Thursday, January29, 2015

Lowering Humidity Below Deck on Papoose

This is the story of my struggles with humidity and mold on my boat Papoose. This page includes several tools for determining and calculating humidity, due point, and calibrating hygrometers. Enjoy the story or just use the tools.


After many experiments with various ways to control moisture below, I settled on trying to maximize ventalization by leaving all hatches open an inch, and using a StorDry to encourage air circulation. This dried out most of the areas below that used to have water dripping. My humidity readings of 70% were not that much different than I got running a heater all day (expensive) and I was able to dry out more areas.

The Struggle

With the conditions below deck in winter growing mold which was literally eating my hats, I needed to do something about the humidity below. For the mold, I washed everything down with a hose (yes, everything) and sprayed Chlorox (4oz per quart -- maybe a little strong) wearing a gas mask and wearing gloves and wiped down all surfaces. It burned my eyes but it got rid of the mold. That was at the beginning of the season. I bought one of those StorDry round 90 watt with fan low watt warm air circulators that is supposed to solve the problem. link Result, I didn't have water dripping from the surfaces so it wasn't 100% humidity below, it was only 97% according to the humidity gauge built into Weems and Plath Barometer/Thermometer/Hygrometer. That reading gave me great concern but as you will see below, probably needlessly but it lead to some good things. Hopefully, reading this article will lead to some good things for your boat as well.

Humidity is moisture in the air so the first thing I did this winter was to address all the leaks that were bringing in water that had no business being there. . No use allowing more water below than what the air wants to bring in. I noticed a couple of leaks in the ports. Fixed them.

I was still relying on my faulty hygrometer, which made me want to get the humidity down. Knowing that air can hold more water when hot than when dry and thus the humidity for a given amount of water in the air is lower if the air is heated, I added a 90 Watt flood lamp to bring the air temperature up. A few days later it was 52 degrees F and 92% humidity. Not good enough, I thought not knowing yet that the reading was faulty.

Next I thought I would try and remove some of the water from the air chemically. I bought some DampRid closet hanging dehumidifiers. link I have had one with the coats this year and it has filled with water so needed replacing anyway. I added a second one. Result, next day no water in the DampRid pockets and the boat smelled like DampRid, same RH. That was a failed experiment.

The obvious solution is a dehumidifier. Defender has one for $500. link It weighs 38 pounds and is almost the size of a case of beer. I have heard people just leave then on the dock when they go sailing -- right. For that money I can run a 500 watt heater for a year continuously. The dehumidifier isn't free to run either. The one in the link uses 385 watts so there really is minimal advantage in the climate I am in where it is cold and humid. If it were hot and humid, that would be a different story.

I bought a Caframo True North Deluxe High Performance Heater. link Issue -- what temperature to run it at.

From this chart, which I modified from an online chart after verifying the numbers, I picked my point at 52 deg F and 92% RH. That puts the dew point at about 51 degrees. Staying on that line, as I am not going to change the dew point with my heater, I see that if I can get the temperature up to about 65 deg F I will be in my green zone (50-60% RH). That is my goal. I am charged for electricity so in the months that follow I can tell how much I am using to keep my boat dry and how good a job I am doing.

Heater was on all night and the readings today were 60degrees F and 92 percent humidity (strange). I heated the cabin up and the humidity dropped, but it did not follow theory so something is off. Perhaps my humidity gauge is wrong? I turned the heater up a notch and will buy an electronic humidity gauge

Humidity Calculators

Here is a calculator that is easier to use than the graph above

RH and DP Calculator