The Far East required submarines able to remain on patrol for long periods and deliver a heavy attack far from their bases. The Royal Navy did not freeze that requirement into one design, but built successive batches and revised them as experience accumulated. The Odin, Parthian, and Rainbow sequence, shortened to OPR, shows this iterative approach to large ocean-going boats. Limited interwar orders formed a continuing design programme rather than three unrelated classes.
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How the Odin, Parthian, and Rainbow classes evolved
Successive Odin, Parthian, and Rainbow-class batches refined the long range and striking power required for Far East patrols.