1929 - 1946: Rainbow Class
Overseas Patrol Submarines
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.
The Odin Class, Parthian Class and Rainbow Class were all fitted with four-cycle blast-injection eight-cylinder diesels, accommodated in the larger engine room and designed to develop a total of 4400 bhp in the Odin Class and 4640 bhp in the Parthian Class and Rainbow Class. 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.
The Oberon Class, Odin Class, Parthian Class and Rainbow Class were designed to dive to a depth of 500 feet. The Oberons were tested to 200 feet and in the three later classes the deep diving trials were to 300 feet, although it is known that some boats went deeper. Rear-Admiral (Submarines) (RA(S)) laid down 300 feet as the test diving depth of boats designed to withstand depth pressure at 500 feet.
It is interesting to review the policy regarding deep diving. In 1928, the Director of Tactical Division (DTD) stated:
The ability to dive to 500 feet was introduced principally in order that pressure hulls of these submarines should be more capable of withstanding the effect of the explosion of a depth charge. Submarine officers do not visualise any intentional diving to such depths as 500 feet though the ability to do so is an asset in the event of an involuntary deep dive which might cause the submarine to go much deeper than ever was intended.
When on patrol, the daily fuel consumption for all classes, allowing 12 hours diving, 12 hours steaming at slow speed and eight hours charging, was given as 2.1 tons per day, except in Orpheus (the only vessel of the Odin Class to be fitted with a Vulcan clutch) where the consumption was 2.6 tons per day.
In the Overseas Patrol Submarines practically the whole of the fuel was carried in external tanks. The tanks were riveted and tested to 20 lb/in2, but they leaked to such an extent that they could be considered a failure. Various reasons, such as defective plating, manhole covers and bad equalising arrangements, were blamed for the leaks, and many fruitless attempts were made to overcome the faults. In fact the trouble throughout had been caused by the rivets and, in an era when welded ship construction was in its infancy, the externals were ultimately successfully rebuilt in welded construction.
Of the 19 Overseas Patrol Submarines built, 18 served in the Second World War; 12 of which were lost on active service. The last submarine of this type to be taken out of service was the Vickers-built Otus, in April 1946.