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Condensation? Weeping on the hull walls

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jpwdesigns
Deckhand

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USA
21 Posts

Initially Posted - 12/29/2019 : 16:27:13

My girlfriend and I slept on the boat last night. It was in the 40s and we ran an electric heater all night. In the morning we had water drips all around the interior of the hull.

Is there anything that can be done to help with condensation or do you think this is a different problem?

250 WK #597

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 12/29/2019 : 18:19:12

Stop breathing and perspiring.

That moisture condenses on the hull liner. In warmer weather the boat will be more open and better ventilated, but in colder weather it's closed up and that moisture has nowhere to escape. Two people on a small boat generate a lot of moisture. It will evaporate during the day when the boat is open, but it will gradually accumulate overnight.

Personally, I can tolerate it for a weekend, but go home for the winter.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

csmcg
1st Mate

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96 Posts

Response Posted - 12/29/2019 : 21:13:30

Hello jpw.

A couple of facts to help describe why certain tactics might help:

1) When the surface of some material drops below the dew point temperature, you will get condensation.
2) The dew point temperature rises as relative humidity rises.

Check out this page for a bit of science and an important chart: http://www.ctio.noao.edu/noao/node/651

When the boat is sealed up, sources of water vapor within the cabin (people, cooking, etc...) will make the humidity rise.

You can follow the example below with a dew point calculator like that found at: https://www.omnicalculator.com/physics/dew-point

For example, lets say that the outside air dew point is 37 degrees, the outside temperature is 55. The inside air temperature is 70, and the relative humidity of both the outside and inside air is 50%. This puts the inside air dew point at 50 degrees.

Lets say that the un-insulated inside skin of the hull tracks with the outside temperature and that the outside temp will stay at 55 all night.

Because the temperature of the inside skin of the hull (55) is warmer than the interior air dew point (50), no condensation will occur.

But... lets put a couple of people into the cabin for the night and close all hatches. The people will add to the water content of the inside air over time and raise the relative humidity. Lets say that the relative humidity reaches 80% during the night.

Now if you re-do the math, at 80% relative humidity, the dew point inside the cabin rises to 64 degrees. Anything colder than 64 degrees will start to collect condensation. The inside hull skin (55), being colder than the dew point (64) will gather condensation and be dripping by morning.

So, what can you do?

* You can try to keep surfaces from being cooled below the dew point. This usually means insulating. You can check out various methods in "The Warm Dry Boat" by Roger McAfee. Sometimes it is simple as closed-cell foam yoga mat sprayed with adhesive and stuck to the interior hull.

* You can reduce the interior air relative humidity, which will reduce the dew point temperature, thus making condensation less likely. There are several ways to reduce the interior air humidity. One way is to ventilate. By mixing in lower humidity outside air with cabin air, you can reduce the cabin air humidity. Leave the companionway hatch open a crack, or the port in the head. Of course, this does increase the heating needs.

Another method to reduce interior air humidity is to run a de-humidifier, which does not necessarily mean additional heating loads.

* Lastly, airflow is really helpful. Vents in lockers and other stagnant air spaces will reduce condensation, moisture, and mold.

Most folks use some combination of tactics.

Regards.

Edited by - csmcg on 12/29/2019 21:16:29

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 12/29/2019 : 21:14:40

Dallas is a different environment from mine�I�ll assume we�re not talking about �real� winter... (I�m in CT.) While it may require a little more work from your heater, a �solar vent� (as from Marinco) will expel much of the moist air, drawing in dryer night air through the companionway board slots�no wiring or battery draining required. Some sailors mount them in the forward hatch, avoiding cutting a big hole in the coachroof. Many of us install them just to prevent night-time cooling in an empty boat from condensing moisture out of daytime air, causing mildew. As Steve says, breathing exacerbates the situation, but a little forced air exchange can help control it.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 12/29/2019 21:36:31

jpwdesigns
Deckhand

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USA
21 Posts

Response Posted - 12/30/2019 : 14:24:52

Stinkpotter, csmcg,

Thanks guys that gives me some food for thought. More projects ahead!