Editing U-boat (section)

==World War II (1939–1945)==

=== Operations ===
{{Main|Battle of the Atlantic}}

During World War II, U-boat warfare was the major component of the Battle of the Atlantic, which began in 1939 and ended with Germany's surrender in 1945. British Prime Minister [[Winston Churchill]] later wrote "The only thing that really frightened me during the war was the U-boat peril."{{Sfn|Churchill||p=529}}

[[File:Base ssmarin stnazaire.jpg|thumb|left|[[Submarine pen|U-boat pens]] in [[Saint-Nazaire]], France]]
As convoying had been key in the defeat of German submarines during World War I, the British began organizing convoys at once in September 1939. The most common U-boat attack against convoys during the early years of the war was conducted on the surface and at night. During 1939 the Germans made a few attempts to attack convoys with their new 'wolfpack' tactic, but these were not successful. The [[Operation Weserübung|invasion of Norway]] in April 1940 halted temporarily all U-boat operations against merchant shipping. During the invasion many technical problems with the German torpedoes were exposed and only in August 1940 could the campaign against convoys be revived. There were now fewer U-boats operational than at the beginning of the war, but thanks to the new bases in France and Norway U-boats could reach their operation grounds far more easily. During the following months the U-boats put their 'wolfpack' tactic against convoy in practice with spectacular results. This period, before the Allied forces developed truly effective antisubmarine warfare <!-- (ASW) --> tactics, was referred to by German submariners as "{{lang|de|die glückliche Zeit}}" or the [[First Happy Time]].<ref name="milHisOnline">{{Cite web |title=Military History Online |url=https://www.militaryhistoryonline.com/wwii/atlantic/uboatwar.aspx |access-date=2019-02-04 |website=www.militaryhistoryonline.com}}</ref>

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 [[Ultra (cryptography)|break]] into German secret naval [[Enigma machine|Enigma]] communications and could henceforth reroute convoys around U-boat concentrations.{{Sfn|Costello & Hughes||pp=154–155}} 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.{{Sfn|Costello & Hughes||p=165}} 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,{{Sfn|Blair||pp=357–358}}{{Sfn|Mason||p=68}} and next, he ordered a substantial force of U-boats to operate in the Mediterranean in order to support the Italians and [[Erwin Rommel|Rommel's]] [[Afrika Korps]].{{Sfn|Mason||p=54}}[[File:Captured German U-boats outside their pen at Trondheim in Norway, 19 May 1945. BU6382.jpg|thumb|upright=.9|Captured [[German Type VII submarine|Type VII]] and [[German Type IX submarine|Type IX]] U-boats outside their pen in [[Trondheim]], Norway, 19 May 1945]]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. There followed a [[Second Happy Time]] when U-boats could extend their successful operation to the [[Gulf of Mexico]] and the [[Caribbean Sea]].{{Sfn|Mason||pp=72–73}} 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.{{Sfn|Blair||p=654}} The renewed offensive against convoys reached its climax in March 1943, when two thirds of all ships sunk, were ships sailing in convoys.{{Sfn|Mason||p=108}} But the Allies put effective countermeasures into effect, and only two months later on 24 May, Dönitz had to stop the campaign due to heavy losses.{{Sfn|Rohwer||p=252}}

By the end of the war, almost 3,000 Allied ships (175 warships; 2,825 merchant ships) had been sunk by U-boats.<ref name="Tread">{{cite book |last=Crocker III |first=H. W. |url=https://archive.org/details/donttreadonme40000croc/page/310 |title=Don't Tread on Me |publisher=Crown Forum |year=2006 |isbn=978-1-4000-5363-6 |location=New York |page=[https://archive.org/details/donttreadonme40000croc/page/310 310] |url-access=registration}}</ref> In total, 1181 U-boats entered service before the German surrender, of which 863 had executed war patrols, and 785 were lost.{{Sfn|Haskell||p=14}}{{Sfn|Middlebrook||p=327}} 222 U-boats were scuttled by their crews and 174 surrendered to Allied forces.{{Sfn|Blair Vol2||p=700}} 121 U-boats were scuttled in 1945-46 during [[Operation Deadlight]].{{Sfn|Rohwer||pp=432-433}}

