The D class and the transition to recognizable First World War submarines

By the 1910s, submarine designs began to resemble the boats that would serve in the First World War. Royal Navy D-class boats from D6 onward carried a forward deck gun, while internal bow and stern torpedo tubes gave them six weapons including reloads. Simply fitting a tube was not enough: early systems that endangered or sank their own boats did not amount to a successful operational capability. The D class could travel roughly 2,500 nautical miles on the surface but only about 45 nautical miles submerged, showing that it was still fundamentally a surface cruiser that dived for attack or survival.