The Parthian-class submarine or P class was a class of six submarines built for the Royal Navy in the late 1920s. They were designed as long-range patrol submarines for the Far East. These boats were almost identical to the Odin class, the only difference being a different bow shape.

The Odin-class submarine was a class of nine submarines developed and built for the Royal Navy (RN) in the 1920s. The prototype, Oberon, was followed by two boats originally ordered for the Royal Australian Navy, but transferred to the RN in 1931 because of the poor economic situation in Australia, and six modified boats ordered for the RN. Three modified boats were built for the Chilean Navy as the Capitan O'Brien-class submarines in 1929.