1951. The programme was completed with the conversion of in February 1954 – June 1956. The conversion was not entirely successful since the metacentric height was reduced, making the boats roll heavily on the surface in rough weather. This was alleviated in 1953 in those conversions which had been completed by increasing the buoyancy by raising the capacity of a main ballast tank by 50 tons. This was done by merging it with an existing emergency oil fuel tank. For the four boats remaining to be converted, increase in buoyancy was achieved by lengthening the extra hull section to be inserted from to . 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. In the meantime, in December 1950, approval was given for the streamlining of five riveted boats. This was a much less extensive process, with the removal of deck guns and external torpedo tubes, the replacement of the conning tower by a "sail" and replacement of the batteries by more modern versions providing a 23 percent increase in power. The work was much more straightforward than the conversion of the welded boats and was undertaken during normal refit. The first riveted boat to undergo this modification was in 1951. The last operational Royal Navy boat of the class was , which was decommissioned on 29 August 1969. The last T-class boat in service with Royal Navy, albeit non-operationally, was , which was permanently moored as a static training submarine at the shore establishment from 1969 until 1974, when she was replaced by . The last operational boat anywhere was the INS Dolphin, formerly , one of three T-class boats (and two S-class ones) sold to the Israeli Navy; it was decommissioned in 1977. Another submarine sold to Israel, renamed , was lost in the Mediterranean in 1969 while on passage from Scotland to Haifa. Although the wreck was discovered in 1999, the cause of the accident remains uncertain. Group One boats These fifteen pre-war submarines were ordered under the Programmes of 1935 (Triton), 1936 (next four), 1937 (next seven) and 1938 (last three). 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. These external tubes were therefore removed from