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Discussion Starter · #1 ·
I am sorry if this is a post that has been redone multiple times.

I live in the Pacific NW- (wet, mild winters) and am wondering about how people leave their boats to ensure that it is dry. I understand that most people do a small electric heater. (or 100 watt dehumidifier)

Curious what you do, why, and how well you think it works.
 

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I used to use a heater but the problem I found was the heat would vaporize the moisture, and then it would condense on cold surfaces such as the engine. That was causing all kinds of corrosion issues.

I switched to a dehumidifier and it is a much better solution. It maintains 50% RH all winter. Dehumidifiers don't work well in freezing weather, but when it gets that cold humidity is low anyway.

We bought a unit that has a built in condensate pump and ran the hose to the nearest sink, so we don't have to worry about emptying the water regularly.

Sent from my SM-G981W using Tapatalk
 

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Discussion Starter · #3 ·
I used to use a heater but the problem I found was the heat would vaporize the moisture, and then it would condense on cold surfaces such as the engine. That was causing all kinds of corrosion issues.

I switched to a dehumidifier and it is a much better solution. It maintains 50% RH all winter. Dehumidifiers don't work well in freezing weather, but when it gets that cold humidity is low anyway.

We bought a unit that has a built in condensate pump and ran the hose to the nearest sink, so we don't have to worry about emptying the water regularly.

Sent from my SM-G981W using Tapatalk
I like using a "real" dehumidifier instead of the heater that claims to be one.... I might check it out- thanks.
 

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I’m in the PNW too (Lake Union). I run a small dehumidifier and no heater and have had no mold issues. I also have a Nicro solar vent that probably helps. Even in our cold snap last winter (down below 20 F), I don’t think the temp ever got below freezing in the boat. The number of people running unattended electric heaters scares the crap out of me.
 

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Even in our cold snap last winter (down below 20 F), I don’t think the temp ever got below freezing in the boat. The number of people running unattended electric heaters scares the crap out of me.
A dehumidifier does not add much heat to the boat, so you shouldn't rely on it to prevent systems from freezing. You should still winterize for freezing weather. During that cold snap last winter we ran the webasto heater constantly because our water systems were still full. ( we use the boat through the winter)

I think running electric heaters on boats is a concern. The constant relatively high amp draw will expose any bad connections and cause them to heat up and melt. Our club has limited shore power infrastructure anyway, so we have been encouraging members to make the switch from heaters to dehumidifiers, and the result has been a huge drop in power consumption on the docks, saving money and wear and tear on the power systems.

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I bought the Davis Marine Air dryer. Its like a 35 watt heater. It doesn't really heat much, but it does keep the boat mold/mildew free. For me in Virginia, it also USUALLY keeps the cabin above freezing. Yes I still winterize.

But for what amounts to of about $100 for the entire winter in power (along with charging my batteries), my boat stays dry).
 

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A dehumidifier does not add much heat to the boat, so you shouldn't rely on it to prevent systems from freezing. You should still winterize for freezing weather. During that cold snap last winter we ran the webasto heater constantly because our water systems were still full. ( we use the boat through the winter)

I think running electric heaters on boats is a concern. The constant relatively high amp draw will expose any bad connections and cause them to heat up and melt. Our club has limited shore power infrastructure anyway, so we have been encouraging members to make the switch from heaters to dehumidifiers, and the result has been a huge drop in power consumption on the docks, saving money and wear and tear on the power systems.

Sent from my SM-G981W using Tapatalk
Correct-it adds basically no heat. But in our temperatures ( generally not less than 25 F), the water the boat is in seems to keep it from really freezing. Having said that, I do winterize the fresh water system. We just bring water in a jug if we sail in winter. I think the situation is completely different if you are in a very cold climate or if you pull the boat out in winter.
 

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Condensation problem or mildew/mold issue? I, too, have mild, wet winters. My boat sits year round, in the warm waters(59 deg. in Feb) of coastal, GA (USA) Condensation issues are the result of differential heating (of surfaces)in the presence of moisture. The closer your interior surfaces temps are in comparison to outside temps the less condensation you will have. For this issue, ventilation is the key. For mildew and mold, I use the Forespar Australian Tea Tree Gel. I use 5 canisters placed about the boat and the stuff seems to work fairly well. The gel in the canister lasts about one year, and I refill by purchasing the gel in bulk.
 

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Condensation problem or mildew/mold issue? I, too, have mild, wet winters. My boat sits year round, in the warm waters(59 deg. in Feb) of coastal, GA (USA) Condensation issues are the result of differential heating (of surfaces)in the presence of moisture. The closer your interior surfaces temps are in comparison to outside temps the less condensation you will have. For this issue, ventilation is the key. For mildew and mold, I use the Forespar Australian Tea Tree Gel. I use 5 canisters placed about the boat and the stuff seems to work fairly well. The gel in the canister lasts about one year, and I refill by purchasing the gel in bulk.
You are correct, ventilation can be a good way to control humidity, provided outdoor ambient conditions are suitable. That said, passive ventilation would not be very effective, you would need to actively ventilate using a fan. One potential drawback to using active ventilation is that you are introducing large volumes of outside air into the boat. That air could contain dust, pollen, mold and fungus spores etc, which would be counterproductive to the goal of controlling mold and mildew.

