A submersible is defined as a neutrally buoyant submarine which relies largely on the buoyancy of the hull rather than gas or liquid filled floats at ambient water pressure. 29 different kinds of submersible have been built in USA and 15 in other countries. One has been built in England. Half of these do not go below 2000 ft and one third do not go below 1000 ft. The minimum safe weight for a vehicle is 5 tons at 1000 ft and 10 tons at 20 000 ft. A useful research or light work vehicle must be twice the minimum weight, and a heavy work vehicle in the 30-100 ton range.
High utilisation factor can only be obtained with special handling and support equipment, underwater navigation aids, high manoeuvrability, and a reserve of speed, life support, payload, and endurance. Principle applications of submersibles are in scientific research, geological mapping, prospecting, the oil industry, dredging, pipe-laying, and search and salvage.