(dam) Damp deck - SailNet Community
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post #1 of 9 Old 08-31-2018 Thread Starter
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(dam) Damp deck

Question: just how dry does the deck need to be before I button it up ?

While installing hinges for a dodger I struck moisture in the cabin top. Top is composed of the typical build up, outer glass, balsa core, inner glass liner. The balsa is white/cream color but moist. Taking a chunk of balsa then squeezing it really hard one can get water from it. The deck is not de-laminated. Using a Ryobi moisture meter E49MM01 on a core taken several years ago when installing a solar vent the meter reads 10%. The worst case reading on the cabin top is 25%. I know how the water got in and steps will be taken to prevent future incursions. I'm tempted to drill several holes from the inside, vacuum bag it and let it run while the sun beats down on black plastic laid on the deck. Maybe that will aid in drying. But the question is this, just how dry does the deck need to be before I button it up and finish installing the dodger? Does the deck need to be 10%? Will 15% do? What happens if I don't do anything? Will the moisture eventually even out over the entire deck making the readings a little higher than 10% but much lower than 25%?

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post #2 of 9 Old 08-31-2018
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Re: (dam) Damp deck

I wish I had a lot of concrete, long term proven answers for you.

The positive is that the balsa is still light coloured and doesn't let go of its water easily. It's not delaminating (structurally sound). Natural moisture content of air dried wood is 10-20% anyway. Liquid water doesn't migrate well in end-grain balsa, so the rate of drying might be forever and you'll only have a wet spot that should equalize with its surroundings eventually.

Vacuum bagging. Sounds like a great idea and couldn't hurt. Once dried, maybe add a bit of fungicide to the wood to help prevent rot. It's already a low-oxygen environment so as long as it doesn't get air voids, you should be ok.

I recored 3/4 of my deck and cabin top and a lot of the core was really bad, brown to black mush. The soaking wet light brown was reasonably sound, firm..but was losing its bond to the upper skin, so I changed it too. The "good" stuff I left sounds a lot like yours and it hasn't caused a problem

My guess is that the core had been wet for many, many years. Good that you're "on it".

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Re: (dam) Damp deck

Just got back in from the boat (nice to have it in the driveway). Yesterday I purchased some refills for DampRid. The refills are calcium chloride which sucks up moisture. Last night I added some of the calcium chloride to the test holes drilled for inspection. The holes where sealed up air tight. Today the calcium chloride had absorbed moisture. One can tell because the crystal had turned mushy. YEA!.

"Vacuum bagging. Sounds like a great idea and couldn't hurt. " ... It was suggested by another person who saw Mads from Sail Life vacuum bag his hull to dry it out. Not sure which is easier, pulling water from balsa or pulling it through fiberglass.
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Re: (dam) Damp deck

I am of the camp that all older boats are going to have some moisture in the deck core. As long as it's just damp and not rotten and mushy, I would just do what you can to reduce the water content, fix the cause of the water entrance, and stop there. I don't think there is any magic number of moisture that you need to get down to. I suspect it will take a very long time to get it even close to dry.

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post #5 of 9 Old 09-01-2018
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Re: (dam) Damp deck

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Originally Posted by JimsCAL View Post
I am of the camp that all older boats are going to have some moisture in the deck core. As long as it's just damp and not rotten and mushy, I would just do what you can to reduce the water content, fix the cause of the water entrance, and stop there. I don't think there is any magic number of moisture that you need to get down to. I suspect it will take a very long time to get it even close to dry.
It definitely does take very very long Jim. I re-cored the lazarette lids on my last boat, they were plywood and soaked through. The plywood was wet but only rotten in a few spots, however due to freeze\thaw cycles up here the lids were completely delam'd. I pulled the core out and threw it in the garbage can in my shop (completely exposed, no glass\resin on it). I remember a couple weeks later going to throw the can out and noting that the core was still wet when I brought it to the landfill. That was in my shop, away from the waterfront. I imagine drilling holes in the underside can't HURT to help dry it, but I definitely would expect it could take years to dry out that way unless it was a very small patch.

