Dehumidifier or not?

[TernVI]

The small boat lives in Cornwall, ashore from about now until March. No dehumidifier, no ventilation beyond a mushroom vent. I get a bit of mildew if she’s left more than a few weeks between visits but nothing to worry about and not on the upholstery.
Never had a problem with the boat in Greece, despite it being warm and wet during the winter. However, she’ll have been shut up for 18 months by the time we get to her next year, so lord knows what we will find....

I think that's the key point.
We keep our boat on a mooring, but visit it regularly over the winter. Keep it clean, open everything up when the sun comes out.
And we have Zero Tolerance of rain leaks.

The boat I race on sometimes, lives on a pontoon with power. A dehumidifier means it dries nicely, even if we've been racing in the rain and left damp sails etc below.

I have a cheap compressor dehumidifier in my garage/workshop. I try to keep the max humidity to about 70%, so I don't get rust on tools and the carpet doesn't start to smell. It uses about 2kWh a day in wet weather, well worth it IMHO.
Around here, the great outdoors is often very humid, and not that cold in winter, my well ventillated garden shed always smells a bit manky.

 

[TernVI]

Just installed a planer hot air blower with timer. Nervous about setting the timer for it to come on for say a hour every couple of days as sure dries the boat quickly. Sitting on a mooring. Any one do this.

Hot air heating is great, but you need to let the hot, damp air out.
Otherwise you will dry the bits of the boat which get warmest, but the coldest bit of the boat will get condensation.
A bit like my old estate car, nice and warm and dry up from, but condensation on the tailgate glass.

I have reservations about a diesel heater unattended. We thought about some sort of overheat fuel shut off and even mounting it in a box with an auto fire extinguisher. My mate used to run one remotely by text message in his LandRover, but that was a Webasto IIRC.

 

[duncan99210]

When we lived aboard full time, we used the Ebersbacher for heating through the winters. That sorted the condensation problems, including all the cooking steam. However, we did tend to leave the top washboard out most of the time as it made access easier as well as providing plentiful ventilation.
If we left the boat for more than a couple of days, we‘d leave the heater on the timer to come on 3 times a week for a couple of hours. The pulled fresh air into the boat and kept things fine. That said, we had neighbours in the marina who kept an eye on things for us including making sure the power supply was working. Don’t forget that the Ebersbachers and the like have pretty effective overheat cut outs.

 

[RJJ]

Do you hang your washing out in a sealed room with a dehumidifier, no you hang it out to dry

"sealed room with a dehumidifier" = tumble dryer.

 

[thinwater]

... Do you have any woodwork? If you do it will completely dry out and start to split.

A. No, I have never noticed that, over several decades of using a dehumidifier.
B. My home furniture doesn't split.

We're not talking about a wooden hull that has soaked for years suddenly being desicated. We're talking about duplicating home conditions on plastic boats with a little wood inside. So perhaps this is an exaggeration.

 

[Whitlock]

A. No, I have never noticed that, over several decades of using a dehumidifier.
B. My home furniture doesn't split.

We're not talking about a wooden hull that has soaked for years suddenly being desicated. We're talking about duplicating home conditions on plastic boats with a little wood inside. So perhaps this is an exaggeration.

Not an exaggeration. If you have a completely sealed and completely dehumidified boat your internal, not hull, woodwork will shrink and crack as all the moisture is drawn out of it at the rate of about an inch a year. If that's not happening then fresh air must be getting in or the dehumidifier is not set to completely dry. Ergo, dehumidifiers on boats are supplied with a constant stream of damp air from outside and only give the impression that they are doing anything useful by producing lots of water.
My comments are tongue in cheek in response to this idea of being able to make a production boat airtight. I should confess that I think that dehumidifiers are a waste of time and the correct way to keep a boat dry and condensation free is to heat and ventilate it.

