Ships in class: HMS Thames, HMS Severn, HMS Clyde
The River class was the last attempt by the Admiralty to produce "Fleet Submarines" that is submarines fast enough to operate as part of a fleet which at the time meant being able to manage somewhere around 20 knots (37 km/h) while surfaced. The previous attempts had been the steam powered K-class submarines and the large 12-inch (305 mm) gunned M-class submarines.
The design compromised on diving depth to keep weight down and speed up. They had a safe diving depth of some 300 feet (90 m) compared to the Odin class before them which had managed 500 feet (150 m). They were powered by two diesel engines delivering 8,000 bhp (6,000 kW). Two Ricardo engines drove generators that supercharged the diesels up to 10,000 bhp (7,500 kW). This gave them a surface speed of 22 knots (41 km/h).