The Type XXI U-Boot, Paperback

The Type XXI U-boot was one of the few types of weapon that, despite being unable to take part in combat, completely changed the character of naval actions at sea. It was the first true submarine in history. The other types of vessels designed so far were basically "diving ships" that might have been underwater for some time, but most of the time they had to be on the surface because their underwater range was severely limited. Under water they were slow and not maneuverable, and often had to emerge onto the surface to charge their electric batteries. The Type XXI was designed from the outset as a true submarine whose natural environment was to be the deep seas.