The Science of Keeping your Boat Dry.

The science of keeping your boat dry.

 

Fall hit us like a hardwood 2x4 across the neck. It was warm, sunny 27 C and suddenly the sun set two Saturdays ago and the next day was wet. The day after Grandview looked like a horizontal waterfall at 5 AM. I was up, looking at it, thinking that, paradoxically, Grandview now being a waterway did not mean more opportunities for sailing. Depressed, I spent a day upside down in the boat, bleeding and changing the oil in everything I could think of that should have oil in it. 

Luckily this called for a trip to Steveston Marine to get a little something to fell better with.
In my case, it was a boat dryer, 120W this time - I have a little 70W one with a fan in already.

This one did NOT have a fan. I was considering whether the 120W would be better than 70W + fan. 

The science of keeping a boat dry is not exactly rocket science. Which is a pity for a rocket scientist.

I used to fly gliders a lot and we had almost the same problem with the transport trailer over the winter. Many things were tried. I had a lovely old Standard Cirrus and every winter, we took the instruments out and made some provision for drying the air in the trailer. A glider trailer is about 8-10 meters long, made of fiberglass and contains about the same amount of air as a 29 foot sailboat. I have some experience keeping the trailer and plane dry, but it never really went 100% well and we had to remove seats, parachute, radio, variometer and what else was in there which could not be dipped in water. In the spring there might be water in the hoses for the instruments working off air pressure and it was not pretty.

Usually we would try the bags of desiccants, but after a short while, all we had was a bucket of suspicious chemicals and a flaccid bag the chemicals used to be in. Ventilation did not seem to do it either. We left the hatch slightly open and put venting holes in the top, but nothing made a really huge difference. People who had farmer friends would store the trailers indoor, which works.

Finally one fellow hooked up a 2000W heater blower. That worked. He was expelled from the club when the board of directors figured out he had hot-wired his electricity meter and the club was paying for the feast. Yet, the effort remains the only successful method at that date. Later, quite a bearable solution was found with the solar powered ventilators. They worked better than most of the methods, except the 2000W heater. I believe this now is the vanilla solution.

Since the Marina has its very own special price for electricity, I do not really feel like putting a 2000W heater in the boat. 190W seems a little more reasonable, so I have the two air dryers running now. 70+120W = 190W of heat. One works by convection, the other with a little fan. I had turned the big one off, as an experiment, last night ... and now with a humidity of 94% outside and a temperature of 14 degrees, I thought I better go turn both on. On the way I passed Canadian Tire and picked up the temperature and humidity meter at the top. Inside the boat was 17 degrees and a relative humidity of 75%. It was time to get scientific about it.




The various temperatures right now are as follows.

Boat: 17C, 75% RH. (Calculated dewpoint, 13 C)
Air: 14C, 94% RH, dewpoint 13C. (as per weather man).
Water: 14.1C (as per my sounder).

The weather forecast says we may hit a low of 11C tonight.

The parameter that does NOT change for air is the dewpoint - the temperature at which water starts coming out of the air, because the air molecules can't hold the water molecules suspended anymore.
Water only changes dewpoint if it manages to suck up water or get rid of water by condensation. 

We note that the dewpoint inside the boat is exactly the same as outside the boat, the air is just 3 degrees warmer, thanks to my 70W heater. The inside humidity CAN be affected by either wet or very dry cushions and so on, but it was not. Any fabric or water absorbing material will create a balance with the air, so if my cushions were bone dry, they will take a bit of the humidity out of the air. I was very quick and careful going down the hatch and closed it immediately after me, so I could get a reading. My sense was correct, my ventilation was adequate - which means the outside air came in with its dewpoint of 13C, but the air was warmed up by 3 degrees .. thus making my stuff drier.

Looking at those numbers, the boat should be fine tonight.
The water is 14C, which is one degree over the dewpoint of the ambient air, which will continue to find its way into the cabin (dewpoint=13C). There is no way for the inside of the hull, below the water line, to become so cold there will be humidity coming out of the air.

The problem may be on the roof. The outside temperature may well drop to 11C, which is two degrees BELOW the dewpoint inside and humidity will then form on the roof of the cabin.

A vent or blower would have done nothing to prevent it.

