Navy. In the absence of a battlefleet, the submarines would be the primary offensive weapon against the Japanese. As such, the replacement "Repeat P" class had to have a similar endurance but be easier to maintain, as well as appreciably smaller in expectation of future treaty restrictions. In drawing up the future requirements for the British submarine fleet, 20 of these new submarines were estimated to be required for a total tonnage of 20,000 tons. Rear Admiral (Submarines), Rear Admiral Noel Laurence, one of the most distinguished British submariners of World War I, also pushed for a strong torpedo armament. He was convinced that a British submarine facing a powerful Japanese surface force would have difficulty penetrating the destroyer screen, and only a large torpedo salvo would be able to ensure the required hits at longer ranges, if necessary using only ASDIC data for a firing solution. On 27 February 1934, the Director of Naval Construction (DNC), Sir Arthur Johns, was asked to investigate designs for a 1,000 ton displacement patrol submarine. Two DNC designs were drafted (DNC 'A' and DNC 'B'), forming the basis of the November 1934 preliminary staff requirement. These designs called for an armament of six internal 21 inch torpedo tubes, two external tubes, and one 3-inch (or 4-inch gun, if stability permitted), and a patrol capability of 4,000 nmi at 11 kn plus sufficient fuel for a 28-day patrol (corresponding to a range of 5,500 nmi at 11 kns). Submerged endurance was to be 15 hours at 2 kn or eight hours at 5 kn. A maximum submerged speed of 9 kn, surfaced speed of 15 kn, and diving depth of 300 ft were specified. An alternative proposal by Rear Admiral Laurence suggested using a double hull for greater survivability under depth charge attack. However, his proposal was rejected by the DNC in favour of a more conventional single hull with saddle tanks. Subsequently, in 1935, the "Repeat P" design was modified to reduce the displacement to 1,000 tons in compliance with treaty limitations. In order to do so, Design 'C' had to sacrifice machinery space, reducing the surfaced speed to only 14.5 kn and the surfaced endurance to 8,600 nmi at 8 kn. The design was again modified with Design 'D', eliminating fuel stowage in external tanks (which had proved problematic and prone to leakage on the O, P, and R classes) in favor