Though its origins went back to 1928, the 'S' class was highly successful during the war and, with 62 completions, was the Royal Navy's most prolific class, Ostensibly 'H' class replacements, the performance of the 'S' class boats was required to be enhanced to allow operation in the Baltic and Mediterranean. A tight, 600-ton surfaced displacement target was set to produce a small submarine which, nevertheless, was required to be able to transit 805 km (500 miles) to and from its patrol area, where it was expected to remain up to 10 days. Any increase in the 805-km (500-mile) range meant having to find space for very much larger radio equipment. The specification was later to be altered drastically, calling for 1930-km (1,200-mile) passages at not less than 9 kts and eight days on station.
Initially, a class of four ('Swordfish' type) was built; launched between 1931 and 1933 by Chatham Dockyard, these displaced 640 tons despite every effort at weight control. The design was really too tight and was relaxed to 670 tons for the eight lengthened 'Shark' type boats of 1934-7, Though it was planned to terminate the class at 12, war saw the design stretched further and constructed in series.
To save on topweight a 76,2-in (3-in) gun was fitted but, with the extra hull length, a further torpedo tube was worked-m aft on some boats. Others traded both of these for a single 101.6 mm (4-in) gun. With only 12 or 13 torpedoes aboard, the gun was a useful means of disposing of 'soft' targets which, while not warranting the expenditure of a torpedo, were often reluctant to sink. Earlier boats had fuel tanks within the pressure hull, but later units supplemented these with external capacity, which allowed them to work even in the Far East.
Interestingly, eight units were lost from the original 12, the same number as were lost from the following 50. All of the first group losses occurred before February 1941, while the first hull of succeeding groups was not launched until October 1941. Submarine operations in European waters during the early months of the war were clearly hazardous.
Specification 'S' class (later boats) Displacement: 860 tons surfaced and 990 tons submerged Dimensions: length 66,14 m (217.0 ft); beam 7.16 m (23.5 ft); draught 3,20 m (10.5ft) Propulsion: two diesels delivering 1416.8 kW ( 1,900 bhp) and two electric motors delivering 969,4 kW (1,300 hp) to two shafts Speed: 15 kts surfaced and 9 kts submerged Endurance: 13897 km (8,635 miles) at 10 kts surfaced Armament: one 101.6-mm(4-in)gun and six 533-mm (21-in) torpedo tubes, or one 76.2-mm (3-in) gun and seven 533-mm torpedo tubes Complement: 44