Torpedo development required long testing cycles

When did Britain begin replacing its 21-inch Mark IV torpedoes?

Replacement work began early and overlapped: later Marks were developed and tested over years, with the Mark VIII program properly taking shape in the mid-1920s.

Successive British Marks ran from designs begun before the First World War through later interwar programs. New torpedoes took years to design, test, and enter service, so replacement work often began soon after an earlier weapon arrived. Extensive testing was an advantage for a global navy able to fire weapons near overseas bases in peacetime. Long development cycles left several generations of torpedoes in the inventory at once.