I also think the numbers that moisture meters crank out are tough to rely on. I'm not saying I have a better way on how to read levels, but I feel like they aren't as accurate as everyone gives them credit for. Just my opinion though!

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Re: (dam) Damp deck

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I also think the numbers that moisture meters crank out are tough to rely on. I'm not saying I have a better way on how to read levels, but I feel like they aren't as accurate as everyone gives them credit for. Just my opinion though!
And I agree with you completely Guy from the north that moisture meters don't give absolute numbers. What I think they do is give relative numbers and are repeatable. If an absolutely dry piece of wood (0 moisture) was measured and the reading was 10% then all pieces of wood that measured 10% would be moisture free assuming the density was the same for all pieces of the measured wood. In my case the core that was stored in my air conditioned basement measured around 9 to 10% so when I measure the boat and find 25% and the core sample at 25% is damp/wet/moist and the core sample where 10% was measured is dry as a bone I have confidence in the meter.

I haven't taken any readings on the hull thru the copper laden bottom paint. Since the paint isn't uniform in thickness then I would be a leery of the readings.

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Re: (dam) Damp deck

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And I agree with you completely Guy from the north that moisture meters don't give absolute numbers. What I think they do is give relative numbers and are repeatable. If an absolutely dry piece of wood (0 moisture) was measured and the reading was 10% then all pieces of wood that measured 10% would be moisture free assuming the density was the same for all pieces of the measured wood. In my case the core that was stored in my air conditioned basement measured around 9 to 10% so when I measure the boat and find 25% and the core sample at 25% is damp/wet/moist and the core sample where 10% was measured is dry as a bone I have confidence in the meter.

I haven't taken any readings on the hull thru the copper laden bottom paint. Since the paint isn't uniform in thickness then I would be a leery of the readings.
Seems like you have a good handle on moisture meters. Any bottom with copper, or any of the oxides will cause questionable readings. I discovered the Ryobi unit several years ago and have not used my $700 GE unit (or any of my others) since.
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Re: (dam) Damp deck

You can't pull moisture through fiberglass with anything you can rig. And, i'd be wary that using calcium chloride was going to release some of that into the damp wood--leaving possible damage from the chloride in the wood.

IIRC lumber is kiln dried to around 6%. Whether your meter says 6% or 10% is a matter of accuracy, I guess, but that's the ballpark. Anything more is leaving moisture in the balsa to do future damage. This is one of those jobs where trying to rush it or cut corners, is just going to mean doing a bigger longer job in the future. Putting a heating pad below the deck, and sucking a vacuum through multiple holes on top of it, might be the fastest simplest way to make sure it gets dried. Even that won't be "fast".
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Re: (dam) Damp deck

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Originally Posted by hellosailor View Post
You can't pull moisture through fiberglass with anything you can rig. And, i'd be wary that using calcium chloride was going to release some of that into the damp wood--leaving possible damage from the chloride in the wood.

IIRC lumber is kiln dried to around 6%. Whether your meter says 6% or 10% is a matter of accuracy, I guess, but that's the ballpark. Anything more is leaving moisture in the balsa to do future damage. This is one of those jobs where trying to rush it or cut corners, is just going to mean doing a bigger longer job in the future. Putting a heating pad below the deck, and sucking a vacuum through multiple holes on top of it, might be the fastest simplest way to make sure it gets dried. Even that won't be "fast".
There are US Mil Specs for marine lumber. For end grain balsa the high end is 13%, For marine ply it is 18%.

A decent moisture meter is very accurate on wood. A quality meter (expensive) will come with a list of calibration settings for hundreds of species of wood. There are no settings for wood covered with unknown laminate schedules of various resins and glass types/thicknesses, gelcoats, polyurethane paints, epoxy paints, anti-fouling paints etc. Once all/any of these variables are introduced the meter numbers become relative and percentage numbers are meaningless.
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Last edited by boatpoker; 09-03-2018 at 03:40 PM.
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