A Physics Brain Teaser

Discussion in 'All Things Boats & Boating' started by Man Overboard, Jan 25, 2007.

  1. stonebreaker
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    stonebreaker Senior Member

    Did the door act like a wing? Or did the barrel dump when it surfaced and fill up with water again?
     
  2. jehardiman
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    jehardiman Senior Member

    A couple of points, assuming that all the structural shapes/connections were up to it

    1) the adibatic expansion of the air in the barrel as it rises could cause an ice plug due to air cooling (Lake Superior = fresh water thats kinda cold)

    2) the bubble volume that the barrel rises in could lower the water column density which prevents the barrel from floating all the way up, it would then sink again as the density returns to normal and recompression of the air occurs.

    3) The classic submarine breaching problem, where the pressure is lowered in the barrel to ~1 atm as it breaches the surface of the water due to upward velocity, then when the barrel sinks back down, recompression occurs as the air volume compresses until it is unable to float the weight
     
  3. Man Overboard
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    Man Overboard Tom Fugate

    A lot of good answers, Danish Bagger from Denmark is close, can you be more specific?
    By the way, the scuba tank was not attached to the tank, I held the hose in one hand, and controlled the valve with the other.

    Poida is correct about the drum, it is not really all that strong, so you can’t attach the hose without a pressure relief valve. I decided to just let the air escape out the bottom hole. When you release compressed air from the scuba tank, it expands, and fills the drum with air, rising to the top of the drum the drum quickly begins to float in an upright position; as you continue to fill the drum it will force the water downward and out the bottom through the same hole where the air is going in. water simply runs past your hand where you are holding the hose inside the hole in the bottom of the drum. For those of you who said the drum would rocket to the surface, this is true, and not being real sure how to keep this from happening I chose the approach of get it lifting, and get out of the way. The volume that the drum displaced was more than needed. You fill the drum with air until the whole unit just starts to lift; (leaving some water in the drum) the air in the drum continues to expand as the drum rises to the surface, forcing the rest of the water out of the drum creating more lift at the same time. None of this was a problem. When at the surface, the weight of the door keeps the drum from tipping, so that also was not a problem.

    Here is a summary of events:

    1. After diving out of boat with tank under my arm, descend to salvage area.
    2. Drum is sitting about 10 feet from door, so move drum over to door.
    3. Attach chain securely to door with locking clevis.
    4. Fill drum with air until unit starts to lift.
    5. Get out of the way
     
  4. Man Overboard
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    Man Overboard Tom Fugate

    That is interesting, I guess that could certainly have happened, but as there was plenty of reserve buoyancy, the unit didn’t sink back down much after surfacing. Maybe it was a good thing I had a little bigger drum than needed
     
  5. jehardiman
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    jehardiman Senior Member

    There are other structural issues that I and others alluded to above. I.e. the volume of mass flow of the water out the bung cannot keep up with the expansion of the air volume and the barrel over-pressurizes, possibly causing a rupture. This also is a submarine problem. The flood grates in the MBTs must be sized to avoid this problem...but I just assumed that the balloon...I mean barrel.... held.
     
  6. gerard baladi
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    gerard baladi Junior Member

    You made it to the surface, you still have to get it to the boat (which was moored away!) and then lift the door onto the boat.
    I hope, for your sake, that you had help and a hoist on board.
     
  7. Man Overboard
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    Man Overboard Tom Fugate

    Yep, the barrel held up O-K, in fact I still have it. I had painted it red, with a diagonal white stripe; the same as a divers flag. As far as lifting it into the boat; we just left it in the water and towed the whole thing to shore where a couple of guys could get a hold of it.

    You are correct, I needed help, but not with lifting it into the boat.
    As far as lifting it into the boat; we just left it in the water and towed the whole thing to shore where a couple of guys could get a hold of it.
     
  8. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Is he close here?
    Did the whole assembly have a very long period of vertical movement, a quite small waterplane area compared to the weight?
    Did you have trouble when you towed it?
    Did the door start to act like a keel/foil and steer it's own ways?

    Still curious :)
     
  9. Roly
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    Roly Senior Member

    I should have grabbed two tanks as the one wasn't enough to fill the 44gal drum. As the the wreck was at 90 ft the 80cuft (of free air) became only 80/3
    and did not displace enough water out of the drum to lift the door. So I partly
    filled the drum and bolted up for another to displace enough to lift the door.
    Except that 80/3=26.66cuft*6.25gals=166gals=5drums at 3at... so the math is crap.

    A 30gal drum would give you approx 30*10=300lbs of lift, and an 80cuft tank on the surface would
    fill that drum 80/30/6.25=16.66 times. at 33ft, 8.33 times; at 66ft, 5.55 times etc
    And you must be yankin' our chain? Or is my physics bad........

    Yahoo!....just lazered our waterline.......off to bed
     
  10. jehardiman
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    jehardiman Senior Member

    Actually, I think is has more to do with the stability of a half full barrel before it begins to take up the weight of the door enough to add stability. Look at the problems they had with the salvage pontoons on the S-4 and Squalus.
     
  11. stonebreaker
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    stonebreaker Senior Member

    Did the door catch on the bottom and not let you get to shore? Did you not have enough rope to tie off and row to shore?
     
  12. hansp77
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    hansp77

    Did you pull it out on shore and realise it wasn't a door?:p

    But seriously,
    isn't it time yet?
    Please?;)
     
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    No, I think he said earlier that he managed to lift the door and the barrel up to the surface without any problems, THEN he had some problems:

    "lifting the door went quite smoothly"
     
  14. safewalrus
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    safewalrus Ancient Marriner

    What about you? Up goes the tank and door - wow! So up you go too, like an express train, couple of atmospheres and bingo nitrogen whatsits, sometime known as the 'bends' you in deep poo then! So your mates recover the door etc and tow it up the beach for you whilst you is rushed to the hyperbaric "de bending machine"? Tis a thought and plausable - think out of the box like!
     

  15. Man Overboard
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    Man Overboard Tom Fugate

    Yes, Danish Bagger from Denmark is close. If you recall I stated:

    I calculated that the drum full of water would have about 10 lbs(4.5kilograms) of negative buoyancy, and indeed this is the case. You can hold it out from your body off the lake bottom with one hand.
     
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