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How much weight required to submerge this object?

  1. Dec 30, 2014 #1
    Hi all,

    So A quick question for all the smart minds out there to ponder (plus I'm stumped and need help).
    I'm helping a friend out with a short film and we are trying to be cost efficient.

    I need to submerge a 2000L plastic tank (circular and approx 1.5 high) and then release it to make it look like it's popping out from the ocean.
    The tank we have is 3 metres deep and we have 3000kg of steel weights which we have a friend making a low profile steel rack for stacking the weight and to attach pulleys.
    We will run a line from the tank through the pully attached to the weight stack and back to a forklift to pull the tank under.
    Or we get a turfer and do a direct pull from the weight stack.

    I slightly understand the principle of water displacement and realise that the steel will weigh less under water due to it's volume/water displaced.
    A rough guesstimate would be 600L of displacement so the weights will be approx 2400kg or 2.4 ton underwater.

    I know this weighs more then the object we are trying to submerge so excuse the silly question.....but is that enough weight firstly to pull the tank directly down with the turfer......
    Or if we go with the pulley and forklift method, because it's 2 forces from above, does the weight at the bottom need to be doubled?

    Hope you can understand my explanation and thanks for any help.
  2. jcsd
  3. Dec 30, 2014 #2


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    As long as the pulley doesn't jam, you should be okay. You're remembering the mechanical advantage of two pulleys from way back in school giving you an ability to lift twice the weight of the actual pulling force.
  4. Dec 30, 2014 #3
    Thanks Bystander.......so I'm thinking of the 2:1 principle or something like that.......... but it doesn't apply in this situation because it's a single pulley purely to enable pull from another direction? (so essentially the pulley isn't there and I'm just pulling a direct weight).
    If this is correct, would it not matter at all where the pull once through the pulley was from then? ie...directly above, directly to the side, 45 degrees etc etc.....I assume it doesn't but just checking......

    For some reason known to only my brain, I still think I don't have enough weight on the bottom but that's all I can get!!!
  5. Dec 30, 2014 #4


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    You don't want to hook up to the people who drive in tractor pulls, or the weight will come off the bottom just because your tank won't sink quickly enough, but a nice slow sedate drawdown in quiet water should be very well behaved.
  6. Dec 30, 2014 #5
    Haha.....I'll be the calm driver....thanks again.
  7. Dec 30, 2014 #6
    And just one more question........what happens if you use steel 44 gallon drums filled with water for weights......since the volume is the same as the weight, does this just completely cancel it out and the actual underwater weight is only the steel or is there some other calculation for this scenario?
  8. Dec 31, 2014 #7


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    No magical advantage --- that's why novelists always fill oil drums with cement to dispose of victims.
  9. Dec 31, 2014 #8
    Aha....so it's pointless to use a water filled drum as an underwater weight.....cheers
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