The United States Navy Salmon-class submarines were an important developmental step in the design of the "fleet submarine" concept during the 1930s. An incremental improvement over the previous Porpoise class, they were the first US submarine class to achieve 21 knots with a reliable propulsion plant, allowing them to operate with the Standard-type battleships of the surface fleet. Also, their 11,000 nautical miles (20,000 km) unrefueled range would allow them to operate in Japanese home waters. These rugged and dependable boats provided yeoman service during World War II, along with their immediate successors, the similar Sargo class. In some references, the Salmons and Sargos are called the "New S Class", 1st and 2nd Groups.