Type XXI “Elektroboot”
During World War II, the Kriegsmarine produced many different types of U-boats as technology evolved. Most notable are Type VII, known as the “workhorse” of the fleet, which was by far the most-produced type; Type IX boats were larger and specifically designed for long-range patrols, some traveling as far as Japan and the east coast of the United States. With the Type XXI “Elektroboot”, German designers realized the U-boat depended on submerged ability both for combat effectiveness and survival; this was the first submarine whose design favored submerged performance. The Type XXI featured a revolutionary streamlined hull design, which was the basis for the later USS Nautilus nuclear submarine. Its propulsion system featured a large battery capacity, which allowed it to cruise submerged for long periods and reach unprecedented submerged speeds. A larger battery was possible because the space it occupied was originally intended to store hydrogen peroxide for a Walter turbine, which was unsuccessful on the Type XVII.
Throughout the war an arms race evolved between the Allies and the Kriegsmarine, especially in detection and counter-detection. Sonar (ASDIC in Britain) allowed allied warships to detect submerged U-boats (and vice versa) beyond visual range but was not effective against a surfaced vessel; thus, early in the war, a U-boat at night or in bad weather was actually safer on the surface. Advancements in radar became particularly deadly for the U-boat crews, especially once aircraft-mounted units were developed. As a countermeasure, U-boats were fitted with radar warning receivers, to give them ample time to dive before the enemy closed in. However, at some point the Allies switched to centimetric radar (unbeknownst to Germany) which rendered the radar detectors ineffective. U-boat radar systems were also developed, but many captains chose not to utilize them for fear of broadcasting their position to enemy patrols.
The Germans took the idea of the Schnorchel (snorkel) from captured Dutch submarines, though they did not begin to implement it on their own boats until rather late in the war. The Schnorchel was a retractable pipe which supplied air to the diesel engines while submerged at periscope depth, allowing the boats to cruise and recharge their batteries while maintaining a degree of stealth. It was far from a perfect solution, however. There were problems with the device’s valve sticking shut or closing as it dunked in rough weather; since the system used the entire pressure hull as a buffer, the diesels would instantaneously suck huge volumes of air from the boat’s compartments, and the crew often suffered painful ear injuries. Waste disposal was a problem when the U-boats spent extended periods without surfacing. Speed was limited to 8 knots (15 km/h), lest the device snap from stress. The schnorchel also had the effect of making the boat essentially noisy and deaf in sonar terms. Finally, Allied radar eventually became sufficiently advanced that the schnorchel mast itself could be detected beyond visual range.
The later U-boats were covered in a sound-absorbent rubber coating to make them less of an ASDIC target. They also had the facility to release a chemical bubble-making decoy, known as Bold, after the mythical kobold.