condensation in sealed "water tight" hulls

Discussion in 'Boat Design' started by jedkins, Dec 29, 2006.

  1. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    You pick the boat up and shake it to hear if there is water in there. You got it right about the contracting air. If the amas or whatever are sitting in the sun, they get heated like an oven. Rain immediately cools them down and if you have a 'watertight' hatch with a rubber seal, water will sit in the channel between the hatch cover and and its frame and get sucked right past the rubber seal. Each time it happens water goes in but not out, so it adds up. It would be interesting to connect a vacuum gauge to an ama and see how much is developed with a hot hull and a cold rain. Sam
  2. jedkins
    Joined: Jul 2006
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    jedkins Junior Member

    Purpose made material to reduce condensation

    That would be too complicated for me. I have noticed that condensation can be reduced in other boats by storing cushions and other absorbant materials in the hulls. I wonder if there is a purpose made material or product that acts in the saame way?.
  3. RAWRF
    Joined: Dec 2006
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    RAWRF Junior Member

    Unless you have foam, you are always going to have moisture inside a sealed container, and the foam absorbs moisture eventually. I would put one way check valves in the amas and have float actuated bilge pumps as backup. I had this same concern with a pontoon boat once and I did the above. Those sealed pontoons can actually get a lot of water buildup pretty quickly, perhaps half a gallon a day under fluctuating temperatures.
  4. longliner45
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    longliner45 Senior Member

    if you want the airtight compartments for safety,,in case the boat swamps or is damaged by reef,,,the bildge pumps will create another hole for water to enter through the (exaust or exiting water) ,,we glassed our airtight compartments and then blown polyurethan foam into them ,,they lasted a good 6 years and when we replaced the deck ,,we replaced the foam,,,or better yet if you can measure the compartments,,and make the foam the same size and glass it over completely,,and slid them into the compartments,,this will make sure they dont soak up any water.
  5. Frosty

    Frosty Previous Member

    When the boats not in use open up the hatch and let the air dry it out. when you go out it-- shut them. Look at the big plywood speed boats at the Boat Lagoon, there hatches are open when not in use.

    I think its a problem that no one has got around or worried too much about.
  6. longliner45
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    longliner45 Senior Member

    Im sorry ,,,misread your question ,,your not talking airtight compartments ,,you mean water tight hull,,,,my bad,,,,,,,,longliner
  7. Frosty

    Frosty Previous Member

    No Longliner I think air tight is what he wants. I was suggesting putting some hatches in so that he could dry them. Although I can see I didnt make that clear.
  8. Poida
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    Poida Senior Member

    What about those crystals they put in packaging to absorbe moisture.
    A hatch with a strainer attached to the lid, place in some water absorbing crystals and change them at frequent intervals.

    Can any material be 100% water tight especially when it is submerged with weight on it forcing water through microscopic pores?

    So it is impossible to completely water tight a compartment and absolutely impossible to get water out.

    To my way of thinking it would be better to have a hatch that might let a little water in because it is easier to get it out.
  9. monrosm@shrewsb
    Joined: Apr 2006
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    monrosm@shrewsb Junior Member

    Hull problem, possible answer

    It seems to me that people are suggesting very complex and most of them physical ways to stop water getting to your hulls.
    Firstly let me explain why the condensation forms as many other people in their reply’s have partially done. This is as im sure you know due to a significant change in temperature on the inside of the hull than on the outside. Now there aren’t many practical ways to stop this change in temperature, in fact its almost impossible, without going to great lengths and expense.
    Now, this condensation would not form if the water vapour was not inside your hull, so the question is 'How do I get rid of it', well most people would state the obvious, create a vacuum in the hulls.....however most people say this before they think about it. If you did this you remove the substance that makes your hulls float, which is of course...'Air' meaning one thing they wont float. Even replacing it with foam, still wouldn’t work (or in some substances, give very minimal flotation) as air in-between the foam pores make it you need a better but probably far simpler solution.
    If you visit this website '' you will find one possible solution to your problem. This is an oxygen absorber, firstly you might be questioning why this would help....Well look at the chemical composition of water....H20, now, if you remove one element from here....the Oxygen, it makes it impossible for the water to form. All you are left with is a hull full of H, Co2, N2, and other minor gasses in the air...This is all ok as i assume nothing is going to be doing any breathing in your hull. This stops the condensation problem, so if you hull is truly air water tight and has no waster residue when you seal it, it should stay like this. However if there is a leak, this oxygen absorber would only work until a certain volume of water is in the hull (depending on how much absorber you put in) at this point it becomes saturated and will not absorb any more Oxygen, for this I don’t have a very good solution, other than using Silica Gel, to absorb the moisture, although it doesn’t absorb much...
    Hope this helps
    Stefan Monro
  10. Frosty

    Frosty Previous Member

    Oh dear-- oh dear-- oh dear
  11. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    And your's is simple?

    I think we have to be realistic,
    you can keep the water out, but not the vapor in the air.
    This is how modern rain jackets work, the are water tight but not air tight.

    So make at least one small hatch for inspection and ventilation.
    I think the cheap plastic inspection hatches is quite water tight.
  12. PAR
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    PAR Yacht Designer/Builder

    Stefan, boats float, as a result of a long since dead Greek spilling some water out of his bath, which he calculated was equal to his weight. Granted, his water logged butt had considerable air in it, but even if he didn't, his shape would have still displaced sufficient water, to support his mass.
  13. Man Overboard
    Joined: Oct 2006
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    Man Overboard Tom Fugate


    “If you did this you remove the substance that makes your hulls float, which is of course...'Air' meaning one thing they wont float”

    I don’t mean to nick pick here, but because this is a boat design forum, I thought I might point out that air is not what causes objects to float. Objects float if the weight of fluid displaced is greater than the weight of the object. So, in fact, if you remove the air from a container, the container will weigh less, floating higher out of the water. (Less wetted surface area.)

    Another example: My 100 cubic foot scuba tank weighs 40.2 lbs. When empty it has 3.5 lbs of positive buoyancy, so it floats. When filled with 100 cubic feet of air it has 3.9 lbs of negative buoyancy so it sinks (8” in diameter 26” long) The air adds 7.4 lbs.
  14. Poida
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    Poida Senior Member

    It would have only been equal to his weight if he was floating in the bath.
    Archimedes actually had at the time of his famous bathing, realised how to calculate the purity of gold. Believe it or not.


  15. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Trimaran amas

    Hello all,

    When I wanted to learn a bit of boatbuilding I went to work (for free mind) for one of Brisbane's best multihull builders. He had built a 40 ft schooner tri (won lots of races) with sealed floats. He put water sensors in the floats and left them for years. This was in Brisbane, a somewhat similar climate to Thailand.

    I was told by Robin Chamberlin (great designer down here) to put a small amount of fungicide in every compartment after sloshing the whole lot in oodles of epoxy. I had to cut into one of the compartments a while ago and was gratified to see the ply in good nick. The idea is that the copper napthalate forms a fog of fungicide so if any water is in the air (as it must be) than you get air laced with fungicide rather than straight water.

    Build it well with no bolts or protuding steel inserts and cover it after you put the copper napthalate in is my call.


    Phil Thompson
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