Lifting underwater
One litre (a quart) of water weighs 1 kg (2 pounds). If you can displace this amount of water with 1 L (1 quart) of air underwater, you will obtain about 1 kg (2 pounds) of lift.
Any open-ended vessel can be used as a lifting device provided you can attach a line to it. It must be open ended because the air within it will expand on the way to the surface and the open end will allow this expanding air to escape. Attach a line to the vessel so you can tie on the load.
A better solution to a lifting problem is to use a lifting bag designed for the job. These are available in a range of lifting capacities based on their volume when inflated. A good lifting bag is teardrop shaped so that it provides stable support when it reaches the surface and the wave action does not cause the air to spill from it. The best ones also have a dump valve like the one on your BC. This allows you to release air so that the lift is reduced or removed completely. It also allows you to take the bag away from the lifted object once it has been moved to a new location.
You will need to calculate the amount of lift required to move the object. This is worked out by finding the object’s density and volume and then subtracting the weight of the water it displaces.
Using lifting bags
You should use a lifting bag of the right size. If you use an oversized lifting bag, the air in it will expand, causing the load to accelerate as it ascends. With a bag of the appropriate size, the excess air will spill out and the speed of the lift will remain constant.
The bag is securely attached to the load, and air from either the diver’s tank or a second tank is used to fill it. A good system is to use two lifting bags. A larger one, which is insufficient to lift the object alone, is first filled. After that a smaller one is attached to the load and filled with air. The smaller one is used to control the lift and, if necessary, air can be dumped from it so that the load is carefully controlled. In this way, massive loads can be balanced so that it takes only the power of one swimmer to move them. When the object reaches the surface, it can be towed to its new location or to where a crane is available to lift it out.
The massive power provided when air displaces water can also be used to remove objects that are embedded in a muddy seabed. In this case, lifting bags will have far too much capacity to perform a lift safely. Instead, they are attached to the object at the end of long ropes so that they start their journey upward from very near the surface.
Once the bags are filled with sufficient air to wrench the object free of the mud, they move upward only a short distance before reaching the surface, and the object is left dangling. At this time, a lifting bag of a size appropriate to a properly controlled lift is attached.
