U-boat

This contrasts with the cylindrical profile of modern nuclear submarines, which are more hydrodynamic under water (where they spend the majority of their time), but less stable on the surface.

The Germans hoped to break the deadlock by starting an unrestricted submarine campaign against shipping in the waters around the British Isles.

Despite this, the deadlock situation on the continent frontlines demanded even greater results, and on 1 February 1917, Germany restarted the unrestricted submarine campaign around British Isles.

Those in home waters sailed to the British submarine base at Harwich, after which the vessels were studied, then scrapped or given to Allied navies.

[citation needed] Twelve U-boat crewmen were decorated with the Goldenes Militär-Verdienst-Kreuz, the highest bravery award for noncommissioned officers and enlisted men.

[22] In 1935, Britain sought to control the increasingly apparent breaches of the Versailles Treaty and it concluded in 1935 the Anglo-German Naval Agreement.

This ended officially the limitation of the Versailles Treaty and allowed Germany to build ships in a 100:35 tonnage ratio to the British fleet.

Based on these views he requested that the remaining 11,500 tons be used for building twenty-three medium submarines, which were in his opinion the ideal type for the commerce war against British convoys.

Raeder however did not share these beliefs and opinions and opted for a more balanced expansion of the submarine fleet:[25] Twenty-one of these twenty-three U-boats were commissioned before the start of World War II.

Again, Raeder decided that the extra 7,785 tons would be divided between medium and large U-boats:[27] During 1938, Hitler changed his attitude towards Britain.

This period, before the Allied forces developed truly effective antisubmarine warfare tactics, was referred to by German submariners as "die glückliche Zeit" or the First Happy Time.

[39] In the beginning of 1941 British countermeasures began to take effect: in March 1941 the three leading U-boat aces were sunk during convoy battles.

In May 1941 the British were able to break into German secret naval Enigma communications and could henceforth reroute convoys around U-boat concentrations.

[40] When American warships started to escort Atlantic convoys, the U-boats were restricted in their operations as Hitler wanted to avoid possible conflict with the US.

[41] The campaign against merchant shipping received further impediments when Hitler interfered on two occasions: first he insisted that a small force of U-boats be kept on station in the Arctic as a precaution against a possible Allied invasion in Norway[42][43] and next he ordered a substantial force of U-boats to operate in the Mediterranean in order to support the Italians and Rommel's Afrika Korps.

[44] When the US entered war, the focus of U-boat operations shifted to the Atlantic coast of the United States and Canada, where no convoys were organized and anti-submarine measures were inadequate.

[45] By mid 1942 an adequate defense was organized in these regions and then U-boats returned to their original and crucial hunting grounds on the North Atlantic convoy lanes.

[50][51] Of the 154 U-boats surrendered, 121 were scuttled in deep water off Lisahally, Northern Ireland, or Loch Ryan, Scotland, in late 1945 and early 1946 during Operation Deadlight.

During the first eight months of the war, torpedoes often ran at an improper depth, detonated prematurely, or failed to explode altogether – sometimes bouncing harmlessly off the hull of the target ship.

This was most evident in Operation Weserübung, the invasion of Norway, where various skilled U-boat commanders failed to inflict damage on British transports and warships because of faulty torpedoes.

In 1940 the Germans made successful tests with the V-80 experimental submarine featuring a new type of propulsion: on the surface it used the classic Diesel engines but submerged it used a revolutionary hydrogen peroxide air-independent propellant system designed by Hellmuth Walter.

The Germans did not possess the resources and plants to produce sufficient hydrogen peroxide to operate a fleet of Walter submarines.

[63] It was far from a perfect solution: problems occurred 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.

Advances in convoy tactics, high-frequency direction finding, referred to as "Huff-Duff", radar, sonar, depth charges, anti-submarine weapons such as "Hedgehog" and "FIDO", the intermittent cracking of the German Naval Enigma code, the introduction of the Leigh Light, long range patrol aircraft, escort carriers and the enormous US shipbuilding capacity, all turned the tide against the U-boats.

A team including Alan Turing used special-purpose "Bombes" and early computers to break new German codes as they were introduced.

Because West Germany was initially restricted to a 450-tonne displacement limit, the Bundesmarine focused on small coastal submarines to protect against the Soviet threat in the Baltic Sea.

The Germans sought to use advanced technologies to offset the small displacement, such as amagnetic steel to protect against naval mines and magnetic anomaly detectors.

Small and agile submarines were built during the Cold War to operate in the shallow Baltic Sea, resulting in the Type 206.

Germany has brought the U-boat name into the 21st century with the new Type 212; it features an air-independent propulsion system using hydrogen fuel cells.

This system is safer than previous closed-cycle diesel engines and steam turbines, cheaper than a nuclear reactor, and quieter than either.