 

[JumbleDuck]

I have a cheap compressor dehumidifier in my garage/workshop. I try to keep the max humidity to about 70%, so I don't get rust on tools and the carpet doesn't start to smell. It uses about 2kWh a day in wet weather, well worth it IMHO.
Around here, the great outdoors is often very humid, and not that cold in winter, my well ventilated garden shed always smells a bit manky.

I've been tempted, but my workshop is a 200 year old smiddy and leaks air everywhere - including through a large vent in the roof which I think let the smoke from the hearth out. Luckily things in it seem to stay pretty dry and I haven't had significant issues with rust, although the column of my milling machine and bed of my lathe both needed a good clean up after I abandoned them for ten years.

I suspect that for workshops one either wants lots and lots of ventilation or somewhere airtight enough for a dehumidifier. I added an open fronted barn a couple of years ago and things stay pleasantly dry in there as long as they are more than 6' from the entrance and so don't get rained on.

 

[thinwater]

Not an exaggeration. If you have a completely sealed and completely dehumidified boat ....

Right there is an exaggeration. No boat is air tight, and all dehumidifiers are adjustable, even if just by using a timer.

 

[Railbob]

On my previous boat I'd run a dehumidifier over winter as I always found her to be a bit of damp boat, even with heaters and good ventilation. On my current boat for the first year we had her I ran the dehumidifier over the winter but didn't really notice any discernible difference in humidity , so didn't use it the next year and she was perfectly dry and I haven't used it since. For me I think the difference is that my new boat is bigger and has better ventilation.

 

[KompetentKrew]

Not an exaggeration. If you have a completely sealed and completely dehumidified boat your internal, not hull, woodwork will shrink and crack as all the moisture is drawn out of it at the rate of about an inch a year. If that's not happening then fresh air must be getting in or the dehumidifier is not set to completely dry. Ergo, dehumidifiers on boats are supplied with a constant stream of damp air from outside and only give the impression that they are doing anything useful by producing lots of water.

To be useful the dehumidifier need only remove water at a faster rate than it gets in.

We can see from the graph posted by @jdc in post 44 that the humidity rises when the companionway hatch is opened and workmen spend the day going in and out of the boat, yet the dehumidifier lowers the humidity again once they leave. Surely this indicates that it is having some effect?

 

[pessimist]

An update. Bit the bullet and purchased the recommended Meaco - the DD8L Junior. Seems to work very well in the bathroom so should sort the boat. Superb service from Marine Superstore - ordered (via Ebay) one day delivered the next. One thing to be aware of - to "save the planet" they no longer supply the hose required for continuous draining, so the thing is useless on an unattended boat until a source of tubing can be found. Doesn't stop Meaco touting their PBO and Sailing Today awards though.

 

[Sandy]

"sealed room with a dehumidifier" = tumble dryer.

Tumble dryers use heat, a different technology.

I've only seen dehumidifiers work successfully once at The National Outdoor Center in Wales about 35 years ago. Goodness knows how much that cost to run but the dehumidifier units were massive.

 

[Lucy52]

Whitlock said, ".....I think that dehumidifiers are a waste of time and the correct way to keep a boat dry and condensation free is to heat and ventilate it."

So, whereas with a dehumidifier you are trying to dry out the marina, then with heat and ventilation you are trying to heat the marina? No?

I think if you are living on the boat in cold Northern climes, with breathing and cooking on the inside, unless you live a very rufty tufty existence then I think a closed hatch and a dehumidifier is the way to keep the damp away.

An empty boat left for the winter may be quite different.

 

[jdc]

I came across this little infographic which might be informative. Just an 'infographic' so hardly science but probably about right.

 

[Whitlock]

Whitlock said, ".....I think that dehumidifiers are a waste of time and the correct way to keep a boat dry and condensation free is to heat and ventilate it."

So, whereas with a dehumidifier you are trying to dry out the marina, then with heat and ventilation you are trying to heat the marina? No?

I think if you are living on the boat in cold Northern climes, with breathing and cooking on the inside, unless you live a very rufty tufty existence then I think a closed hatch and a dehumidifier is the way to keep the damp away.

