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