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Hi All,

We've had our boat now about 2 months. Have NEVER had an issue with condensation or water in the boat. Yesterday I was doing some serious work inside (replacing waste tank drain line and vent), lots of grunting and sweating, with the companionway top shut and curtain hanging off the back like a tent flap. Had a small electric heater on and off to keep the wife warm (I was down to a T shirt). It was 50 or so outside and rained hard most of the day. By days end we had water running down the skylights, mirrors, and everything felt damp. Today when we went back, my tools were covered in "sweat". Because this has never happened to us yet (and we've had plenty of rain), I'm perplexed as to what triggered this crazy amount of moisture in the boat?

Dave
 

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you warmed up the air so it can hold more moisture from the rain. then the boat cooled down with the moisture was trapped inside the boat and it condensed on any cold surface
 

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sunfish?junior?
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ventilation or lack of it with high humid moist air. when you heated the air it could no longer keep the water it held. Some magazine just did a good article on this and heating sail boats. Insulation can also help
Good day, Lou
 

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All that heavy breathing as you were fighting with the fittings, plus the heat, etc... presto, RH off the scale...
 

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overboard got it. Warm air holds more moisture. Once it cooled, it could no longer hold the moisture and it condensates out.

The Dew Point is a mathematical temperature where the current amount of moisture would no longer be able to be held in suspension, if temperature were to decline to it. The higher the dew point, meaning the closer to the ambient/room temperature, the more humid it must be. As soon as the room temp drops to the dew point, water will condensate out. The surfaces of the hull and metal, such as tools, will often give off heat first, so they have a boundary of air that cools faster than the room and condensation forms on them first.

Insulation is a method to keep the surface of the interior of the hull from dropping below the dew point, when the cabin air is heated, but would not have helped this situation. The entire room was allowed to drop below the dew point.

There are only two ways to approach the problem. Control humidity or control temperature.

Temperature is controlled by keeping the temperature of both the air and surfaces inside the boat above the Dew Point. This is very difficult, unless you run heat 24/7.

Humidity is controlled by either electronically dehumidifying the air or limiting the addition of moisture. You breath moisture into the air, so its impossible to eliminate the addition. Showers and cooking will add extraordinary amounts, but just your breath is enough to raise the humidity. Even if you control these, you are fighting a battle when its raining outside, ie. 100% humidity or dew point nearly equals ambient temperature. The only good way to deal with the issue is to remove moisture with an electric dehumidifier (chemical desiccants will not remove enough).

Many refer to ventilation as the solution. This works when the outside air has less humidity than the interior air, probably because you've been breathing! Still, the air inside has to remain above the dew point and when it's raining outside, there is no chance that you'll be ventilating in air with less moisture.

Dehumidifier is the only real solution.
 

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This year I've started using a small Eva-Dry dehumidifier in my boat. It is making a huge difference in keeping condensation down. The dehumidifer appeared to collect 1 cup of water per day on average out of the atmosphere.

I have this model:
Eva-Dry 1100 Petite Dehumidifier

I carefully removed the cover of the water container and added a drain line that goes from the dehumidifier into my sink. Otherwise I'd need to visit the boat every 2 days to empty it.

I also put the dehumidifier onto a simple mechanical timer that is set to run for 4.5 hours, then turn off for 1.5. This will prevent it from icing up too much as the temp drops.

As I said it has made a huge difference. It is much more effective than those "dehumidifiers" that are really just small heaters, but takes less power than they do. It does take enough power that I need to stay hooked up to shore power.
 

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Discussion Starter #7 (Edited)
After reading your responses and experiences (and other searching) and going through all the options, we found this last night on e-bay and jumped on it. It's a "real" marine grade "made for boats" stainless steel dehumidifier. From what I've read, pretty hot ticket, but as one owner said "too bad they're out of production". Let you know how it pans out.
 

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Over Hill Sailing Club
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If tied to a dock where you can use a dehumidifier, that would be the way to go. The plastic surrounding the cabin is a pretty good moisture barrier. Once there is a lot of humidity in the cabin, it's difficult to get it out unless you open up and vent to get the ambient, drier air in. If the ambient air is saturated, you're out of luck:) Condensation, as mentioned above, is a function of temperature and relative humidity. Water forms on cold surfaces first and will form on the cabin sides as the outside air cools those surfaces. Being a pretty good molecular moisture barrier, water vapor is trying to go to cold, can't and H2O forms on the barrier surface. This is also a major problem with using moisture barriers incorrectly in houses. The key to thinking about water formation is: molecular moisture always migrates warm to cold and cold surfaces that block that flow will form water when the rel. humidity is high enough. If you can keep the cold and the high humidity air apart there is no water. "Burying" the dewpoint inside insulation is a basic principal of super-insulating passive solar houses. No way to do that on a boat.
 
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