Eric asks:

"Why is the inside of my boat (20 foot wooden powerboat) so damp?"

Scott answers:

After spending 5 years working on a boat in the arctic and substantial time living in a tent in below-freezing conditions, I can understand and sympathize with your situation. There are three fundamental problems:

1) Every time you cook with propane, you are pumping water vapour into the cabin. A pound of propane is going to produce about a pound and a half of water vapour, and that's a lot. Plus, every time you exhale you are also emitting water vapour. The result is that the air in the cabin is saturated with water vapour.

2) In cooler weather there are many objects on the boat that are below the dew point of this saturated air. Windows are the obvious culprit, because they are at roughly the same temperature as the outside air. But cabin sides, dishes, metal objects, clothes, etc. can also be cold. When these objects cool the moist air below its dew point, the moisture condenses out of the air as water, settling on the cool object. The result is that everything on the boat feels damp and clammy, because it is in fact slowly getting covered with water.

3) This is exacerbated because you have no source of heat that will raise the dew point of the air.

The solutions are actually pretty simple.

1) You need to make sure that you control the sources of water vapour. The best way to do this is to put a ventilation fan in above the stove. Use a computer fan and a Nicro-fico fitting. This will move a ton of air and should be run whenever the stove is burning. I know that it's a bit counter-intuitive, that running a vent fan will make you "feel" warmer, but it's true. I have 4 of these fans on the boat, I leave them running 7X24 when I am at the dock. Keeps the boat fresh-smelling. I have three exhaust fans (vented to the outside), one in the shop, one in the forward head, and one over the stove. I also have two solar vent fans, one in the lazarette and one in the mid head. I also have one that draws from the shaft alley into the engineroom, which has several vents.

Nicro Fico makes a bunch of vents. See . You can buy one with a 12 volt fan if you want, or you can buy the unpowered one (model 500). If you buy a 3 inch computer fan (less than $10) you can glue it on to the inside and it will work like a charm. Make sure you get a quiet fan. You can also add a 100 ohm rheostat for variable speed control.

2) It's pretty hard to add insulation to the boat after the fact, especially to windows. Probably not worth it unless you live aboard. Even then don't replace the windows, just use the clear insulating heat-shrink film (eg This works surprisingly well.

3) The real solution is to provide a source of DRY heat inside the cabin. Candles, hurricane lanterns, catalytic heaters, etc. are all useless, because they just dump water vapour into the cabin atmosphere. There are only two possibilities on your boat. At the bock you can use electric, but that's not too practical when you are away from the dock. You can also use some form of vented heater, such as a wood-burning fireplace or a diesel stove. But by far your best solution is a direct-vented propane heater like this one: This draws combustion air from the outside and then returns the products of combustion back out through the same pipe. This has the dual benefit of reducing the water vapour and also keeping the warm air inside the cabin (it's safer too).

So, here's the bottom line: put in a vent fan over the stove, and a propane heater on the aft bulkhead. Figure out how to wedge a 20 lb propane tank into a seat locker so you don't run out of propane. And cruise to your heart's content. If you do decide to head up to Princess Louisa for Christmas, consider plastic film on the windows. And run the fan when you are not at the dock, the draw is about 2 watts.


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    About Scott

    I'v been boating since I was 6 years old, and I'm passionate about doing things the right way. Hopefully you'll find these posts useful, and if you disagree with then, I'm always ready to learn new things, feel free to comment or email me.


    March 2011