With the steam-driven 'K' class boats and the experimental XI, the Royal Navy had attempted to produce submarines with characteristics appropriate for operations with the surface fleet. Unfortunately, the 'K' boats were disastrously problem-prone and the XI remained a one-off. The requirement still existed, therefore, for a boat to fill the role while avoiding the weaknesses of the 'O' class. Adhering to the limitations of the Geneva Conference it was decided to build 20 boats, each of the maximum allowable surface displacement of 1,800 tons, with the combined functions of fleet work and long-range patrols. In the event, with the surface fleet becoming faster, policy was changed and only three of the class were completed between 1932 and 1934 as the 'Thames' class units HMS Thames, Severn and Clyde. The boats were only 1.83m (6ft) shorter overall than the monstrous 'K' class boats and were actually beamier overall, despite a narrower pressure hull. In cross-section the hull was carried downward at the keel to meet the line of the outer hull. Little oil fuel was carried inboard, most of it being stowed in spaces above the main ballast tanks. Weepage was apt to be into the main hull through started rivet-heads.
Only now were diesels available of the size and power to match the 'K' class legend speed of 23.5 kts. Engine design was by the Admiralty, and turned out to be lighter than forecast. This was fortunate as the boats were highly weight-critical. For their extended patrols, for instance, 41 tons of fresh and distilled water were carried, some two per cent of surface displacement. General submarine policy of substituting 101.6-mm (4-in) for 119.4- mm (4.7-in) guns saved some 6 tons, while a further 8 tons was gained by burning fuel of a lower specific gravity.
In the Norwegian campaign of 1940, the Thames was lost on a mine, while the Clyde succeeded in damaging the Gneisenau by torpedo, The Clyde ran an invaluable 1,200 tons of supplies to a beleaguered Malta and sank several enemy merchantmen while working out of Gibraltar. The Severn was active for a time in the little-known Levant operations.
