The Dehumidifier vs Ventilation Debate - Bah, phooey and humbug!

Simondjuk

Simondjuk

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We've been spending quite a bit of time on the boat. Two days ago our dessicant dehumidifier messed the bed. We immediately adopted maximum ventilation measures. Tonight, on our return to the boat, the deckhead is still covered in beads of moisture and the duvet is damp to the touch. Previously, all was dry and warm.

Dehumidifier 1 : Ventilation 0

End of story.

All countering arguments considered.
 
rapid diurnal changes in temperature and humidity are responsible in this case. I'd stick with a dehumidi thingy till the electric runs out.
 
Ventilation is very very effective when there's heat involved too. Suck air in, warm it up, it then sucks in moisture, and then exits the boat.

Balanced correctly this keeps boat warm, dry and fresh and beats the hell out of any dehumidifier, and once you get to know how best to do it it frees you up to live off grid, which is what being on boats is all about!

But having said that we do have a Meaco Dessicant (which also wet the bed recently) for when we get the above wrong.
 
I leave the boat in the water all winter, leave all the upholstery in place, don't have a dehumidifier or background heating, and don't seem to have any problems. But the boat does have good ventilation.
.
 
CaptainBob said:
Ventilation is very very effective when there's heat involved too. Suck air in, warm it up, it then sucks in moisture, and then exits the boat.

Balanced correctly this keeps boat warm, dry and fresh and beats the hell out of any dehumidifier, and once you get to know how best to do it it frees you up to live off grid, which is what being on boats is all about!

But having said that we do have a Meaco Dessicant (which also wet the bed recently) for when we get the above wrong.
Click to expand...

Which is great, and indeed true to a point, but if I heat and ventilate simultaneously, I'm basically trying to heat Hampshire, which doesn't work too well.
 
pvb said:
I leave the boat in the water all winter, leave all the upholstery in place, don't have a dehumidifier or background heating, and don't seem to have any problems. But the boat does have good ventilation.
.
Click to expand...

We have a dorade open, all four deck hatches in the vent position and two human beings cooking and breathing aboard. The outside air is already saturated, I'm not sure how it's supposed to carry any of our moisture production away.
 
Update: Just gave the machine a technical shake (don't want to take it apart as it's still under guarantee) and it's running for now. Unless it sets fire to the place, there may be no shower in bed in the morning.
 
A friend's boat was hurriedly tied up and left in an Essex marina at the end of a month's jaunt. He was then unexpectedly overtaken by domestic events, ended up working abroad etc. and unable to return to the boat. He had it moved to a nearby mud berth after a few months. The boat then sat there. When we went aboard again after something like four years everything, including the teatowel I had forgotten to hang up, was exactly where we had left it. There wasn't the slightest sign of mould, staining, or smell of damp. No dehumifier had been used. The boat was unusually well equipped with dorade vents, a couple of very small portlights had been were left open, and there was an air gap at the top of the washboards.

On the other hand, trying to keep the interior of a small boat damp-free by ventilation alone when in active use in the English winter is a major challenge!
 
Simondjuk said:
Update: Just gave the machine a technical shake (don't want to take it apart as it's still under guarantee) and it's running for now. Unless it sets fire to the place, there may be no shower in bed in the morning.
Click to expand...

You could also try spending the night on board without breathing or cooking- that might help?
 
Simondjuk said:
True. Letting copious amounts of wet air in won't help, but might make things cooler.
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Simon, that not how it's suppose to work. To dehumidify with saturated outside air you need to heat this incoming air up.
There is a chart called a psychrometric chart that shows the relationship between air temperature and moisture. If you take outside air at zero degrees C and 100 saturated and heat it to 20degC the moisture content of this air would be about 26%. This would feel very dry and would have the ability to absorb lots of moisture from the boat. By comparison, the humidity levels in a house in summer would typically be 70%.
If you have saturated air ventilating a boat all winter without heating it there is no way this air can remove moisture. There will be times over winter when the air is not at 100% saturated so there would be times when this air would assist in removing moisture.
The secret really is heat and ventilation. Our last boat had a Dickenson drip feed heater and four large dorades. We could comfortably live on that boat all winter with minimal condensation and dry bedding. The boat was fully insulated so the hull stayed above dew point.
 
on my old boat - I used only ventilation. I was often on board for up to a week at a time throughout the year and never had any problems with damp. I always left the hatches on vent and if I pulled the companionway hatch forward, it created an inch wide gap - so plenty of ventilation.

