Something that we struggled with during our trip up the inside passage was how to keep the relative humidity low enough inside that our walls weren't dripping wet from condensation every morning. It's something that I've heard many fellow cruisers struggle with. In this post, I'll share the strategy that we found (through trial and error) worked best for us.
The Problem
First, a quick overview of the problem for those that aren't familiar with this issue. Relative humidity is the amount of water vapor in the air, expressed as a percentage of the total amount of water vapor the air could hold. Warmer air can hold more water vapor. Humans breathing and sweating, wet clothes drying out in the cabin, heating liquids on the stove, and more all add water vapor into the air. In watertight areas like the cabin of a boat, this water vapor accumulates with no place to escape. When warm air (carrying lots of water vapor) hits a cold surface, like the hull of a boat that's sitting in cold water, or the metal rim around a window that's gotten cold overnight, then this water vapor condenses into water droplets. In the short-term, it's annoying to wake up to water dripping on your face. In the long-term, this causes mold and rot throughout all those crevices of a boat that don't have good ventilation.
The Challenge
At a marina, the solution is simple: buy a dehumidifier capable of sucking large quantities of water out of the air and run it so that the dew point inside the cabin always stays below the outside water temperature (basically keep the relative humidity at or below 50%). We have this one that removes up to 30 pints a day that we find works well. However, these machines plug into 115-volt ac household outlets and use more power than our house bank of batteries could handle, so they're only available for use while at a marina and plugged into shore power.
Our Off-The-Grid Strategy
First of all, two pieces of equipment are needed. First, you need to be able to measure the relative humidity in your cabin. We bought this hygrometer mainly because it's cheap and simple. The 24-hour temperature range readout stopped working soon after we bought it, but that's ok because I don't really care about that. There are probably nicer hygrometers out there but this one works for us. Second, you need some sort of cabin heater. This is a major project, but I'm guessing anyone going to Alaska would be installing one of these anyway. We're very thankful that De Novo came with a Wallas forced-air diesel heating system that was installed by the previous owners.
What we found worked best when the humidity got high was to turn on our heater to max heat to bring the cabin up to a comfortable room temperature. Once the temperature was just right, we'd turn the heater down to low and open a ceiling deck hatch. If it was raining out (which it often was in these situations), we'd open the companionway hatch since that is covered by the dodger (we found this is a great reason to have a dodger). Since hot air rises, and warm air holds more water vapor, doing this allows all that warm, moist air to escape the boat and take lots of water vapor away with it. Keeping the heater on at a low setting maintained room temperature (even though all the warm air was escaping) and it also helped circulate air around the cabin. We'd keep the hatches open until the humidity levels got down to a reasonable level. Typically we'd do this in the evening once we've dropped anchor and were relaxing before going to bed. We found that we could get the humidity to drop from 70% (with air temp around 60°F) to 45% (with air temp around 70°F) after an hour or two.
This strategy isn't enough to keep rot and mold from forming over the long-term; it's nothing compared to running our big dehumidifier while at the dock. There were a couple times we paid for a marina slip primarily to run our dehumidifier and electric space heater after multiple days of wet and cold weather making the humidity more than we could handle off-the-grid (pulling into Hartley Bay immediately comes to mind). But it's the best short-term solution we found to prevent waking up with water dripping on our faces when anchored in the temperate rainforest wilderness found throughout the Inside Passage.
