Type XXI U-boat

Type XXI U-boats, also known as "Elektroboot" (German: "electric boats"), were the first submarines designed to operate primarily submerged, rather than as surface ships that could submerge as a means to escape detection or launch an attack.

The key improvement in the Type XXI was greatly increased battery capacity, roughly triple the Type VIIC. This gave these boats great underwater range, and dramatically reduced the time spent on or near the surface. They could travel submerged at about 5 kn (5.8 mph; 9.3 km/h) for two or three days before recharging batteries, which took less than five hours using the snorkel. The Type XXI was also far quieter than the VIIC, making it harder to detect when submerged.

The Type XXI's streamlined and hydrodynamically clean hull design — even featuring a small, pioneering form of the later bulbous bow at the ventral end of the stem as designed — allowed high submerged speed. The ability to outrun many surface ships while submerged, combined with improved dive times (also a product of the new hull form), made it far harder to chase and destroy. It also gave the boat a 'sprint ability' when positioning itself for an attack. Older boats had to surface to sprint into position. This often revealed a boat's location, especially after aircraft became available for convoy escort. The new hull design also reduced visibility by marine or airborne radar when surfaced; whether this was a goal of the design or coincidence is still debated.

Type XXI U-boat in The War That Came Early
The Type XXI U-boat was a new Kriegsmarine U-boat type that did not see service during World War II but became available shortly after. The boat's hull and conning tower were streamlined without a deck gun and no angles anywhere. It did have two 30mm gun turrets, one abaft the periscope and the other forward of it for anti-aircraft fire. It could travel at about sixteen knots on the surface, same as previous models, but also maintain that speed while submerged. If the speed was kept below five knots, then hydro-phonic listening devices would not detect it.

Kapitan zur See Rochus Mauer, who had worked on the development team, showed a model to Lt. Commander Julius Lemp. He mentioned that while the streamlined hull had been worked out a couple of years earlier, Dr. Walther had come up with a new hydrogen peroxide-powered engine which would have revolutionized propulsion if it worked properly. However, with Walther's political connections with people in the government, the development team had wasted time on trying to come up with ways to prevent the new engine from catching fire. He lamented they had wasted two years on boats with good hulls and death-trap propulsion systems.