6in increase in the engine room length. Odin was approved by the Board in August 1926, and was the first of a class of six: the others being Oswald; Osiris; Otus; Olympus and Orpheus.
In the 1927 Programme, six Parthian Class submarines were ordered (Parthian; Perseus; Poseidon; Proteus; Pandora and Phoenix), and in the following year six Rainbow Class (Rainbow; Regent; Regulus; Rover; Rupert and Royalist) were also ordered. Of the latter, Rupert and Royalist were subsequently cancelled. Although slightly larger than the O boats, the main particulars of these two classes were practically the same. But, in an attempt to make living conditions more pleasant on long surface passages to the Far East, the Rainbow Class were fitted with a galley on the upper deck and a shower in the conning tower.
