During the 1930s, the construction of overseas patrol submarines waned considerably. However, in 1934/35 the Admiralty decided to build a new ocean-going boat to replace the Oberon, Parthian and Rainbow Classes that had not lived up to expectations.
Requirements for the new class (defined as Patrol Submarines) demanded that they had a strong armament and a patrol duration of at least 42 days. Restricted by the limitations imposed by the London Naval Treaty that allowed a total of only 16 500 tons of new construction submarines, the class was designed to have a displacement of about 1000 tons so that a sufficient number could be built. The first-of-class, approved in the 1935 Programme, was built at Vickers and entered service in December 1938 under the name of Triton.
Fifty-three T Class submarines were eventually constructed, making it the largest class of ocean-going submarines ever built for the Royal Navy. The original order for the class was made under the growing threat of war, which forced the Admiralty to open its purse-strings, and no fewer than 21 riveted-hull T boats, built between 1937 and 1941, followed Triton.
Displacing almost 400 tons less than the O, P and R Classes, this first group of 22 T Class submarines was noted for its simplicity of construction. They were superior to the O, P and Rs in that they had greater submerged speed, better surface and underwater handling and more torpedo tubes. However, because the displacement limitations restricted the size and power of their engines to 2500 hp surfaced (1450 hp submerged), maximum surface speed was reduced.
The first T Class submarines were 275 feet long and displaced 1327 tons surfaced. Their 'surface' armament included one 4-inch gun and three 0.303-inch machine guns, which were later replaced by, or supplemented by, one 20mm Oerlikon cannon. They were the last Royal Navy submarines designed for overseas patrol to have insufficient range for the Pacific.
One of the most distinguishing features of the Group 1 T boats was their high number of torpedo tubes: six bow tubes, which were reloadable from inside the pressure hull; two external bow tubes, contained in a bulbous bow casing; and two external tubes situated amidships, and so arranged as to fire ahead. This gave the class the phenomenal bow salvo of ten torpedo tubes which, the Royal Navy believed, would compensate for the inevitable errors that accompanied long-range attacks. As an alternative to this armament, a load of 18 mines could be carried.
Perhaps the best known of the early T Class submarines was the ill-fated Thetis. Sailing in Liverpool Bay on the morning of 1st June and prior to handover, Thetis had on board her a 53-man crew and 50 passengers (shipyard and Admiralty men concerned with the trials). For her trial dive, Thetis was reluctant to submerge, so her six bow tubes were checked. When Numbers 1 to 4 were correctly found empty, Numbers 5 and 6 were tested to confirm that each contained sea-water. The test cock of No.6 tube squirted water but, strangely, the test cock of No.5 did not, and so was apparently empty. As there was only one way to be sure, the door was opened - and the sea roared in. Jammed by one of its clips, the watertight door couldn't be closed and, as two compartments flooded, Thetis nose-dived to the sea-bed 160 feet below.
Triumph was lost on active service in the Aegean Sea in January 1942.
