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Buoyancy question

  1. Aug 10, 2008 #1
    question: I have a closed container filled with helium and I can't change the shape or size of the container. How can I achieve maximum buoyancy in order to make the container seem as light as possible? Would I want to jam pack the container with helium so that the pressure inside container is high, or have the helium in the container be at the same pressure as the outside air, etc?

    This isn't a HW question, its for something I want to build.

    I would really appreciate any help.
  2. jcsd
  3. Aug 10, 2008 #2


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    Net buoyancy is about volume displaced vs the mass of the object doing the displacing. So to have maximum buoyancy, you want the object to be as light as possible.
  4. Aug 10, 2008 #3
    exactly, I want the container to be as light as possible, but how should I use the helium in order to make the container lighter (like the way a hotair balloon works)?
  5. Aug 10, 2008 #4


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    erm … fill it with helium … then remove all the helium! :smile:
  6. Aug 10, 2008 #5

    Are you saying that the container will have maximum buoyancy if it's a vacuum?
  7. Aug 10, 2008 #6


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    Yup! :smile:

    Of course … it'll probably implode! :biggrin:
  8. Aug 10, 2008 #7

    now that I know a vacuum provides maximum buoyancy, I have another question: The container will be rectangular (say 18x5x2 ft) and it will be made of a fiberglass/kevlar composite (which would have really good mechanical properties); do you think this would be rigid enough to keep from imploding at sea level pressure?
  9. Aug 10, 2008 #8


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    It is possible, it we won't be able to tell you from that information. You'll just have to find out!

    The thing is, though, helium is pretty light, so the extra buoyancy to be gained by evacuating it vs filling it with helium isn't very much.
  10. Aug 10, 2008 #9
    so im guessing that keeping helium in it won't effect buoyancy that much, but it should help the container from imploding.

    I could actually weigh the container with and without helium, and see how much the buoyancy is effected.

    The important thing is that with the project I want to do, implosion is a matter of life and death, so it's obviously a top priority.
  11. Aug 10, 2008 #10


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    The density of helium is 0.011lb/cu ft and the density of water is 62.4 lb/cu ft. So the difference in buoyancy between a helium filled and evacuated vessel is .02%.

    For that matter, air's density is only .075 lb/cu ft...
  12. Aug 11, 2008 #11
    With a rectangular vessel, pressurizing with helium would likely increase the volume slightly, yielding more bouyancy by displacing slightly more water. If you are planning to submerge the vessel to great depths, you might consider filling it with oil to preclude implosion. Oil being only marginally lighter than water will not offer as much bouyancy as a light gas, but it will not compress.

    Since you are evidently planning to submerge whatever is attached to this bouyancy device, you will in fact have to add weight or pull it down with a cable attached to the sea floor. Which is it going to be?
  13. Aug 11, 2008 #12
    isly ilwott,

    I said rectangular just to give everyone a rough idea of the dimensions. It's actually going to be a boat. I need the boat to draft in as little water as possible. So I plan on giving the bottom of boat as much surface area as possible, but not so much that it will affect the performance of the boat.

    Also, the boat will be very rigid (fiberglass and kevlar) so i don't think the helium will affect the shape at all. Since it's going to be very rigid and it will be at sea level pressure, I think I can make the hull of boat a vacuum to achieve as much bouyancy as possible.
  14. Aug 11, 2008 #13
    Incidently, a friend of mine used to build party barges with two or three rectangular floats made of sheet steel. Each float was divided into air-tight chambers, each of which was fitted with an air check valve (as on most all car wheels) to pressurize each cavity to about 4 psi. The air made the flat sides "pooch out" a bit, making the raft float a bit higher in the water. This system also aided in the detection of leaks below the waterline....you see bubbles...you've got a leak.
  15. Aug 11, 2008 #14
    The weight of air removed will be negligible. Consider filling with foam as is done with the Boston Whalers. That way, when the integrity of the hull in compromised by a sharp rock, you will still float. I've seen a Boston Whaler sawn in half...each half remained afloat...even with an outboard attached and a heavy man in each half.

    Even with many structural trusses between the two surfaces of a relatively flat fiberglass structure, the flat surfaces will become slightly concave when a vacuum is introduced between them. I think you are wasting your time trying to gain a smidgeon of bouyancy by vacuum.

    ...and a floating boat will displace its weight, regardless of the vacuum between the two hulls. Not only that, but with a vacuum, a pinhole leak will causes water to be forced into the cavity due to atmospheric pressure on the outside of the cavity.

    Are you making a "bonefisher" boat to run in the shallows?

    If you really want to save weight, leave the beer cooler at home.
    Last edited: Aug 11, 2008
  16. Aug 11, 2008 #15
    yeah, I want to make an ultralight flats boat ( I live in florida).

    From what you said, making a vacuum is pointless so i'm scratching that out.

    Compared to having regular air in the hull, do you think having helium will make a noticable difference in the weight of the boat?
  17. Aug 11, 2008 #16
    Building a boat hull from those materials and then having a large delta P is madness. Just fill it with a medium density foam like airplane wings and be done with it.
  18. Aug 11, 2008 #17


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    … balloons …

    Hi ucf-fisher21!

    If you have access to plenty of helium, why not lighten the boat by attaching helium balloons to it? :smile:
  19. Aug 11, 2008 #18
    I am confident that you could not tell the difference without an extremely sensitive scale. The amount of helium I visualize between the two hull shapes is of the order of ounces, not pounds. Carrying an extra rod and reel would probably add more weight than the helium saves over the weight of air.

    Again, even pressurizing the hull cavity will do no good when the hull is cracked open. You will lose pressure, fill with water and sink as if the protection was never there.

    Use foam. There are many products that expand like crazy when injected into such cavities. Any one of them that is not affected by fuel will last as long as you are expected to live and will keep water out of the cavity even when the cavity is cracked open.

    What horsepower will you use to turn the screw(s)? [Engineers love redundancy...I'd carry two or more outboard motors if I were doing it...especially in the ocean, where currents can carry you miles from where you want to be. They don't have to be the same size, just each be long enough to reach below the surface...and if money is not a problem, I'd use inboard engines with jet drives. You can adapt something from a jet-ski to do that...then there's no prop to submerge below the bottom of the boat, allowing passage through very shallow waters...and you can slide over manatees without slicing them up.]

    A hand-held marine radio is recommended also...in case you need assistance.
    Last edited: Aug 11, 2008
  20. Aug 11, 2008 #19
    tiny tim,

    that would work, but I don't think i have the self courage to be out on the water with a bunch of balloons attached to my boat! plus it would make running the boat more difficult.
  21. Aug 11, 2008 #20
    If the helium thing won't work, ill just maximize the surface area of the hull to displace as much water as possible, and use the best weight reducing construction (lightest materials and vacuum infusion).
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