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During World War II, U-boat warfare was the major component of the Battle of the Atlantic, which lasted the duration of the war. Germany had the largest submarine fleet in World War II, due to the Treaty of Versailles which limited the surface navy of Germany to six battleships (of less than 10,000 tonnes each), six cruisers and 12 destroyers. Although Winston Churchill, the United Kingdom's Prime Minister wrote "The only thing that really frightened me during the war was the U-Boat peril", evidence later accumulated showed that 98% of convoyed British ships in the first 28 months of the war crossed the Atlantic safely, and at no time was the U-boat force close to a successful blockade of the United Kingdom.^[1]

In the early stages of the war, the U-boats were extremely effective in destroying Allied shipping, ranging 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. 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 most common U-boat attack during the early years of the war was conducted on the surface and at night. This period, before the Allied forces developed truly effective antisubmarine warfare (ASW) tactics, was referred to by German submariners as the "happy time."

The U-boat was essentially a sophisticated launch platform for its main weapon, the torpedo. World War II torpedoes suffered from many limitations: they could only be launched from periscope depth or while surfaced; until later on in the war they were completely unguided, travelling in a straight line until they either collided with an object or ran out of energy. Torpedoes were fitted with one of two types of fuses, one which detonated the warhead upon impact with a solid object and another which detonated magnetically, upon sensing a large metal object nearby. Ideally, when using magnetic torpedoes, the commander would know the target ship's draft (how low its keel sits in the water) and set the torpedo's depth so that it passed just underneath, detonating via the magnetic fuse. The explosion would create a temporary gas bubble underneath the suddenly unsupported keel, which would split as a result. In this way, even large or heavily-armored ships could be sunk or disabled with a single well-placed hit. In practice, however, both the depth-keeping equipment and magnetic fuses were notoriously unreliable in the early part of the war. Torpedoes would all too often run at an improper depth, detonate prematurely or even bounce harmlessly off the target's hull. Many commanders lost all faith in their torpedoes and the problems took some time to be fully rectified.

In the later years, Germany developed an acoustic homing torpedo. These were primarily designed to combat pursuing warships, which normally took the strategy of making erratic changes in speed and direction in order to make a hit with straight-running torpedoes nearly impossible. The acoustic torpedo was designed to run straight to an arming distance of 400 meters and then zero in on the loudest noise it heard. This sometimes turned out to be the U-boat itself, and at least two submarines were probably sunk by their own torpedoes. Additionally, it was found that the torpedoes were only effective against ships moving at at least 15 knots. U-boats also adopted "pattern-running" torpedoes which ran 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 in case the weapon missed its primary target.   Counter-measures

Advances in convoy tactics, the full entry of the U.S. into the war with its enormous ship-building capacity, high frequency direction finding (referred to as "Huff-Duff"), radar, sonar (called ASDIC in Britain), depth charges, ASW spigot mortars (aka "hedgehog") the cracking of the German Enigma code, the introduction of the Leigh Light, and the range of escort aircraft (especially with the use of escort carriers), all turned the tide against the U-boats. In the end, the U-boat fleet suffered extremely heavy casualties, losing 743 U-boats and about 30,000 submariners (a 75% casualty rate).

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. With the type XXI "Elektroboot", German sub designers realized that the U-boat depended on its submerged ability both for survival and lethality. The XXI featured a revolutionary streamlined hull design and propulsion system which allowed it to cruise submerged for long periods and reach unprecedented submerged speeds.

Throughout the war an arms race evolved between the Allies and the Kriegsmarine, especially in the arenas of 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 detection antennas which were intended to give them ample time to dive before the enemy closed in. U-boat radar was also developed, but many captains chose not to utilize it 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 went underwater 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 serious problem as the U-boats spent extended periods without surfacing. Speed was limited to a meager 8 knots, lest the device snap from stress. The schnorchel also had the effect of making the boat essentially blind, noisy, and deaf. Finally, allied radar eventually became sufficiently advanced that the profile of the schnorchel itself could be detected from a distance.