Dealing with air pressure differences inside a sealed hull

Discussion in 'Boat Design' started by W9GFO, Mar 14, 2019.

  1. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    I am about to put the deck on a 25 ft hull (< 1 ft beam). It is going to be a two person pedal powered stabilized monohull much like Rick W's boats. It will have several water tight compartments and no storage within - trying to keep weight down as much as possible.

    My concern is that when the Sun is beating down on it the increase in pressure will deform it.

    My current plan is to put a very small hole in the middle of each bulkhead so that pressure can equalize between the compartments and one drain plug somewhere on deck.

    Maybe there is a better way?
     
  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    A reasonable concern.
    Tell me you didn't paint the boat a dark colour?
    How small a hole, 1/64"
    You may as well put one near each top corner of the bulkheads.
    You could then deck-mount a small low-pressure valve ( 2-way, in and out ) protected under the seat or pedal pod.

    Or, fit a length of surgical tubing (rubber hose) sealed at one end, to a small through-hull in the same area.
    Varying hull pressure will expand and contract the tubing accordingly.
    You can also lay the hose out inside but need an access hole for it.

    (When you built the bulkheads you could have installed circular rubber diaphragms ( <1/2 the beam in diameter )
    in each bulkhead allowing for equalization while maintaining a seal... also making the bulkhead lighter.)

    Have you calculated the actual expansion pressure?
    Is there really a need given the small volume?
     
  3. W9GFO
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    W9GFO Senior Member

    1/64" is about what I was planning on. If the boat is holed, a hole in the middle of the bulkhead should retain partial buoyancy. A hole near the top would allow it to completely flood.

    My research indicates that the volume of air will change +/- ~10% with a 50 deg F change either direction from 70 degrees. Too much for diaphragms or surgical tubing.

    I did consider the diaphragm idea though, I like that it keeps each compartment sealed. There would need to be tubes lead from each compartment to a central one to be able to vent for large changes.

    Going from 60F to 120F is 3+ psi increase in pressure. 20F to 120F is 6 psi, volume is irrelevant. Just going from 60F to 80F is more than a 1 psi increase. That's a lot for a flat sided boat made of 3mm ply.
     
  4. JamesG123
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    JamesG123 Senior Member

    This concern is why most closed off buoyancy chambers are foam filled.

    A hole in the top will negate any buoyancy if that compartment develops a leak. Put the relief hole at the bottom. Then even with a chunk taken out of the hull it will still be an air chamber once filled past the port and any sweating or condensation within the compartment will drain out.
     
  5. Rumars
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    Rumars Senior Member

  6. Doug Halsey
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    Doug Halsey Senior Member

    As a data point for comparison: the Mach2 Moth has a vent tube more-or-less below the mast step. As close as I can tell, its ID is ~1/16".
     
    Last edited: Mar 14, 2019
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  7. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    McMaster Carr sells waterproof vents about 3/8" in diameter-fairly cheap.

    D4Z Foils  5-14-15 010.JPG
     
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  8. BlueBell
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    BlueBell Ahhhhh...

    W9GFO,

    It sounds like you've already got this all figured out.
    I have built a similar vessel but SOF and it wasn't an issue.
    Don't forget it's water cooled.
     
  9. W9GFO
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    W9GFO Senior Member

    With the hole in the bottom, it will retain buoyancy IF it remains right side up. That's why I was opting for the middle - in any orientation (except vertical) there would be at least partial buoyancy. It's a moot point now since I learned that I can get waterproof vents for under $4 each.
     
  10. W9GFO
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    W9GFO Senior Member

    The problem is when it is out of the water, which will be most of the time.
     
  11. W9GFO
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    W9GFO Senior Member

  12. Blueknarr
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    Blueknarr Senior Member

    I left James's entire post because I feel it needs repeating.

    I run a repair shop. 70% of my income is from boats which have traveled over insufficient water. 25% from water intrusion into "watertight/proof" areas. Water will always find a way in. IMO all compartments should have inspection access. Leave the inspection ports open while vessel is not in use. Air expansion won't be a issue for day trips.
     
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  13. JamesG123
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    JamesG123 Senior Member

    Besides "what Blue said", uh, no. If you have a hole in the "bottom" and a vent hole also in the bottom and you capsize, both holes are now in the topside.
     
  14. W9GFO
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    W9GFO Senior Member

    I would agree that this is the proper way for most boats. However, in this case adding 11 inspection ports would add more weight (and cost/potential for leaks) than I am comfortable with. Remember, this is a lightweight human powered boat.

    Also, we can experience a 50 degree temperature change throughout the day. I think that could present a problem, even for a day trip.
     

  15. W9GFO
    Joined: Dec 2014
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    W9GFO Senior Member

    I was saying that a vent hole in the center of the bulkhead would allow for partial buoyancy whichever way the boat sits. I never suggested a vent hole at the bottom (or top).
     
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