When Thetis was raised in November 1939, an investigation into the cause of this tragic accident revealed an incorrectly-wired bow cap indicator - showing the bow cap to be shut when it was open - and that the vital test cock was blocked with paint. To avoid any suggestion of a jinx on the boat, the Admiralty refitted and recommissioned her, in November 1940, as Thunderbolt. As an epitaph to her 'previous life', she entered service with a diagonal rusty line on her hull that could not be hidden.
THE WAR YEARS
Under the 1940 War Programme came the decision to build nine slightly modified T Class submarines. These modifications were made in the light of experience gained with the first group of T boats, the main changes being in the number and disposition of torpedo tubes, the outer hull shape and the use, in most of the modified vessels, of electrically-welded rather than riveted construction. The latter change assisted deeper diving, improved the resistance to depth charge attack and enabled the shipbuilder to adopt the new technique of prefabricating the hull in sections in the shops and assembling large units at the building berth.
To this modified group of submarines was fitted, at the extreme stern, an additional external torpedo tube, whilst the two tubes amidships were repositioned aft of the conning tower, angled to fire astern. These changes altered the shape and silhouette of the class, as did the removal of the bulbous bow casing. This had created a notable bow wave which, when running at periscope depth, hampered visibility and the correct trim of the boat. As a result of these alterations, the second group of boats were more streamlined, and the openings for the two external tubes were more clearly visible.
In addition to their 21-inch torpedo tubes. Group 2 T Class submarines were fitted with a 4-inch gun, a 20mm Oerlikon cannon on a platform aft of the periscopes and three 0.303-inch machine guns on removable mountings.
Additional orders in the 1941 and 1942 Programmes meant that a total of 31 modified T Class submarines entered service between 1942 and 1946, 21 of which were laid down at Vickers (although a number of these were ultimately completed at other yards). Of the 22 Group 1 submarines constructed, eight were built exclusively at Barrow.
Along with other British submarines, the T boats ordered in the 1941 and 1942 Programmes were fitted with surface and air search radar sets.
During the Second World War, T Class submarines operated. successfully in all the theatres in which the Royal Navy was committed and many of the Group 2 boats were further modified for employment in the Far East. Several ballast tanks were changed into fuel tanks, thereby increasing the fuel load from 132 to 230 tons and surface range from 8000 to 11 000 miles at 10 knots. In a theatre where it took up to a week to sail from base to the operational area, this increase in range, together with increased stores capacity, enabled long patrols to be carried out - the record being 56 days by the Barrow-built Tantalus, 40 days of which were spent in the patrol area.
