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Standard buoyancy calculations refer to atmospheric air pressure. However, the effect of the weight of air underneath the waterline is not included as well as displaced air in tanks, structure and equipment above the waterline. A review of the theoretical mistakes in calculating buoyancy which includes comparison tables of data of different ship types. Suggestions for correcting the apparent mistakes are made. While per mille values for ships are small, per mille buoyancy is a few thousand kN. Documentation of real ships is used for correction of the actual mentioned air weights, thereby yielding precise ship weight and draft values.

The range of underwater vehicles from classical large submarines to deep sea vehicles (bathyscaphes, submersibles and submarines) is analyzed in this paper. Attention to the requirements for surface and underwater conditions including submerging/emerging processes is made. With classical submarines, buoyancy is satisfied by the inherent large pressure hull and corresponding appendages. However, deep sea vehicles which have smaller sized pressure hulls in order to satisfy strength and economic issues make use of syntactic foam. This paper provides a qualitative explanation of the development of chosen underwater vehicles. Special comparison of “Trieste”, “Deepsea Challenger”and “Mir” submersibles are discussed. Additionally, the experience of the main author’s collaboration in the preliminary design of the RONIO submersible is presented and its relevance to the topic of manned submersibles is explained.