely due to the stanchion fasteners leaking and moisture getting in to the core there. If you haven't removed the stanchions, potted the fastener holes and re-bedded the stanchions—you should probably do so soon—before it does progress into worse problems.

IIRC, for the core to rot, three things are generally needed: a core made of wood—balsa or plywood; water; and oxygen. If the bacteria that rot wood are deprived of any of the three... they can't do their work. By sealing the core off, you've cutoff additional water and most of the oxygen.

dvideohd · Nov 7, 2008

Plywood deck core....

for deck repair.......

what kind of plywood? suggestions?

I need to start getting supplies in.... :dance:

--jerry

sailingdog · Nov 7, 2008

dvideohd-

If you've got a problem with a deck core where hardware is going to be mounted to the deck, you're far better off IMHO, replacing the core material with all solid fiberglass. Re-coring areas where deck hardware goes through is kind of a pointless exercise in futility IMHO.

Solid fiberglass is a much better thing for areas that have highly loaded hardware.

If you're replacing the core in areas without deck hardware, like stanchions, cleats, winches, etc, then using a better core material, like foam or end-grain balsa makes much more sense IMHO... the boat will be stiffer and lighter for it.

If you really have to go with plywood, which is a lousy core material and was generally used either as a shortcut to making a more solid "backing plate" for deck hardware, or was a cheap way out to build a boat... use marine-grade plywood.

dvideohd · Dec 16, 2008

Using the CT33 has worked out pretty well... The results were not as I really "expected"... but that is why you get it... I could identify moist.... wet.. and CENTER of wet pretty good.....

It really has been useful for estimating what I need to do.... I went through a LOT of magic markers...

For the moment - I am taping over "problem spots" where water seems to be getting into the deck. I am using "outdoor duct tape (3M)" and it does look funky...

I think a retired Jedi Master could make a good living selectively chopping up decks with a light saber... My "human" equivalent is a 4 sanders (7" variable sander, two 5" (8 hole) rotary sanders, and a corner sander), a multitool (Fein equivalent), and a rotary cutter. I think the shop vac will see a lot of dust this spring....

-jerry

Sailorbob · Dec 17, 2008

The moisture meters in question rely on resistance and do not provide a fixed consisent value but need to be interpreted in relationship to other areass. Take for instance you've just hauled your boat and power washed the slime off it. It's warm and sunny and the surface dries quickly. You go to a specific spot and using your new fandangled moisture meter take a reading that Whoa is way up on the scale. come back the same day it has dropped dramatically. You you have a moisture problem? probably not. If you don't have the luxury of time (most surveyors don't), then find an area that you're pretty sure is dry (high on the topsides). Compare these readings to the rest of the boat. be smart. Concentrate on penetrations first. Anytime a fastener passes through the gelcoat it creats a path for moisture to follow. The Gougoen Brother's Book on Boatbuilding captured the cure nicely by overdrilling all fasteners and filling with epoxy/filler. then redrilling/fastening. The moisure cannot get to the core, no more problems.

A boatyard trick, when removing fittings temporarily, put a squirt of bathtub caulk to keep moisture out of the holes. Duct Tape breaks down with UV. The caulk is easily removed with a long sheetrock screw thraded into the caulk then pulled out of the hole like a ready made cork

Good Luck Take Pics Share Info!

Sailorbob

DIY's Buying & Using Moisture Meters

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