However, now we are virtually liveboards on our much bigger boat (2 adults + 1 pet) we need a dehumidifier. We also have 4 dorade boxes + plus 4 vents, plus the hatches on vent, plus the companionion open a zillion times a day. We do not shower on board, nor dry clothes. However, after 3 months we had wet under the mattress, wet behind the dressing gown hanging on a hook, and all the clothes felt damp and clammy. Got a meaco, shut all the vents, and extracted gallons and gallons in first few days (astonishing).

My conclusion:
if you are not liveaboad and have several days for the boat to dry out in between stays = you don't need a demudifier.
If you liveaboard in the UK = you probably do.
 
ChattingLil said:
on my old boat - I used only ventilation. I was often on board for up to a week at a time throughout the year and never had any problems with damp. I always left the hatches on vent and if I pulled the companionway hatch forward, it created an inch wide gap - so plenty of ventilation.

However, now we are virtually liveboards on our much bigger boat (2 adults + 1 pet) we need a dehumidifier. We also have 4 dorade boxes + plus 4 vents, plus the hatches on vent, plus the companionion open a zillion times a day. We do not shower on board, nor dry clothes. However, after 3 months we had wet under the mattress, wet behind the dressing gown hanging on a hook, and all the clothes felt damp and clammy. Got a meaco, shut all the vents, and extracted gallons and gallons in first few days (astonishing).

My conclusion:
if you are not liveaboad and have several days for the boat to dry out in between stays = you don't need a demudifier.
If you liveaboard in the UK = you probably do.
Click to expand...

I wouldn't disagree. Our current boat has lots of dorades. We are living onboard at weekends. The eberspacher and electric heater run at weekends and we have put the blanking plates in the dorades and turned on the dehumidifier over most of the winter. We do shower and cook onboard so lots of dampness but we have a dry boat with dehummer running 24/7
 
Simondjuk said:
We've been spending quite a bit of time on the boat. Two days ago our dessicant dehumidifier messed the bed. We immediately adopted maximum ventilation measures. Tonight, on our return to the boat, the deckhead is still covered in beads of moisture and the duvet is damp to the touch. Previously, all was dry and warm.

Dehumidifier 1 : Ventilation 0

End of story.
Click to expand...

Similar experience.
Tried ventilation, humidity traps, ... nothing worked.
That is to say, nothing worked like our B&Q dehumidifier (£19.99).
It just completed it's fifth winter :encouragement:
 
So what's the solution for a boat without mains electricity? I'm returning to mine tomorrow, last visit early December, and I am hoping that the solar powered PC fan running beneath a mushroom vent in a hatch will have helped matters. I calculate that at 400 l/min ( http://www.chegg.com/homework-help/...pter-5-problem-11p-solution-9780077295462-exc) it will replace the air in the boat ((10m x 4m x 2m)/2 ~40,000 l) in less than 3 hours of running, accounting for flow inefficiencies.
Very crude calcs and this won't help very much at night when temperature drops sharply, but I hope seasonal damp will be prevented.

Any other suggestions for long term off-grid damp proofing?
 
We've spent many days off grid at all times of year in the UK.

Simple plan... place heat producing item (radiating and conducting to air, creating convection) low, beneath an opening out of which heated rising air can escape, as close to one end of the current space you're occupying.

As near as possible to the other end of the same space, also have an opening.

This causes air to rise and exit above the heat source which causes air to be sucked in at the opposite end of the space.

This creates a warm, dry, dehumidifying airflow and no condensation development anywhere in the same space.

if you also open a vent midway between the two ends, things are spoiled. Things get damp.

Your own body can be that heat source too. In a bunk, an opening at your feet, and one at your head, and you have a dry cabin.

It works as a very effective dehumidifier by losing a little of the generated heat - you can't dehumidify off-grid without some losses after all. Air comes in and is warmed and is then able to hold more water - so it's relatively dry and so sucks in moisture. It then flows out of the boat before cooling/condensing. You create your own dehumidifier.
 
HiAndDri said:
So what's the solution for a boat without mains electricity?....

Any other suggestions for long term off-grid damp proofing?
Click to expand...


A cockpit cover helps a lot in keeping drips away from the main hatch and cockpit lockers. You can then keep your cockpit lockers slightly open which promotes a flow through the boat. I have a pet theory that the cover promotes a greenhouse effect when/if the sun shines, which also helps, a lot - if it is true.

A box shaped contraption affixed over the fore hatch gives security and prevents rain blowing in, whilst allowing the hatch to be left opened.

It you are ashore take the valves our of all your Blakes seacocks.. and grease them and replace before launching. Open the boat up and remove as much organic stuff as possible, etc, etc.


This normally keeps my boat ok over a typical winter but the 35 days of continuous rain this time has proved a challenge.
 
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