T-class submarines: construction priorities and a stern-firing salvo

The T class replaced older patrol submarines, with Triton in the 1935 programme and only three boats completed when war began. More boats were wanted, but the need for numbers diverted some construction toward the easier U and S classes. T-class orders ceased once preparations for the A class were sufficiently advanced. Complete welding improved hull strength, and conversion of main tanks to fuel stowage increased endurance for Far Eastern work. Revised bow casings reduced the blunt shape around the forward external torpedo tubes. Amidships external tubes changed from firing forward to firing aft, and a later additional tube at the rear of the casing strengthened the stern-firing salvo.