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.Raven (1988), p. 179.
At the start of the Second World War the T class was, with the British S and U class, Dutch and German Type VII, one of the most advanced submarine classes in service.van den Pol (1989), p. 352.
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 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 over-complicated. The Washington Naval Treaty of 1922 required that these submarines be retired after 13 years of service. would have to be paid off in August 1940 (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 , a maximum standard surfaced displacement of for any boat, and maximum gun armament of . The Americans had proposed a limit of for the 1935 London Disarmament Conference, but this was rejected by the Admiralty since it would exclude the mine-laying submarines. The Admiralty proposed retaining the limit of , hoping that the rival naval powers would build fewer but larger submarines as a matter of national pride, which would be easier to hunt than numerous smaller submarines. The National Government of Stanley Baldwin had also proposed banning the submarine altogether or imposing an individual displacement limit of 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 Imperial Japanese Navy. In the absence of a battle fleet, the submarines would be the primary offensive weapon against the Japanese. 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 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), Noel Laurence, one of the most distinguished British submariners of the First World War, 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 displacement patrol submarine. The DNC designed (DNC 'A' and DNC 'B'), forming the basis of the November 1934 preliminary staff requirement. These designs called for an armament of six internal torpedo tubes, two external tubes, and one (or , if stability permitted) gun, and a patrol capability of at plus sufficient fuel for a 28-day patrol, corresponding to a range of at . Submerged endurance was to be 15 hours at or eight hours at . A maximum submerged speed of , surfaced speed of and diving depth of were specified. An alternative proposal by Rear Admiral Laurence suggested using a double hull for greater survivability under depth charge attack but his proposal was rejected by the DNC in favour of a more conventional single hull with saddle tanks.
In 1935, the "Repeat P" design was modified to reduce the displacement to in compliance with treaty limitations. Design 'C' had to sacrifice machinery space, reducing the surfaced speed to only and the surfaced endurance to at . The design was again modified with Design 'D', eliminating fuel stowage in external tanks (which had proved 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 to without unacceptable reductions to endurance and the displacement was allowed to rise to .
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 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 , with additional fuel 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, particularly problematical in heavy weather. Some of the Group One boats were fitted with closed cab-type bridges to resolve this problem; this became standard in the Group Two boats, but wartime experience found that the greater visibility from the open bridges was more important in action 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 .Bishop, in The Complete Encyclopedia of Weapons of WW2, p.450, gives 12,665nm at 10 knots.
Propulsion The T-class boats used a variety of diesel engines depending on where they were built. Vickers-built boats 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 equalled by any of the other T-class boats, which usually managed about .
The Vickers 6-cylinder 4-stroke injection diesel engines fitted to the majority of the T class proved to be very reliable, although less advanced than the diesels used by the German U-boats. The engine could continue running even if one cylinder failed, by disconnecting the cylinder from the crankshaft. The 12 boats completed at the Royal Dockyards fitted with Admiralty diesel engines proved equally reliable although the engines were somewhat more complicated than the Vickers ones.
The MAN diesels proved to be rather troublesome; they were built under license, and once the Second World War broke out in 1939, technical support from the German MAN company stopped. By 1943, only two of the T-class boats with MAN engines were left ( and ), and they were relegated to training. Even when the Royal Navy in the Far East was facing a critical submarine shortage in March 1944, Tuna was not sent there due to her untrustworthy engines. The Cammell Laird Sulzer 2-stroke engines received mixed reviews; some boats, including and , were perfectly satisfactory, while the engines caused problems on others. They were insufficiently engineered for running at full speed, and the cylinder rings and blocks tended to crack.
Submerged propulsion was provided by a 336-cell battery driving two Laurence Scott electric motors. These provided an endurance of 48 hours at , or only one hour at the maximum submerged speed of . The battery proved vulnerable to shock damage from depth charge attacks. This contributed to the loss of in 1942, which sustained depth charge damage from the that ruptured her battery tank and filled the submarine with chlorine gas, forcing her to surface and eventually surrender. This problem was resolved by strengthening the battery compartment and fitting rubber shock absorbers. In contrast to Tempest, the modified survived a prolonged depth charge attack from Japanese escort vessels which rendered her hull a constructive total loss but caused no damage to her battery cells.
Weaponry
Torpedoes