Now of course I have 190W of heat going, which is blown upwards towards the roof. At least in the main cabin, I will have no trouble at all. I am pretty positive the two will keep the temperature of the cabin at 3-4 degrees above the outdoor temperature, so the fresh air coming in will have a dewpoint that is at least 3-4 degrees below the temperature it will be heated to. At the same time the roof has lots of circulation from the two driers.

If air is cooled down to the dewpoint or below, the dewpoint moves down with the temperature as the water comes out of the air, so if the outside temperature drops below 13C, so will the dewpoint.

I could however have a problem in lockers over the waterline - and if the water was colder than the dewpoint, I would certainly have a problem if I did not have circulation of the air.
For condensation to happen on the inside of a fiberglass hull, the INSIDE of the fiberglass has to be below the dewpoint. If there is plenty of circulation, the inside of the hull will warm up towards the air temperature inside the boat and the entire temperature drop happens inside the fiberglass. If there is very little motion in the air along the inside, the temperature drop may very well happen in the first few mm of air next to the wall ... and then it starts raining from the mm closest to the wall. 
Here ventilation is great - but it has to get to ALL the places that we need to keep dry.

Of course the white Naugahyde wallpaper that was so modern in the early 70's will completely thwart that  effort. It has a layer of insulation behind, which will have have unmoving air in it .. and if that air is humid, it will create rainy weather behind the Naugahyde. Then the Naugahyde falls down and it's black behind it and the cleanup crew has to come in.

So does ventilation help?
YES, but only if the dewpoint of the air outside is lower than the dewpoint of the air inside.
Otherwise it is a completely futile effort. If there is no heating of the cabin added, the air vented out is the same as the air vented in, except if the boat is wet inside. In which case the dewpoint inside is higher than that of outside and rule 1 applies.

Ventilation combined with a slight heating works, since heating the outside air creates a dewpoint buffer of a couple of degrees. With a fan inside the cabin, the surfaces will be warmed up as well as the air and condensation will not happen because the temperature drop happens inside the fiberglass hull, not in the air just over the inside wall.
It is still possible to exhaust that temperature buffer, if the air temperature outside drops fast or the water is cold. The places with still air will be raining first.

Remember also that on clear nights, the temperature of the top of the boat will drop UNDER the air temperature because of radiation exchange with the Universe, which is -57C in terms of IR radiation. That is why the ground gets cold and the grass gets dew on it.  The surface can easily be a several degrees colder than the air just over it. In that case a fan ON TOP OF the boat would work. Except people would think you are silly fanning your boat.

The only really solid idea that works all the time is to lower the dewpoint inside the boat. That can be done by means of the dehumidifiers sold at home and hardware stores, Canadian Tire etc. They have a compressor and a fan inside them, which cools a surface inside the machine down to below freezing. This immediately gets the rain going, but inside the machine ... the air that comes out of it has a dewpoint of, say, -10C or whatever the cooling element can get it down to.  The rain is collected in a bucket and the machine continues to do so until the relative humidity is below a setpoint chosen with a +/- selector. Usually the larger dehumidifiers has a hose option, which can be dropped into the bilge or into a sink. Now THAT's good news for a control freak.

You can for instance set the machine to 60% relative humidity. At 15 degrees, that corresponds to a dewpoint of only 7 degrees. At 10 degrees, the dewpoint at 60% RH is 2.5 degrees.
Even freezing water outside will not be able to condensate water on the inside of the hull.
Another great thing is that a medium sized dehumidifier uses about 500W and blows a storm out of the top of it. It will not run all the time, but it WILL increase the temperature in the cabin by about 5 degrees, at least.

DO NOT set the dial lower than 60% ... 50% will dry out your teak and it will end up warped and cracked.

If the boat's vents are closed, it does not have to work nearly as hard and you can save some power.
This also means it heats the air less... so when you can't dry out your teak, perhaps let in a tiny bit of air it can be allowed to suck dry, so it, as a by product, heats up the air a bit. Both are beneficial effects. Hotter air = bigger temperature buffer.

The humidity will be completely under control inside the boat, the papers crisp and the Naugahyde will not be naughty. The downside is that those dehumidifyers are fairly big and noisy, but coming out to a boat that's heated up a couple of degrees, smells nice and has no mildew ... priceless.

Kent