Anchor lifting bouys

Discussion in 'Projects & Proposals' started by valvebounce, Apr 16, 2017.

  1. valvebounce
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    valvebounce Senior Member

    Would a 25ltr drum be capable of lifting a 7kg anchor?I would of course attach a ring etc.
    My boat has a 14ft fibreglass planing hull with an anchor roller.
    The anchor would have the chain attached at the bottom with a zip tie through the top hole.
    Looking for an inexpensive alternative to an anchor bouy.
    Any opinions will be appreciated.
     
  2. gonzo
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    gonzo Senior Member

    25 liters will displace about 25 kg of water. Deduct the weight of the line, chain, drum, and the rest is its lifting capability.
     
  3. valvebounce
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    valvebounce Senior Member

    Thanks G,so if the setup weighs 15 kg's what is the common denominator to start with?
    ie,what would a 25ltr drum exert in pull?
     
  4. gonzo
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    gonzo Senior Member

    25-15=10Kg of reserve buoyancy
     
  5. Barry
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    Barry Senior Member

    As the 15 kg of steel will displace 1.9 litres of water, the weight of the chain and anchor under water will be the 15 kg less 1.9kg or say 13 kg

    If the anchor is on the bottom then the amount of weight that the drum has to hold up does not then include the weight of the anchor or the amount of the chain that is on the bottom.
    But we need some constraints, and lets assume that you are wondering if the drum will hold the weight of the anchor and chain with the anchor being off the ground. Ie the worst case scenario with no wind effects.

    So we know the downward force on the drum, 13 kg.

    You know that a 25 litre drum will offer a vertical force of 25 kg. BUT the drum will weight something. If it is steel and heavy, then you need to determine its weight. And deduct this off the 25 kg that the drum could lift. Then you have to account for the fact that at 13 kg of down force, the drum will be submerged and then calculate the amount of volume that this shell displaces and add back a bit for this amount of water displaced.

    An easy question, a complicated answer to be entirely accurate
     
  6. valvebounce
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    valvebounce Senior Member

    Thanks Barry,
    the drum is plastic/nylon,capable of carrying fuel or water,(pretty strong)
    The idea is to use it as an anchor lifting bouy by driving the boat to pull the anchor up.
    After comparing the air capacity in a large bouy used for this purpose,I think I am right in assuming
    a 25ltr drum has more capacity.
    In case the anchor gets stuck,it will have the chain fixed at the bottom of the anchor and a zip tie holding it at the anchor top eye.I expect the upward force of the drum will be capable of snapping the zip tie,so the anchor can be upended and release itself.
    Thanks for your input,it's much appreciated "V"
     
  7. gonzo
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    gonzo Senior Member

    Maybe I misunderstood that the weight given was submerged weight. If it is not, a deduction for the buoyancy of the chain, etc. must be made as Barry indicated.
     
  8. TANSL
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    TANSL Senior Member

    Maybe you're not understanding anything. The submerged weight is the same as the weight without submerging. Another very different thing is the apparent weight of a submerged object, or semi submerged, in a fluid. This weight depends not only on the object but on the specific weight of the fluid.
     
  9. Barry
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    Barry Senior Member

    This is not correct. Attach a simple spring scale to 15 kg of steel that would read 15 kg in air, submerge it and the scale will read close to 13 kg when submerged. (so long as I did the math correctly)
    Only the mass would be the same of the suspended object

    In this case, the OP is wanting to know if his barrel will hold the anchor up when he pulls it up. The force that the drum will exert vertically up on the anchor will be the 25 litres of water displaced by the drum, the resistance that the drum will make when he starts to accelerate his boat, and the vertical component of the vectors that the geometry of the boat to float, and the float to anchor makes.

    I would go with mainly rope with some chain to enable the rope to slip smoothly through the loop.

    A friend of mine runs 6 prawn traps on one skate and he pulls them up with his boat. He has a "toggle" attached to the mainline about 4 feet ahead of the first trap. When this toggle slips through the loop, he stops and the toggle which is attached to the rope offset from the center, flips up by gravity, and stops the mainline from going down through the loop. He then just backs up to the trap line, and hand bombs them in. As these come from 300 - 350 feet of water, this is a slick operation, with little sweat
     
  10. TANSL
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    TANSL Senior Member

    Barry, the weight is the mass multiplied by the acceleration of gravity. If the mass does not vary, the weight can not vary.
    What varies is the apparent weight of the object submerged in a fluid.
     
  11. Barry
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    Barry Senior Member

    Tansl

    Stand on a water proof bathroom scale if there is such a thing.
    Say you weight 80 kg

    Now put the scale in a swimming pool at one meter depth and stand on it. It will show your weight at say 50 kg

    Assume you are 1.82 meters tall, now put the scale at 1.82 meters in the pool, assuming you just float, the scale will read zero kg

    The weight varies due to the vertical upward force from bouyancy
     
  12. valvebounce
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    valvebounce Senior Member

    Just to add my 10%,haha,
    if I pull a heavy fish up,(bordering on the max line strength in the water)
    it will more than likely snap the line if I lift it above the water.
    So,metal or fish,it weighs less in the water than out,or at least requires less force to pull it in below the water.
     
  13. TANSL
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    TANSL Senior Member

    Yes, that's correct.
    And forgive me if I insist, the weight of the object does not change.
     
  14. Barry
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    Barry Senior Member

    What is the weight of 60 kg in space??
     

  15. valvebounce
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    valvebounce Senior Member

    I suppose it's the surrounding fluid and depth that supports the object,so,if I weighed a fish at 1 att,
    you are saying it would weigh the same as it does out of the water?
    I suppose bouyancy comes into the equation somewhere.ie,there is no bouyancy in a lead ball.
     
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