The S class balanced Atlantic and Mediterranean demands

After concentrating on long-range Far Eastern submarines, the Royal Navy still needed replacements for ageing H-class boats. The S class was large enough to carry a useful punch in the Mediterranean and capable enough for Atlantic work, yet more affordable and numerous than the Pacific boats. Its first orders came during the treaty era and the Ten Year Rule, when money was tight and submarine technology was changing quickly. Improvements in batteries, valves, and internal arrangements meant that even externally similar boats could differ greatly from First World War predecessors. The class was a deliberate middle course rather than a reduced version of the long-range fleet submarine.