Dealing with air pressure differences inside a sealed hull

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

  1. JamesG123
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    JamesG123 Senior Member

    You were discounting a bottom location, thus my retort.

    If your boat gets holed and capsizes, and the crew spaces fill, it could sit low enough in the water, esp. if the crew are using it as flotation for your middle vents to be submerged. Murphy also loves fancy one-way valves too.

    If you have 11 separate watertight compartments in a small 2 person boat, you should probably rethink your design.

    The weight difference between air and foam is negligible. A human powered boat is not an airplane.
     
  2. W9GFO
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    W9GFO Senior Member

    Well, I am confused. You pointed out that a hole in the bottom would become the top in a capsize, sounded like you were also discounting a vent hole at the bottom - for the same reason that I discounted a vent hole in the bottom.

    There are no crew spaces in this boat, it is 25 ft long by less than 1 ft wide. If the middle vents are submerged, that's fine. Half the volume of air remains trapped in the compartment - what I referred to as partial buoyancy.
     
  3. W9GFO
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    W9GFO Senior Member

    A human powered boat, one that is intended to be efficient, must keep weight to an absolute minimum. Filling it with foam would not solve the pressure/temp problem unless the foam completely filled the space (no air at all remaining) - it would add 34 lbs (2lb/cu ft) to the weight of a hull that I hope to keep to not much more then 75 lbs. That's nearly a 50% increase in weight. That 34 lbs added results in a loss of 0.1 kts speed.
     
    Last edited: Mar 14, 2019
  4. W9GFO
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    W9GFO Senior Member

    Why? Do you feel that some of the bulkheads should have been frames? Or do you think that the bulkheads should have been spaced further apart? Please explain.

    I contemplated cutting large holes in some of the bulkheads but the weight savings would have been only about a pound. Not worth trading a pound of weight for the added flotation of separate compartments in my opinion.
     
  5. JamesG123
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    JamesG123 Senior Member

  6. W9GFO
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    W9GFO Senior Member

    Thanks!
     
  7. Blueknarr
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    Blueknarr Senior Member


    Why so many compartments?

    Just venting between compartments is useless. Must continue ventilation to exterior.

    I would turn most of the bulkheads into ring frames. One inspection hatch would serve three or four of your current chambers. Toss in some cut up pool noodles, Styrofoam or empty plastic soda bottles with lids attached. Lightweight post catastrophe buoyancy doesn't have to break the bank or weight budget.
     
  8. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    This thread has made me smile.

    10 bulkheads over 25 feet.
    That's ~27 inch spacing with less than 6 inch beam on average.
    If it's well built, it shouldn't be an issue.
    What colour is it?
    Seal 'em up on a hot, dry day and store it out of the sun, no?

    One day when you just need to know,
    instal tiny drain plugs top centre on each void space as camera/sniffer inspection holes.

    What does Rick W do on his hulls?

    In my opinion, it should be built to near aircraft standard.

    I had a hand in a 30 foot "stabilized-monohull" that was foam filled.
    It was epoxy-fibre layed up on a foam plug for all but 8 feet, the enclosed cockpit.
    20 inch beam on the fuselage. Pedal drive/sailer.
    It was close to 60 pounds of foam.
     
  9. W9GFO
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    W9GFO Senior Member

    Because each bulkhead is made of 3mm plywood. It is 25 ft long with bulkheads spaced about two feet apart.

    I don't think anyone has suggested that only venting between bulkheads is what is needed, but venting between bulkheads is necessary unless each compartment is independently vented to the outside.

    What is the benefit to opening up some bulkheads to join compartments? Saving weight? There is very little weight to be saved by doing that, and more weight would be added back with the inspection hatches and pool noodles; and I would still need to add a vent. I am having trouble understanding why this would be a better solution than using waterproof vents first mentioned in post #5.
     
  10. W9GFO
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    W9GFO Senior Member

    Actually a little over 6" beam on average. It has a Cp of just under 0.64.

    I intend it to be well built, what "shouldn't be an issue"? Needing to be vented? Each section of the hull between bulkheads is about two sq. ft - which would be 288 lbs of force pushing on it with just a 1 psi change in pressure (20 degrees F).

    The deck isn't even on yet, but it will probably be either red or white.

    Hot, dry days are a few months away. Even so, that would prevent excessive pressure from building but it would have the problem of low interior pressure when the weather cooled down.
     
  11. fallguy
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    fallguy Senior Member

    My cat has 9 watertights in one hull and 8 in the other.

    All the watertights have inspection holes.

    I did not have inspection holes and we put one hull outside under a black tarp and my first watertight at the bow blew up and damaged a bit of laminate. Looks like stretch marks and I now have repairs.

    I am using a 1.5" pvc female threaded insert and a cover. I will keep them loosely fitted always. If the hull is holed, the first response would be to tighten them. If the boat were to capsize, they would be at the top amd behave somewhat like a container with an air bubble I believe and hold air above the waterline. As they leak; only so much water would enter them.

    They double as a coast guard inspection hole.

    If anyone thinks my idea is wrong, shoot holes in it.

    My whole hulls are getting post cured, so we have also made sure we habe no other air traps on the boat and have added additional holes where needed. For example under the transom stepsides, etc.

    Also painting the boat white.

    Boat is being built at 68 degrees F. The post cure is 145F or a 77 degree heat rise.
    P*V/T = PV/T

    Or for any fixed volume.

    P/T = P/T

    Or

    P/75 = P(2)/145

    P(2) = 145/75 * P

    Or the pressure inside each space is 1.933 the original pressure I believe.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    I built some models a few hours before the weather changed and they all were sealed hull/decks. I neglected to drill and air hole and all four boats pulled themselves, caving in like scrunched aluminum foil in the process. As soon as I drilled a hole in each deck(and with a little heat from a light)they popped back out so no damage was apparent. Weird and a good lesson.....
     
  13. Rumars
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    Rumars Senior Member

    W9GFO why would you want to vent between compartments? Each compartment gets it's own vent. You install a threaded insert (plastic, glued with epoxy) and screw the vents in, they seal with an O-ring. If you want to inspect the cavity you unscrew the vents and look trough the insert. If you have water inside from condensation you turn the boat over. I would prefer vents with holes or slits on the side not on top.
    If you feel you must connect the chambers to save money or weight on vents then please don't forget to line the holes with epoxy.

    Vents are calculated to match the chamber volume to function reliably. I gave the Gore Vent link for a reason, read the FAQ.
     
  14. BlueBell
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    BlueBell Ahhhhh...

    What does Rick W do for hull compartment venting on his line of sit-on-tops?
     

  15. ondarvr
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    ondarvr Senior Member

    If you have a small piece of uncoated thin plywood on top of each compartment it will breath and still be watertight.
     
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