I suspect the conditions in Georgia are quite different to the conditions found in the Pacific Northwest. Washington State gets 2 or 3 times the annual rainfall compared to Georgia.

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You are correct, ventilation can be a good way to control humidity, provided outdoor ambient conditions are suitable. That said, passive ventilation would not be very effective, you would need to actively ventilate using a fan. One potential drawback to using active ventilation is that you are introducing large volumes of outside air into the boat. That air could contain dust, pollen, mold and fungus spores etc, which would be counterproductive to the goal of controlling mold and mildew.

I suspect the conditions in Georgia are quite different to the conditions found in the Pacific Northwest. Washington State gets 2 or 3 times the annual rainfall compared to Georgia.

Sent from my SM-G981W using Tapatalk
Condensation in the winter can be quite independent of annual rainfall. As was pointed out earlier is is attributable to large difference in exterior vs ambient internal temps.
 

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Condensation in the winter can be quite independent of annual rainfall. As was pointed out earlier is is attributable to large difference in exterior vs ambient internal temps.
Well, yes and no. It is not so much the difference in temperature as it is humidity. The moisture content of the air is what matters, which is why I advocate for dehumidification rather than heating or ventilation. It doesn't matter what the temperature differential between inside the boat and outside the boat is as long as surfaces don't drop below the dewpoint temperature of the air inside the boat. Dewpoint temperature is determined by the relative humidity of the air.

Running a heater on a boat may give the appearance of keeping the boat dry, but the reality is, the moisture is still inside the boat, it is just in vapor form. For example, the air could be at 60°F and 85%RH. When exterior temperatures drop, un-insulated surfaces get cold. If their surfaces drop below the dewpoint temperature of the air, condensation will form. In this example the dewpoint temperature of the air in the space is 56.5°F, so a mere 3.5°F temperature drop could result in condensation.

If you are using passive ventilation the interior air may be similar to the exterior air , but you have no control over the humidity of that air. If the ambient temperature drops significantly overnight you could get condensation inside your boat the same way dew forms on the exterior.

In terms of how much rainfall affects moisture inside a boat, you have to consider another factor. In wet climates like the PNW rainwater leaks are a constant battle. Most older boats have some leaks. Rainwater finds its way down deck stepped masts and into the bilge etc. That extra moisture isn't going anywhere u less you can physically remove it. Keeping the relative humidity low encourages that moisture to evaporate. Once it evaporates it can be removed by a dehumidifier.

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We have been boating and living aboard in the NNE Pacific for decades.

We keep the interior of our vessel dry using dessicant dehumidifiers when at the dock [typically only a few months each winter.] From experience [and manufacturer specs] dessicant dehumidifiers will continue to operate to nearly freezing temps [~ 34°F] whereas compressor dehumidifiers start icing up well above freezing [~ 42°F].

When onboard we also run heat for our own comfort, but it gets well below freezing this far north... The interior of the boat is always dry even though we have pilothouse windows. [We have many posts about living on a boat in cool climates...]

If I couldn't run a dehumidifier, I would winterize the boat and ventilate enough so there was little difference between inside and outside temperatures while I was away.

Either way, familiarization with the psychrometric charts helped me better understand the process of keeping the interior of the boat dry.

In case this is helpful.

Cheers! Bill
 

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Discussion Starter · #13 ·
We have been boating and living aboard in the NNE Pacific for decades.

We keep the interior of our vessel dry using dessicant dehumidifiers when at the dock [typically only a few months each winter.] From experience [and manufacturer specs] dessicant dehumidifiers will continue to operate to nearly freezing temps [~ 34°F] whereas compressor dehumidifiers start icing up well above freezing [~ 42°F].

When onboard we also run heat for our own comfort, but it gets well below freezing this far north... The interior of the boat is always dry even though we have pilothouse windows. [We have many posts about living on a boat in cool climates...]

If I couldn't run a dehumidifier, I would winterize the boat and ventilate enough so there was little difference between inside and outside temperatures while I was away.

Either way, familiarization with the psychrometric charts helped me better understand the process of keeping the interior of the boat dry.

In case this is helpful.

Cheers! Bill
Thank you- I will need to do some research, I didn't really understand the chart based on that article. But I bet once I figure it out, that will make a big difference!
 

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Thank you- I will need to do some research, I didn't really understand the chart based on that article. But I bet once I figure it out, that will make a big difference!
The simplest exercise.
  • Pick a temperature.
  • Go up to the relative humidity.
  • Go straight across to the temperature you have heated or cooled the air to (without adding or removing water).
  • The relative humidity curve shows the relative humidity at the new temperature.
You can also use the chart to decided how much water must be removed to reduce the humidity at the same or different temperature. BUT, there is also water in the materials of the boat itself.

Another exersize is to determine how dry a heated (occupied) boat must be inside to avoid condensation on a cold window frame.
 

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The simplest exercise.
  • Pick a temperature.
  • Go up to the relative humidity.
  • Go straight across to the temperature you have heated or cooled the air to (without adding or removing water).
  • The relative humidity curve shows the relative humidity at the new temperature.
You can also use the chart to decided how much water must be removed to reduce the humidity at the same or different temperature. BUT, there is also water in the materials of the boat itself.

Another exercise is to determine how dry a heated (occupied) boat must be inside to avoid condensation on a cold window frame.

All of these can be done with one chart.
 
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