S-class submarines: welding, armament, and Far Eastern endurance

S-class submarines were designed for offensive patrols in confined waters such as the North Sea and Mediterranean. Small prewar orders allowed repeated improvements, and early wartime experience supported further construction because the class suited North Sea operations and was economical to build. Progress from riveting to welding improved hull robustness and permitted greater diving depth. An added stern-firing external torpedo tube was omitted from some boats when a 4-inch gun replaced their 3-inch weapon. Far Eastern operations required more endurance, leading to the conversion of some main ballast tanks into fuel tanks. Riveted external tanks had leaked when structures worked or ships bumped alongside, risking an oil trail that could reveal a submarine. Welded tanks largely removed that obstacle to carrying fuel outside the pressure hull.