An empty boat left for the winter may be quite different.

I do live on a boat in cold northern climes, with breathing and cooking on the inside, and have done for many years so I speak with knowledge on this subject.
We are really very comfortable with all modern conveniences and our lives are not at all "rufty tufty". Neither do we live with open hatches. Our boat is as comfortable as any house or apartment and we keep it warm and ventilated in winter in exactly the same way.
Would you run a dehumidifier in your house? If you do then you must have a serious problem.

 

[Mudisox]

Similar boat and also on the Dart. 24 hrs alongside with electricity with the unit on the draining board and pipe to galley sink does the job, with or without the heater on.
It makes a difference in the feel of bedding, seats and also the condensation on the windows. Particularly relevant in this weather. Only do it nw and again.

 

[JumbleDuck]

Would you run a dehumidifier in your house? If you do then you must have a serious problem.

Or an old house. My place was built, 200 years ago, on the assumption that there would be draughts all over the place. With double glazing and well-fitting doors, even the two chimneys can't get rid of the moisture which wicks up through 3-foot thick rubble-filled stone walls with nothing like modern foundations and absolutely no chance of a damp-proof course.

 

[Lucy52]

Jumbo. It sound like you need to dig a good French drain all round and run it off to a soak-away. Reinstate the rattly widows drafty doors for ventilation and keep a big log fire in the grate. Some sheeps wool insulation in the loft will help. Cable knit jumper, wing back chairs and a hot cup of coco are optional.

 

[johnalison]

Not an exaggeration. If you have a completely sealed and completely dehumidified boat your internal, not hull, woodwork will shrink and crack as all the moisture is drawn out of it at the rate of about an inch a year. If that's not happening then fresh air must be getting in or the dehumidifier is not set to completely dry. Ergo, dehumidifiers on boats are supplied with a constant stream of damp air from outside and only give the impression that they are doing anything useful by producing lots of water.
My comments are tongue in cheek in response to this idea of being able to make a production boat airtight. I should confess that I think that dehumidifiers are a waste of time and the correct way to keep a boat dry and condensation free is to heat and ventilate it.

You are doing what people so often do on forums, which is to assert that something is the only right way of doing something. There may be a few places where this is the case, and I can think of instances within electrical systems where this might occur, but in general sailing and boat management it suggests a rigid approach and lack of tolerance. I would not assert that a dehumidifier is always necessary, only that I have found it useful, as have others. The suggestion that damage to woodwork would inevitably result shows that your knowledge of the subject is limited.

 

[ip485]

Not an exaggeration. If you have a completely sealed and completely dehumidified boat your internal, not hull, woodwork will shrink and crack as all the moisture is drawn out of it at the rate of about an inch a year. If that's not happening then fresh air must be getting in or the dehumidifier is not set to completely dry. Ergo, dehumidifiers on boats are supplied with a constant stream of damp air from outside and only give the impression that they are doing anything useful by producing lots of water.
My comments are tongue in cheek in response to this idea of being able to make a production boat airtight. I should confess that I think that dehumidifiers are a waste of time and the correct way to keep a boat dry and condensation free is to heat and ventilate it.

I dont follow the logic.

My humidfier reads the % mositure content. It I havent run it for a while in the winter it will typical be at 75% to 80% and will fall to around 40% after a good run. The boat feels very different, after a shower the windows dont instantly stream with water, and especially behind sofas and so on the compartments are dry with no moisture on the walls. I could therefore accept your first premise if the results were not more widely evident. The dry air will remove dampness from soft furnishing, compartments etc.

I do feel that many humidifiers are barely up to the job of significantly altering the humidity. A smaller one I had, had little impact on the RH, but produced plenty of water. You do need a pretty powerful humidifier to produce a tangible result.

My woodwork is all teak and I have had no damage whatsoever. In fact well made cabinet work should deal with changes in both temperature and humidity with issue. I would agree rapid changes in either may not be an ideal combination is some circumstances.