When underwater performance began to define submarines

When did navies begin building true submarines rather than surface boats that could submerge?

The shift began with late-war and postwar underwater-optimized designs, but nuclear propulsion made submerged performance fully decisive.

The panel distinguishes boats that primarily cruised on the surface and submerged for attack from designs centered on underwater performance. The Royal Navy's U class receives an early technical mention, while the German Type XXI is the familiar transitional example despite its defects. Postwar A-class and comparable American boats pushed the design further. Nuclear propulsion finally made sustained submerged speed and endurance the normal expectation of a true submarine.