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Buoancy air ballons

  1. Sep 8, 2009 #1
    If there is a shipwreck that i am trying to raise and i try to calculate the number of ballons to attach to the ship to raise it to the surface, do i have to just the weight of the ship given it's mass (all the steel it's made of; all the stuff inside...) and the mass of the water trapped in it? Assume it's a rectangular ship sitting upright - don't i need to calculate the weight of all the water directly above it all the way to the surface? That weighs down on the ship doesn't it?
  2. jcsd
  3. Sep 8, 2009 #2


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    Gold Member

    The water above the ship exerts a pressure on it, that is true, but the additional pressure force pushing it down is canceled by an additional pressure force acting up on the bottom of the ship. By this I mean that the net effect of the pressure force on the ship is the same no matter how deep it is. You can disregard it, except for the effect it will have on the volume of the balloons.

    There will also be a viscous force from the surrounding water acting downward when the ship starts to rise, but this can only slow it, not stop it from rising.
  4. Sep 8, 2009 #3


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    Be careful about the water trapped inside it - since you aren't lifting the ship above the water, the water inside does not affect the lifting force requirement. It has its own buoyancy.

    You also need to subtract the buoyancy of the metal in the ship from the lifting force requirement.
  5. Sep 8, 2009 #4


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    To be clear, you can include the water in the calculation, but as long as you include both the additional weight and additional volume, it will make to difference.
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