The Odin, Parthian and Rainbow Classes were all fitted with four-cycle blast-injection eight-cylinder diesels, accommodated in the larger engine room and designed to develop a total of 4400bhp in the Odins and 4640 bhp in the Parthians and Rainbows. In each case, the main engines were made by the building Yard.
By the late 1920s, the ability to dive quickly had become a major consideration but it was believed that a submarine could not submerge faster than about two feet per second. The minimum time to dive from full buoyancy to periscope depth appears to have been of the order of one minute. The range of sea-water density in which submarines of the period were designed to dive was quite limited, and it wasn't until late in the building of the Odin Class that a requirement to be able to dive in fresh water was introduced - resulting in changes to the compensating water tanks. The ability to dive in waters with a specific gravity of 1.00 to 1.30 remained for all submarines until the Second World War, after which it decreased to 1.015 to. 1.03.
