Air permeable watertight membrane

Discussion in 'Materials' started by DogCavalry, Sep 7, 2021.

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    I'm about to bond down the cockpit sole on Serenity, sealing in 6 separate watertight compartments. The idea of those compartments rupturing on a really hot day sounds like a great YouTube video. But less fun in real life. I'd like some kind of mfmbrane that air pressure could equalize through, but not allow condensation build up. Thoughts?
     
  2. Rumars
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    Rumars Senior Member

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  3. DogCavalry
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    DogCavalry I aim to misbehave.

  4. BlueBell
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    BlueBell "Whatever..."

    You can also screw in a tiny through hull with a capped length of rubber surgical hose attached.
    The pressure will equalize through the hose expanding and contracting (hose goes inside).
    However, I believe it has been debated at length on here that there is no need if the void spaces are small.
    You could take the ambient atmospheric pressure the day you seal it up into consideration however.
    Just saying.

    If you're really committed to this you may want to consider a fitting large enough to get an inspection camera through. But, I wouldn't bother, you've already got your inspection port lights lined up, no?

    EDIT: Remember, you've got a 10 degree celsius air conditioner working 24/7 on the outside of the hull (when it's in the water).
     
  5. BlueBell
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    BlueBell "Whatever..."

    Submersible Threaded Breather Vents

    Wow, and they're cheap!
    You could put these between chambers and then just have to vent one chamber outside!
     
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  6. DogCavalry
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    DogCavalry I aim to misbehave.

    Exactly!
     
  7. fallguy
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    fallguy Senior Member

    The main issue is if you get boarded for drug inspection.

    Someone want to help me here?

    P/T = P/T

    take a volume of space like one of the watertights and call it a cubic foot

    Close it at atmospheric pressure 14.7 psi and a temperature of 70 degrees F

    raise the temperature in the box to 100F, what is the pressure inside the chamber

    if the temp goes up to 100

    14700/70 = 21 or about 50% more

    pressure goes to 21?

    What is wrong with my math? It can't be right, can it?
     
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  8. Barry
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    Barry Senior Member

    When using the ideal gas laws, you must use Absolute pressure and temperature.

    So to 70F you add 470 to get a temp of 540R (Rankine scale) And then 1o0F is is 570R

    The new pressure would then be around 15.5 psi, calculated from an ideal gas calculator on the net. BUT, this assumes that the physical size of the container is rigid. So that if there is a bit of flex, ie increase in volume due to the .8psi increase in pressure, then the actual pressure would be slightly smaller, splitting hair here
     
    Last edited: Sep 8, 2021
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  9. fallguy
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    fallguy Senior Member

    thank you Barry, I knew it was wrong; forgot my chemistry rules

    so, the chamber goes from 14.7 to 15.5 psi or an increase of 0.8psi is a 5% rise in pressure

    hardly seems like a reason to make equalizing chambers because all of the chambers would be doing the same business and the volume is meaningless ftmp

    in my boat, based on this 5% business which I recall as being less somehow, I did NOT want the chambers joined as one gets wet; they all get wet

    but, I did add threaded pvc cleanouts to each chamber; there are 11 of them, and if the boat ever faces extreme temperature swings (it will here in Minnesota), I can open the chambers-need to add that to my winterizing plans formally

    Here, we can go from a chamber being say 120F to -30F which is 150 degree swing or plus 50 to minus 100 from 70F

    '
    Not trying to babble on here on DC's thread, but the issue is very real for me. The 5% becomes about 15% in cold weather. And I did experience a problem where the boat got to 140 before I added the vents and the laminate got stretch marks in the unvwnted chamber. I postcured the whole hull(s) in a shipping container and some of those chambers were heated to 150F (all closed areas were vented after I realized the issue).

    Anyhow, I see little reason to equalize the Seasled chambers, but some purpose in putting pie eyes or inspection ports. Of course, all of my watertights can be atmospherically equalized, so if this is what DC wants and he has more than 11 chambers and some are under the sole only- I get it. But equalizing them doesn't do much if they don't open to atmosphere..
     
  10. Barry
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    Barry Senior Member

    The .8 psi does not seem like much though but over say a 24 x 24 inch panel, this does equate to a total of a 460 pound force total force. Also this uniform pressure can create bending stresses in the panel. I chose 24 x 24 just as an example
    If you gave the measurements of your largest unsupported, (unsupported between say a panel's perimeter) we could come up with the bending force in the panel you could figure out the induced
    bending stresses. I would expect them to be small enough to ignore but??
     
  11. BlueBell
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    BlueBell "Whatever..."

    They are small enough to ignore.
    Plus, we get constant 10 degree celsius cooling/heating of the hull here year 'round.
    And, these pressure numbers say nothing of panel deflection, which reduces them.
    And what of the temperature/pressure at the moment of sealing?
    29.92 inches of mercury (1013 millibars) and 15 degrees celsius being standard.
    Are you more concerned with expansion or contraction?

    DogC, what did your yard Naval Architect have to say about your concern?
     
  12. fallguy
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    fallguy Senior Member

    I don't want to poo on DCs thread, but I experienced a laminate problem when one of my well sealed chambers (first one in an axe bow hull) blew up like a balloon in what I assume was about 130F from install at 65-70F.

    Not gonna happen in a plywood boat.

    For me, my bigger worry is winter, where the install temp of 70, or the close and seal watertights with teflon tape temp of 70 can drop to minus 30, which is 100 degrees. I had considered gore vents, but went away from them. That company wanted drawings of my setup?

    Anyhow, if that seasled is headed to Alaska ever, I'd try to open up. Vancouver, BC is a why bother.
     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    0.8 psi, times 144 square inch per square foot, times 50 square feet. Total force is 5760 pounds. Or 2616kg. Seems significant to me.
     
  14. DogCavalry
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    DogCavalry I aim to misbehave.

    And McMaster-Carr won't ship to Canada. That's disappointing. I need to find a similar product from someone else.
     
    Last edited: Sep 9, 2021

  15. BlueBell
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    BlueBell "Whatever..."

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