British T-class submarine

The Royal Navy's T class (or ''Triton'' class) of diesel-electric submarines was designed in the 1930s to replace the O, P, and R classes. Fifty-three members of the class were built just before and during the Second World War, where they played a major role in the Royal Navy's submarine operations. Four boats in service with the Royal Netherlands Navy were known as the ''Zwaardvisch'' class.

In the decade following the war, the oldest surviving boats were scrapped and the remainder converted to anti-submarine vessels to counter the growing Soviet submarine threat. The Royal Navy disposed of its last operational boat in 1969, although it retained one permanently moored as a static training submarine until 1974. The last surviving boat, serving in the Israel Sea Corps, was scrapped in 1977.

Development

The design of what was to become the T class began in 1934 in order to create a replacement for the first British postwar submarines, the O, P, and R classes. These similar classes of submarines had proved unsatisfactory, being mechanically unreliable, large, slow, and overcomplicated. Furthermore, the Washington Naval Treaty of 1922 required that these submarines be retired after 13 years of service. Thus, would have to be paid off in August 1940 (in the event, the outbreak of war in 1939 kept her in service).

The 1930 London Naval Treaty restricted the British submarine fleet to a total tonnage of 52,700 tons, a maximum standard surfaced displacement of 2,000 tons for any boat, and maximum gun armament of . The Americans had proposed a limit of 1,200 tons for the 1935 London Disarmament Conference, but this was rejected by the Admiralty since it would exclude the mine-laying submarines. The Admiralty itself proposed retaining the limit of 2,000 tons, hoping that the rival naval powers would build fewer but larger submarines as a matter of national pride, which would be easier to hunt down than numerous smaller submarines. The Conservative government of Stanley Baldwin had also proposed banning the submarine altogether or imposing an individual displacement limit of 250 tons, but the Admiralty correctly predicted that the other nations would not accept such strict limitations and continued with the design of what was then known as the "Repeat P" or "Replace P" class of submarines.

The O, P, and R classes had been designed with the Pacific in mind as a counter to the increasingly powerful Imperial Japanese 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 of stowage within the pressure hull. It proved impossible to reduce the displacement back to 1,000 tons without unacceptable reductions to endurance, and the displacement was allowed to rise to 1,075 tons.

After a slight reduction in the length and the fresh water diving requirement of the design, the final design was agreed upon in May 1935. On 24 June 1935, the designation "Repeat P" was formally dropped by the Admiralty, and it was decided that the submarines would all bear names beginning with the letter T. Finally, on 3 September 1935, the name was selected for the lead ship of the class. Final approval for the design was given by the Admiralty Board on 13 February 1936. The Vickers, Cammell Laird, and Scotts shipbuilding companies were invited to submit tenders on 5 December 1935, and on 5 March 1936, the contract for ''Triton'' was awarded to Vickers Armstrong under the 1935 Programme.

Design

The design of the T-class submarines was dictated by the requirements of an extremely large forward torpedo salvo capability and long patrol endurance for operations in the Pacific against Japanese warships, as well as the need to comply with various treaty restrictions. These extremely challenging requirements led to many compromises in the design. Operational experience before and during the war led to many alterations and modifications to the class, and individual boats often differed noticeably from each other.

Hull and superstructure

To accommodate the external forward torpedo tubes, most of the T class had distinctive bulbous bows. The original bow shape of the Group One boats adversely affected the speed while surfaced and two of the Group One boats had the external bow tubes omitted during refitting ( and , formerly ''Thetis''), resulting in a finer bow shape. Group Two boats had the external bow tubes moved further back, allowing for a finer bow shape that ended the speed loss. They also had two of the external torpedo tubes reversed to face aft along with an additional rear torpedo tube, resulting in a characteristic hump. The final Group Three boats had the bows further fined and the casing around the conning tower and rear-facing torpedo tubes flattened, resulting in a smoother profile.

Pre-war T-class submarines were of riveted construction. The riveted hull proved remarkably strong, with many of the T-class boats exceeding the rated diving depth of during combat. , survived a dive to on 23 April 1940. Welding in the hull construction was officially sanctioned by the Admiralty, after much hesitation, in July 1942 for the pressure hulls of the Group Three boats, later extended to the entire hull. Welded hulls were considerably stronger, allowing a diving depth of and for fuel to be carried in external ballast tanks for increased endurance. Partly welded Group Three boats had riveted external ballast tanks; these were welded up before being sent to the Far East to prevent telltale oil leaks betraying the submarine's presence. The estimated crush depth was .

The lead ship, ''Triton'', was completed with a very high open bridge, which was very draughty. The following Group One boats had a slightly different bridge shape but these too were exposed, especially during heavy weather. Some of the Group One boats were fitted with cab-type bridges to resolve this problem, which were subsequently standardised in the Group Two boats. Wartime experience would eventually show that the greater visibility from the open bridges was more important in combat than the better habitability of the cabs and the final Group Three boats reverted to the open bridge.

T-class boats had eleven main (ballast) tanks, two auxiliary tanks for adjusting trim, five compensating tanks for adjusting to changes in water density and the displacement of the submarine as stores were used up and the bow Q tank used for quick dives or rapid changes in depth. Two of the main tanks were converted into fuel tanks in the Group Three boats to increase endurance for operations in the Far East. Diving time from a 50 per cent buoyancy condition was good by British submarine standards at 30 seconds.

The Group Two and Three boats had the fuel capacity increased on many boats to , giving a surfaced range of at .

Propulsion

The T-class boats used a variety of diesel engines depending on where they were built. Vickers-built boats naturally used Vickers engines, while those from the Royal Dockyards used Admiralty diesel engines; Cammell Laird boats used Sulzer engines, while the pre-war Scotts boats had German MAN supercharged diesel engines. These engines drove two shafts, each capable of for a top surfaced speed of about . The lead boat ''Triton'' achieved on her first-of-class trials; this speed was never equaled by any of the other T-class boats, who usually managed about .

The Vickers 6-cylinder 4-stroke 1,250 bhp injection diesel engines fitted to the majority of the T class proved to be very reliable engines, even if they were less advanced than the diesels used by the German