===Torpedo developments===
The U-boats' main weapon was the [[torpedo]], though [[sea mine|mines]] and [[deck gun]]s (while surfaced) were also used.  Early German World War II torpedoes were fitted with one of two types of pistol triggers&nbsp;– impact, which detonated the warhead upon contact with a solid object, and [[Magnetic pistol|magnetic]], which detonated upon sensing a change in the magnetic field within a few meters. Initially, the depth-keeping equipment and magnetic and contact exploders were notoriously unreliable. During the first eight months of the war, torpedoes often ran at an improper depth, detonated prematurely, or failed to explode altogether{{snd}}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. The faults were largely due to a lack of testing. The magnetic detonator was sensitive to mechanical oscillations during the torpedo run, and to fluctuations in the Earth's magnetic field at high latitudes. These early magnetic detonators were eventually phased out. The depth-keeping problem remained problematic, not until January 1942 was the last fault discovered by accident: when ventilating the onboard torpedoes during maintenance, it was possible that the excess internal air-pressure in the U-boat offset the depth setting mechanism in the balance chamber of the torpedo.{{Sfn|Dönitz||p=482}}{{Sfn|Blair||p=485}}[[File:Flachenabsuchender Torpedo type I-firing positions.svg|thumb|The pattern running of a FAT torpedo]]
In order to give U-boats better opportunities against well-defended convoys, several types of "pattern-running" torpedoes were developed. The FAT ('''Flächen-Absuch-Torpedo''' or '''Federapparat-Torpedo''') and LUT ('''LageUnabhängiger''' Torpedo) was an electric torpedo which ran straight out to a preset distance, then traveled in either a circular or ladder-like pattern through the convoy lanes. This increased the probability of a hit. The torpedo had one setting to regulate the length of the prerun, after which one of four other possible settings kicked in and made the torpedo zigzag towards either left or right and either on short (1200&nbsp;m; {{frac|2|3}}&nbsp;nmi) or long (1900&nbsp;m; 1 [[nautical mile]]) legs. When fired, the firing U-boat sent out a warning to the other U-boats in the vicinity so these could dive to avoid being hit by the random running torpedo. The FAT torpedo became available end of 1942 and was in regular use during the convoy battles of March 1943.{{Sfn|Middlebrook||pp=170–171}}{{Sfn|Brennecke|1984|pp=389–391}}

Germany also developed acoustic homing torpedoes. In February 1943, the first acoustic torpedo, the T4 "Falke", was tested on a small scale with moderate success, but this torpedo could only be used against large, slow ships. The acoustic torpedo ran straight to an arming distance of 1000 m and then turned toward the loudest noise detected. Its successor, the [[G7es torpedo|T5 "Zaunkönig]]", was designed to combat small and fast warships, and entered service in September 1943.{{Sfn|Brennecke|1984|pp=391–394}}

===U-boat developments===
[[File:Type XXI section.jpg|thumb|A prefabricated segment of a Type XXI U-boat. The cross-section shows clearly the '8'-shaped hull, where the lower part was used to store large batteries hence the name of 'ElektroBoot']]
In 1940 the Germans made successful tests with the [[German submarine V-80|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 propulsion|air-independent]] [[hydrogen peroxide#Propellant|propellant]] system designed by [[Hellmuth Walter]]. With this Walter-turbine, a U-boat could achieve underwater speeds of more than 20 knots, much more than the 4-knot cruising and 6-knot maximum speed of electrical engines powered by batteries. Four more experimental [[German Type XVIIB submarine|Type XVIIA]] U-boats with Walter turbines were built and tested, but the Germans could not put this design in use for a big frontline U-boat.{{Sfn|Blair Vol2||pp=10-11}}{{Sfn|Costello & Hughes||pp=284–285}} Unlike a classic U-boat that could recharge its batteries with the diesel engines, once a Walter U-boat had consumed its hydrogen peroxide propellant it could not submerge anymore. The Germans did not possess the resources and plants to produce sufficient hydrogen peroxide to operate a fleet of Walter submarines. Despite these limitations, 24 frontline Type XVIIB coastal submarines were ordered, but only three were built and none were operational before the end of the war.{{Sfn|Blair Vol2||p=312}}

