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The renewed German campaign was effective, sinking 1. Despite this, the political situation demanded even greater pressure, and on 31 January , Germany announced that its U-boats would engage in unrestricted submarine warfare beginning 1 February. On 17 March, German submarines sank three American merchant vessels, and the U. Unrestricted submarine warfare in the spring of was initially very successful, sinking a major part of Britain-bound shipping. Nevertheless with the introduction of escorted convoys shipping losses declined and in the end the German strategy failed to destroy sufficient Allied shipping.

An armistice became effective on 11 November and all surviving German submarines were surrendered. Of the submarines that had been built, were lost but more than 11 million tons of shipping had been destroyed. Under the terms of the Armistice, all U-boats were to immediately surrender. Those in home waters sailed to the British submarine base at Harwich. The entire process was done quickly and in the main without difficulty, after which the vessels were studied, scrapped, or given to Allied navies. Stephen King-Hall wrote a detailed eyewitness account of the surrender.

The treaty also restricted the independent tonnage of ships and forbade the construction of submarines. However, a submarine design office was set up in the Netherlands and a torpedo research program was started in Sweden. Before the start of World War II, Germany started building U-boats and training crews, labeling these activities as "research" or concealing them using other covers. Germany had the largest submarine fleet in World War II, since the Treaty of Versailles had limited the surface navy of Germany to six battleships of less than 10, tons each , six cruisers and 12 destroyers.

U-boat pens in Saint-Nazaire, France. In the early stages of the war, the U-boats were extremely effective in destroying Allied shipping, initially in the mid-Atlantic, where there was a large gap in air cover. There was an extensive trade in war supplies and food across the Atlantic, which was critical for Britain's survival.

This continuous action became known as the Battle of the Atlantic , as the British developed technical defences such as ASDIC and radar , and the German U-boats responded by hunting in what were called " wolfpacks " where multiple submarines would stay close together, making it easier for them to sink a specific target. Later, when the United States entered the war, the U-boats ranged from the Atlantic coast of the United States and Canada to the Gulf of Mexico, and from the Arctic to the west and southern African coasts and even as far east as Penang.

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Because speed and range were severely limited underwater while running on battery power, U-boats were required to spend most of their time surfaced running on Diesel engines, diving only when attacked or for rare daytime torpedo strikes. The more ship-like hull design reflects the fact that these were primarily surface vessels which had the ability to submerge when necessary. This contrasts with the cylindrical profile of modern nuclear submarines , which are more hydrodynamic underwater where they spend the majority of their time but less stable on the surface.

Indeed, while U-boats were faster on the surface than submerged, the opposite is generally true of modern subs. The most common U-boat attack during the early years of the war was conducted on the surface and at night, see submarine warfare. U , Birkenhead Docks, Merseyside, England. The U-boats' main weapon was the torpedo , though mines and deck guns while surfaced were also used. By the end of the war, almost 3, Allied ships warships; 2, merchant ships were sunk by U-boat torpedoes. They were fitted with one of two types of pistol trigger: impact , which detonated the warhead upon contact with a solid object, and magnetic , which detonated upon sensing a change in the magnetic field within a few meters.

One of the most effective uses of magnetic pistols would be to set the torpedo's depth to just beneath the keel of the target. The explosion under the target's keel would create a shock wave, and the ship could break in two. In this way, even large or heavily armored ships could be sunk or disabled with a single well-placed hit. In practice, however, the depth-keeping equipment and magnetic and contact exploders were notoriously unreliable in the first eight months of the war.

Torpedoes would often run at an improper depth, detonate prematurely, or fail to explode altogether—sometimes bouncing harmlessly off the hull of the target ship. The faults were largely due to a lack of testing. The magnetic detonator was sensitive to mechanical oscillations during the torpedo run and at high latitudes fluctuations in the Earth's magnetic field.

These were eventually phased out, and the depth-keeping problem was solved by early It was primarily designed to combat convoy escorts. The acoustic torpedo was designed to run straight to an arming distance of meters and then turn toward the loudest noise detected.

U-Boats - The Most Feared Hunters of the Seas

This sometimes ended up being the U-boat itself; at least two submarines may have been sunk by their own homing torpedoes. The Germans, in-turn, countered this by introducing newer and upgraded versions of the acoustic torpedoes, like the late war G7es , and the T11 torpedo.

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However, the T11 torpedoes did not see active service. U-boats also adopted several types of "pattern-running" torpedoes which ran straight out to a preset distance, then traveled in either a circular or ladder-like pattern. When fired at a convoy, this increased the probability of a hit if the weapon missed its primary target. 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. 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.

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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. 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. They also had the facility to release a chemical bubble-making decoy, known as Bold , after the mythical kobold. Advances in convoy tactics, high frequency direction finding referred to as " Huff-Duff " , radar , active sonar called ASDIC in Britain , depth charges , ASW spigot mortars also known as "hedgehog" , the intermittent cracking of the German Naval Enigma code , the introduction of the Leigh Light , the range of escort aircraft especially with the use of escort carriers , the use of mystery ships , and the full entry of the U.

At the same time, the Allies targeted the U-boat shipyards and their bases with strategic bombing. The British had a major advantage in their ability to read some German naval Enigma codes.

Image from page 303 of "The diary of a U-boat commander" (1920)

An understanding of the German coding methods had been brought to Britain via France from Polish code-breakers. Thereafter, code-books and equipment were captured by raids on German weather ships and from captured U-boats.

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A team including Alan Turing used special purpose " Bombes " and early computers to break new German codes as they were introduced. The speedy decoding of messages was vital in directing convoys away from wolf-packs and allowing interception and destruction of U-boats. This was demonstrated when the Naval Enigma machines were altered in February and wolf-pack effectiveness greatly increased until the new code was broken. U was also captured by the British in October , three sailors boarded her as she was sinking, and desperately threw all the code books out of the submarine.

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Two of them, Able Seaman Colin Grazier and Lieutenant Francis Anthony Blair Fasson continued to throw code books out of the ship as it went under water, and went down with it. Gordon Williamson. War Under the Red Ensign. Bernard Edwards. The Red Knight of Germany. Floyd Gibbons. Robert Baden-Powell. Sniping in France Three Years in France with the Guns. Night Action. Peter Dickens. Geoffrey Brooks. Guerrilla Warfare.

The Diary of a U-boat Commander by R.N. Etienne

U-Boat Heinz Schaeffer. Singapore and The Thailand Burma Railway. Alfred E. Commando Men. Bryan Samain.