1915 - 1929: J Class
Designed as a counter to a perceived (but incorrect) threat from high speed German U-boats.
Reports that the Germans had submarines capable of speeds equal to that of surface vessels led to consideration of high-speed submarines by the Admiralty. The idea that submarines could keep up with and work alongside the surface fleet had already been attempted with the troublesome steam-powered K Class but the idea was taken further even though the reported German boats were proved to be non-existent.
This resulted in a diesel-electric design that was unique having three screws, which allowed them to reach a speed of 19.5 knots on the surface, slightly less than the top speed of current battleships.
Eight boats were originally planned, this dropped to six before settling at seven.