U-class submarines: simple machinery and revised bow arrangements

The original U class was intended to replace H-class training boats while also providing short-distance wartime patrols. Its simple diesel-electric machinery helped rapid production, and limited operational capability still proved well suited to Mediterranean work from Malta. External bow torpedo tubes gave the early boats a bluff forward shape that complicated periscope-depth keeping, produced a prominent bow wave, and reduced speed in a seaway. Later boats received a finer bow, while additional operational equipment made it necessary to forgo those external tubes. A quick-diving tank improved diving and control during torpedo firing by allowing the boat to be made deliberately heavy. Later construction used more welding, but the pressure-hull plating seams remained riveted throughout the class.