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.

[3] 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 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 Grampus-class mine-laying submarines.

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.

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.

[5] 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.

[8] 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.

[10] 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.

The riveted hull proved remarkably strong, with many of the T-class boats exceeding the rated diving depth of 300 ft (91 m) during combat.

Welded hulls were considerably stronger, allowing a diving depth of 350 ft (107 m) and for fuel to be carried in external ballast tanks for increased endurance.

She sustained depth charge damage from the Italian torpedo boat Circe that ruptured her battery tank and filled the submarine with chlorine gas, forcing her to surface and eventually surrender.

[21] In contrast with Tempest, the modified Terrapin survived a prolonged depth charge attack from Japanese escort vessels, which rendered her hull a constructive total loss but inflicted no damage to her battery cells whatsoever.

In the face of expected enemy anti-submarine measures any attack would probably have to be made at long range without the aid of the periscope, using only ASDIC.

No armour or overhead protection for the 4-inch gun crew was provided as built due to weight restrictions, except on Tabard, Talent, and Teredo.

Many other T-class boats received improvised gun shields manufactured by depot ships in the Far East, providing some degree of protection.

This proved insufficient and was soon increased; by the end of the war, T boats would often not carry reload torpedoes in favour of taking more gun ammunition.

The unfortunate loss of Thetis on 1 June 1939 along with 99 of the men on board during her trials led to modification of the Royal Navy's submarine escape procedures.

Triton was the only member of the class to undergo full trials, for the outbreak of war meant that the Royal Navy could not afford this luxury at a time when modern submarines were desperately needed.

Triumph was unfortunate enough to run into a mine on 26 December 1939 that blew off her bow section, but miraculously survived and was able to return to Rosyth for extensive repairs.

Firstly, the Italian Regia Marina, almost uniquely among the Axis navies, had devoted a substantial amount of resources and training to anti-submarine warfare.

Equipped with their own version of sonar, the ecogoniometro (ECG), possessing excellent escort vessels, and making extensive use of mines, the Italians were to prove the most successful of the Axis powers at destroying Allied submarines.

Submarines could often be spotted from the air even when submerged, and the shallow waters made deep diving to escape attack difficult or downright impossible.

Having been designed for operations in the Far East, the T boats were substantially larger than the standard German Type VII U-boat, and thus they were more vulnerable to detection and mines.

Conversely, the large sizes of the T-class boats gave them substantially greater endurance and range compared to the smaller standard Royal Navy submarines like the S and U classes.

The other two were awarded to two crew members of Thrasher, T. W. Gould and P. S. W. Roberts, who removed two unexploded anti-submarine bombs stuck in the submarine's gun casing.

Though targets of opportunity were poor compared to where US submarines were operating the class performed reasonably well, sinking numerous merchant/cargo ships and smaller Japanese warships along with large numbers of coasters and small vessels.

With the dramatically reduced surface fleet following the end of the Second World War, he commented that this was one of the few methods the Royal Navy had for "getting to the enemy on his home ground.

The conversion was not entirely successful since the metacentric height was reduced, making the boats roll heavily on the surface in rough weather.

The effect was to lengthen the control room and strict instructions were issued that this space was not to be used for extra equipment otherwise the improved buoyancy would be affected.

The boats originally had a bulbous bow covering the two forward external torpedo tubes, which quickly produced complaints that they reduced surface speed in rough weather.