condensation in sealed "water tight" hulls

Is it possible for condensation to form in sealed air and water tight hulls? I am thinking specifically of beach cat hulls and trimaram amas.

I am building a 20' trimaran and would like to completely seal the ama hulls whereby sea or rain water cannot enter. However, I am concerned that from experience with beach cats such as Hobies that there always seems to be some water within the hulls. I 've had similar experience with ama hulls. The hulls seem to be sealed but there it is - a little bit of water - is there any possibility its condensation or must it always be from a leak? Thanks for any advice

 

If you fill the interior with foam it might stay dry , although the foam has weight .

FF

 

Unfortunately I dont want the extra weight but I wonder what causes the water to form in the first place...?

 

Temperature and humidly changes causes moisture in the air to condense and evaporate repeatedly each day. You could try an inert gas like argon, but keeping it truly vapor tight would prove troublesome.

 

polyuethane foam will absorb moister after time,,and need periodic replacement

 

If its sealed then how do you know there is water in there? I guess you must have an access hatch!!!!!! Probably that leaks a bit, especially as the day cools and the air inside contracts.

A little bit of water is not normaly a problem is it? unless you are building in steel.

 

Yes, That's why I dont a hatch - they always seem to leak. !!(actually a beckton plate) . Water left in the bilges of a wood boat will cause a problem eventually as it obviously causes rot over time. And in Amas, because of the compartmentized bulkhead arrangement water is often difficult to spot unless many plates are installed in deck. Which is a catch 22 as they tend to leak.!

Getting back to my question - Does condensation occur in seal hulls like Amas ? I wonder if anyone can give me a answer to that specific question?

 

If you fill the hull with cold dry air and manage to make it not only water tight, but diffusin tight, then no water will condensate as long as the temperature is allways higher than it was when you sealed it...

But this is theory, I think it's impossible to make it "diffusion tight".

 

This is pretty much true. An enclosed volume in a humid environment that has repeaded daily temperature cycles will "pump" moisture vapor through the walls unless the walls are 100% impermeable. It may take many years but will happen.

That is the theory. I have had no problem with air tanks built of plywood and well coated with epoxy over decades of use.

 

I'm building a boat with 24 air and water tight compartments (I assume). The biggest one is .25 m3 (9 CuFT). They are sealed with Epoxy. Somes sections have fiberglass also. But I live in Canada and temperature vary for -35C to +35C (-30F to +95F). With this calculator http://www.cactus2000.de/uk/unit/masshum.shtml I calculate that if I seal the compartment with 50% humidity at 15C (60F) I capture 6.4 gr (less than .25 oz) of water. If temperature drop to -35C (-30F) 6.2gr of water (or 6.2 ml) will condensate in compartment. If I can seal at 0 C (32F) I capture only 2.4 gr of water so only 2.2 gr can condensate at -35C.

It is not a lot of water.

The compartment need to be really water and air sealled because if new humid air can enter in the compartment at each temperature variation (by variation of air volume) or each variation in air pressure, a lot of water can be accumulate in compartment.

 

If you manage to get a perfectly tight container, you can still get water in the container by way of vapor pressure. To give an example of how powerful vapor pressure is; we have all seen cars that have rust that has formed underneath the paint, causing the paint to bubble. This is not necessarily due to rain, but vapor pressure within the air itself which forces minute molecules of water through the paint where it collects in high concentrations in the pores of the mettle substrate. (As a vapor) With changes in temperature the vapor will condensate into water. You may be able to get around the problem by slightly pressurizing the compartment. A gauge and a location on the compartment for filling with air would do. If you are building with a composite material, than epoxy is the best at reducing to a minimum the effects of vapor pressure.

 

Thanks for your comments. I live in the tropics where the day and night temperature rarely deviates between 27-34c. I am not sure abt our sea temperatures but the water is never very "cold" . However, Humidity is always very high. My amas are ply and are complety sealed with epoxy/glass. I guess I have a sealed area of abt 40 cubic feet. I will try the calcultor you mentioned to see if I can work out how much water I might capture.

 

Drive up in the mointains, fill the hulls with cool dry air (add a little preassure), seal it and drive home

 

No mountains or cool dry air here in Phuket Thailand.......

 

You can make a home-made condensation trap by coiling up an airline hose and stuffing it into a bucket of ice water; after the line passes through the ice water, run it into a small pressure tank. Preferably one with a drain cock. If you can find an old junk truck in your area (one with air brakes) there will be a small tank just like what I describe on the truck somewhere. This setup will also give you dry air for any kind of spray applications you might be using during construction.