Tidal Lift

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billywilliams

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I am considering raising and beaching my mooring using a tidal lift. The ground tackle, which is in salt water, weighs half a ton. Riser is all rope.The idea is to use a builders' sand bag (the cube shaped nylon? ones used to deliver sand by the ton). Will one 40 gallon plastic drum within the bag provide sufficient lift?
 
one litre of sea water weighs 0.975kg
By my calculations you will require 107 gallons or 488 litres of empty containers
 
[ QUOTE ]
one litre of sea water weighs 0.975kg

[/ QUOTE ] no you've got the reciprocal there. It'll be more than 1 kg. According to the data in Kaye and Laby at 20C the density is 1037.44 kg/m³. Thats 1.03744 kg/l But you have got the correct answer at the end!

To be stricty correct of course kg are units of mass, not weight. The unit of weight is the Newton, and weight = mass x the acceleration due to gravity

So 1 litre of sea water has a mass of 1.037 kg but it weighs 10.17 N !

I hope that eliminates any confusion /forums/images/graemlins/grin.gif
 
Those bulk bags are 1m cubed which is approx 1 tonne( =1 ton) of water. Full of air that's one tonne or lift which is what you might need to get the mooring off the bottom.
 
Salt water weighs slightly more than fresh but if you work to the 1kg per litre of fresh water it makes the calculation a doddle and the difference will be negligible. 40 (imperial) gallons is about 182 litres providing this number of kg of buoyancy. Hence you'll need three 40 gallon drums to get the mooring lifted (assuming it's half a metric tonne). It's much easier to work in metric given the definition of 1 kg is the weight of a litre of fresh water.
 
... but an undefined additional amount of lift may be required to unstick the tackle from the seabed.
 
"... but an undefined additional amount of lift may be required to unstick the tackle from the seabed"
Yes - especially if it is buried in mud. Perhaps 10 times as much !
ken
 
Do you intend to attach the buoyancy at low water then wait for the tide to come in and lift the mooring off the sea bed, tow in into shallow water until it bottom out then wait for low water to shorten the ropes and repeat the exercise ad nauseam?

One thing not considered in the above "calculations" is the buoyancy generated due to the displacement of the water.

Concrete has a density in the region of 2300kg/m³ in air however when immersed in water the effective density (sorry Vic) is reduced to 1300kg/m³ so when you say the block weighs ~½ ton lets say 500kg in air the effective weight will be reduced to ~ 283 kg ie to lift the "500"kg block would only require ~ 283 kg of lift if the block was immersed in water, or a lifting bag of approx 283 litres capacity. A "40 gallon" drum is actually 181 litres but nominally 200 litres these days.

Personally I would attach one 40 gallon drum and about 8 x 5gallon drums directly, and as close as possible, to the mooring block. Fill the 40 gallon drum with air (200kg lift) then fill the 5 gallon drums with air one at a time (adding ~20kg lift with each one filled) until I had filled 5 or 6 of them then try to break the suction holding the block down (dig underneath it?) filling the remaining 5 gallon drums if necessary.

Sounds complicated doesn't it? but if you are carring out an air lift and have ½ filled drums (or lifting bags) as the object lifts off the bottom and rises towards the surface the air expands in the drums or bags and creates more lift and the object can wind up going too fast, hit the surface, up end the drums / bags which then flood and the object sinks again. Doing it the way roughly described above means as the block starts to rise the air expands and bleeds out of the full drums and does not increase the lifting force - the only increase in lift comes from maybe one partly filled 5 gallon container - negligable increase in lift.

Your local diving club might even lift your mooring for you as a training "exercise" although a couple of slabs of beer might be appreciated. /forums/images/graemlins/grin.gif

Probably the biggest problem with any lift is breaking the suction.
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lets see new one 44 gallon drum = 205 Litres or about 200+ Kg of lift.
so:
2 drums = 410 kg
3 drums = 615 kg
4 drums = 820 kg
5 drums = 1,025 kg
6 drums = 1,230 kg

How many bags and how much rope did you say you had?

Avagoodweekend......
 
Last time I tried something like this, I used my boat. As the tide rose, the forces rose, but the boat didn't. When the stern was clear of the water and the bows were down to about 6" freeboard my bottle went & I cut the strop. It was a frightening experience. All I was trying to lift was a home made 1/2 admiralty pattern mooring anchor of a few 100 lbs but it was well into very fine sand. As far as I know there are 2 of them still there if anyone wants them! /forums/images/graemlins/laugh.gif
 
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