Lift Bag Calculation

I cannot seem to wrap my head around these calculations. I am obviously missing a step somewhere. I am trying to determine the amount of lift bags(500lb) needed to raise...lets a 3500 lb car...from about 10 ft of freshwater. Can someone give me a formula with an explantion? Any help would be greatly appreciated!

noob024:

I cannot seem to wrap my head around these calculations. I am obviously missing a step somewhere. I am trying to determine the amount of lift bags(500lb) needed to raise...lets a 3500 lb car...from about 10 ft of freshwater. Can someone give me a formula with an explantion? Any help would be greatly appreciated!

Click to expand...

You need to know the volume of water displaced by the object you are going to "lift."

Armed with that number, you can work out the object's apparent weight (it's weight on dry land minus the buoyant effect from the water it displaces by subtracting one value form the other) and from that how may litres of lift bag you need (one litre of water has a mass of one kilo)... at 10 feet (about 3 metres) the density of the air will be 1.3 times what it is on the surface... hence, 100 litres of liftbag will require 130 litres of gas to inflate them

sorry, I don't do we love the queen measurements, but I think one cubic foot of water has a mass of 64 pounds...

You need to work out how much water it displaces...

then think of the weight as the downward force, and the water displacement as the upward force.

I.E if the object weighs 1500kg and displaces liters of fresh water (to change liters of freshwater to kg x1.0) = 500kg
1500-500
the object would be 1000kg negatively buoyant.

this 1000kg translates straight back into liters, so therefore you would need 1000 liters of air in the liftbag

then times this by the density (about 3 meters) 1000 x 1.3 = 1300 liters of air required from your scuba cylinder to fill the 1000 liter liftbag.

You all have ignored the suction of the bottom on the object. It is not uncommon to need 150 to 300% to lift an object. In other words, a 1000 pound (use your mass unit, Kg if you are metric) object would need 1500 to 3000 lbs to break it free of the bottom. The worse bottom (most lift required) is a thick clay, the best (Least lift required) is a large pebble or rock with a sand bottom in between.

To lift a 3000 pound car I would plan to have 4500 pounds of lift bag available and fill them till either the object lifts or all are filled. If all the bags get filled, or in order to reduce the number of bags required, you can try to break the suction by using an air or water jet under the object, but when that suction is broken, do not be in the way of the object as it will head to the surface very quickly.

For the lift you describe being in 10' of water I would also use pillow bags strapped as low on the car as I can get them. If you use balloon shaped bags. you will only lift the car a few feet till the bags break the surface and the lift stops. Also, unless you can do a center pick, one end of the car will always lift before the other, so plan on it and pick the end you want up first, get it up and then lift the other in a controlled manner.

Just a note that if these calculations are for anything other than a hypothetical exercise, to reconsider. Leave the multiple ton objects to the commercial divers. Can very easily kill you.

For a 3500 lb car you want 3500+ lbs of lift. For a 10,000 boat you want 10,000+ lbs of lift. Displacement and all that stuff is fun to work out but when the object break the surface it weighs what it weighs. This is real world figuring. I have been raising cars, boats, etc, for many years now. If you have a 100 lb outboard motor, you can get away with a 50lb bag, because a 50 lb (open) bag will actually give you about 75 lbs of lift and will keep the motor below the surface until you grab it and pull it up. Closed bags for large objects and you have to take into account your placement because the weight of the motor can cause the front of the vehicle to turn down. But, as a basic formula, lift capacity should equal, or exceed, surface weight.

noob024:

I cannot seem to wrap my head around these calculations. I am obviously missing a step somewhere. I am trying to determine the amount of lift bags(500lb) needed to raise...lets a 3500 lb car...from about 10 ft of freshwater. Can someone give me a formula with an explantion? Any help would be greatly appreciated!

Click to expand...

It sounds like you are thinking of trying this ... Don't !

First off, the lift bags will not get your car very much lift in ten feet of water unless the bags are under the car ... Which is a VERY dangerous manoeuvre .

In ten feet of water it would be better to use a crane than lift bags.

If all you are doing is making calculations then I appologise.

Bombay High:

It sounds like you are thinking of trying this ... Don't !

First off, the lift bags will not get your car very much lift in ten feet of water unless the bags are under the car ... Which is a VERY dangerous manoeuvre .

In ten feet of water it would be better to use a crane than lift bags. If all you are doing is making calculations then I appologise.

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With proper bags you can lift a car off of the bottom in 5 feet of water. I had one a few years ago that was swept down a bayou. I had to lift and float it to an extraction point 300 yards upstream in 5' of water. You have to "cradle" the vehicle with straps, or solid attachment points, which hold the bags low and raises the vehicle off of the bottom. It is not as easy as it sounds, due to stretching straps and such, but it is not terribly difficult either. Generally a strap to the vehicle and cable from a tow truck works fine, unless there are obstructions between the object and the bank. Cranes are great, but not feasible for recovering vehicles as they are cost prohibitive. With proper bags or a tow truck it can be done safely, but not without the proper training as there are a lot of dynamics involved. My facebook page has quite a few photo's of jobs, from vehicles to airboats, etc. The big thing is to be careful and don't take on something that you are not positive you can do safely. You need the right equipment for the job.

Mark Michaud | Facebook

Spd 135:

With proper bags you can lift a car off of the bottom in 5 feet of water. I had one a few years ago that was swept down a bayou. I had to lift and float it to an extraction point 300 yards upstream in 5' of water. You have to "cradle" the vehicle with straps, or solid attachment points, which hold the bags low and raises the vehicle off of the bottom. It is not as easy as it sounds, due to stretching straps and such, but it is not terribly difficult either. Generally a strap to the vehicle and cable from a tow truck works fine, unless there are obstructions between the object and the bank. Cranes are great, but not feasible for recovering vehicles as they are cost prohibitive. With proper bags or a tow truck it can be done safely, but not without the proper training as there are a lot of dynamics involved. My facebook page has quite a few photo's of jobs, from vehicles to airboats, etc. The big thing is to be careful and don't take on something that you are not positive you can do safely. You need the right equipment for the job.

Mark Michaud | Facebook

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Hi Mark,

I am no expert on shallow water salvage, and personally find it to be amongst the most dangerous salvage because of the great instability introduced in shallow water lifts, when the lift bag is very close to the CG of the load. You obviously need great expertise and caution using the method you describe.
And i do agree, cranes are very expensive, but in my opinion the safest way to do such an extraction.

Nice pictures.

I used to recover several vehicles per year and I kept it both fairly simple in terms of lift calculation and employed an overkill approach. I'd use a 1500 pound closed circuit pillow bag on each corner securely attached to the frame and if possible using the same location intended for DOT hooks.

I also used a manifold and surface supplied air and ran hoses to each bag and in most cases I'd be on the surface during a zero viz lift, or be mid water and offset well to the side in a lift with visibility.

Once on the surface, I'd rig it for towing to the boat ramp and then be met by a tow truck. Once it was over the ramp and connected, I'd set it on the ramp, remove the bags and clear the area before the tow truck took any significant strain. That last procedure became standard after a tow cable broke and narrowly missed me on that final stage of the recovery.

It's definitely something you want to be very well schooled in before you attempt it.




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