The Walter U-boats had very large hulls in order to store the fuel for submerged propulsion. Once it became clear these Walter U-boats would not be operational in time, the Walter U-boat hull design was reused with a different approach: the space for the hydrogen peroxide tanks was used to store much larger batteries. With the much increased battery power U-boats were also able to reach much higher speeds and endurance when submerged.{{Sfn|Breyer||pp=7–13}} Based on the design of an Atlantic Walter U-boat, the [[German Type XXI submarine|Type XXI]] "{{lang|de|Elektroboot}}" was designed to boost submerged performance. Smaller [[German Type XXIII submarine|Type XXIII]] coastal {{lang|de|Elektroboote}} were also taken into production. These {{lang|de|Elektroboote}} were mass-produced, with prefabricated segments constructed at different sites and then assembled at the bigger shipyards.{{Sfn|Blair Vol2||pp=312–313}}{{Sfn|Costello & Hughes||pp=285-287}}
[[File:Submarine snorkel.png|thumb|The {{lang|de|Schnorkel}} mast and air flows]]
After the [[German invasion of the Netherlands]] in 1940, the Germans captured some Dutch submarines equipped with a [[Submarine snorkel|''Schnorchel'']] (snorkel) but saw no need for them until 1943. The {{lang|de|Schnorchel}} was a retractable pipe that supplied air to the diesel engines while submerged at [[periscope depth]], allowing the boats to cruise submerged on diesel engines and recharge their batteries.{{Sfn|Breyer||p=8}} 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. Speed was limited to {{convert|8|kn|km/h}}, lest the device snap from stress. Whilst running submerged with the {{lang|de|Schnorchel}}, the [[Gruppenhorchgerät]] was useless because of interference with the noisy diesel engines. But the {{lang|de|Schnorchel}} allowed the old Type VII and IX U-boats to operate in waters which were previously denied to them.{{Sfn|Costello & Hughes||p=284}}

===Classes===
* [[German Type I submarine|Type I]]: first design for a large 750-ton U-boat. Only 2 built as the design was not very successful.
* [[Type II submarine|Type II]]: small coastal submarines used mainly for training purposes. The latest subtype IID had saddle tanks which gave it a range to operate in the Atlantic, which it did until 1941
* [[German Type VII submarine|Type VII]]: the "workhorse" of the U-boats with 709 completed in World War II
* [[German Type IX submarine|Type IX]]: these long-range U-boats operated as far as the Indian Ocean with the Japanese ([[Monsun Gruppe]]), and the [[South Atlantic]]
* [[German Type X submarine|Type X]]: long-range minelayers but mainly used to resupply other U-boats
* [[German Type XIV submarine|Type XIV]]: unarmed U-boat, used to resupply other U-boats; nicknamed the {{lang|de|Milchkuh}} ("Milk Cow")
* Type XVII: small experimental coastal submarines powered by experimental hydrogen peroxide propulsion systems, not put into service
* [[German Type XXI submarine|Type XXI]]: known as the {{lang|de|Elektroboot}}. The design was taken into mass production, but only two set out for a war patrol before the end of the war
* [[German Type XXIII submarine|Type XXIII]]: smaller version of the XXI used for coastal operations. operated on a small scale during 1945
* [[Midget submarine]]s, including [[Biber (submarine)|''Biber'']], [[Hai (midget submarine)|''Hai'']], ''[[Molch]]'', and ''[[Seehund]]''
* [[Uncompleted U-boat projects]]