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U-boats are naval submarines operated by Germany, including during the First and Second World Wars. The term is an anglicized form of the German word U-Boot , a shortening of Unterseeboot (). Austro-Hungarian Navy submarines were also known as U-boats.
U-boats are most known for their unrestricted submarine warfare in both , trying to Commerce raiding towards the UK and force the UK out of the war. In World War I, Germany intermittently waged unrestricted submarine warfare against the United Kingdom: a first campaign in 1915 was abandoned after strong protests from the US but in 1917 the Germans, facing deadlock on the continent, saw no other option than to resume the campaign in February 1917. The renewed campaign failed to achieve its goal mainly because of the introduction of convoys. Instead the campaign ensured final defeat as the campaign was a contributing factor to the entry of the United States in the First World War.
In World War II, Karl Dönitz, supreme commander of the Kriegsmarine's U-boat arm (Befehlshaber der Unterseeboote), was convinced the UK and its convoys could be defeated by new tactics, and tried to focus on convoy battles. Though U-boat tactics initially saw success in the Battle of the Atlantic, greatly disrupting Allied shipping, improved convoy and anti-submarine tactics such as high-frequency direction finding and the Hedgehog anti-submarine system began to take a toll on the German U-boat force. This ultimately came to a head in May 1943, known as Black May, in which U-boat losses began to outpace their effect on shipping.
Some 50 years later in 1903, the Friedrich Krupp Germaniawerft dockyard in Kiel completed the first fully functional German-built submarine, Forelle, which Krupp sold to Russia during the Russo-Japanese War in April 1903.
At this time, the German commander of the Navy Alfred von Tirpitz was building the High Seas Fleet with which he intended to challenge the supremacy of the UK Royal Navy. He focused on expensive and there was no role for submarines in his fleet.
Only when Krupp received an order for three U-boats from Russia, did Tirpitz order one submarine. The was a completely redesigned Karp-class and when the Imperial German Navy commissioned it on 14 December 1906, it was the last major navy to adopt submarines.
The U-1 had a double hull and a single torpedo tube. It used an electric motor powered by batteries for submerged propulsion and a Körting kerosene engine for charging the batteries and propulsion on the surface. The 50%-larger was commissioned in 1908, had four 45 cm torpedo tubes and a much larger battery capacity. But the boat was a failure due to problems with both the kerosene and electrical engines. The next two U-boats of the Type U 3-class, ordered on 13 August 1907, were more reliable.
March 1907, the Germaniawerft received an order from the Austro-Hungarian Navy for two U-boats and in October Norway ordered a similar U-boat.
These foreign U-boats were based on an improved U-1 design. Between 1908 and 1910, the German navy ordered fourteen boats with four 45 cm torpedo tubes and two reload torpedoes. These boats used a kerosene engine which was safer than gasoline and more powerful than steam, but the white exhaust of the kerosene betrayed the presence of the U-boats, robbing them of their primary asset, their stealth. Diesel engines did not have that disadvantage, but a powerful and reliable diesel engine was still under development. As some equipment could not be delivered within the specified weight limits, there was some variation in the total weight of each U-boat. Usually this was solved by reducing the number of battery cells, which affected underwater performance. The last two of these U-boats, the Type U 17, was designed to receive diesel engines but due to delays in developing a lightweight diesel engine, these U-boats were equipped with kerosine engines.
Between 1910 and 1912, twenty-three diesel U-boats were ordered when diesel engines finally became available: four Type U 19 U-boats on 20 November 1910 from KWD with MAN SE engines and four similar Type U 23 U-boats from Germaniawerft with Germaniawerft engines on 18 March 1911. These boats were larger to accommodate the diesel engines, and were equipped with torpedo tubes. On 12 Februari 1912 a further four similar Type U 27 were ordered from KWD, and although Germaniawerft experienced problems with its diesel engines, it received an order for eleven Type U 31 U-boats. Due to these problems, delivery of these U-boats was delayed up to eight months and ran into 1915.
At the start of World War I in 1914, Germany had 48 submarines of 13 classes in service or under construction.
| U 1 | 1 | 1906 | 2 x Korting | 2 x | |||
| U 2 | 1 | 1908 | 2 x Daimler | 2 x | |||
| Type U 3 | 2 | 1909 | 2 x Korting | 2 x | |||
| Type U 5 | 4 | 1910-11 | 4 x Korting | 2 x | |||
| Type U 9 | 4 | 1910-11 | 2 x Korting + 2 x Korting | 2 x | |||
| Type U 13 | 3 | 1912 | 2 x Korting + 2 x Korting | 2 x | |||
| U 16 | 1 | 1911 | 2 x Korting + 2 x Korting | 2 x | |||
| Type U 17 | 2 | 1912 | 4 x Korting | 2 x | |||
| Type U 19 | 4 | 1913 | 2 x MAN | 2 x | |||
| Type U 23 | 4 | 1913-14 | 2 x GW | 2 x | |||
| Type U 27 | 4 | 1914 | 2 x MAN | 2 x | |||
| Type U 31 | 11 | 1914-15 | 2 x GW | 2 x | |||
| Type U 43 | 8 | 1915-16 | 2 x MAN | 2 x |
| Forelle | 1 | 1904 | 2 x 45 cm | -- | -- | |||
| Karp-class | 3 | 1907 | 1 x 45 cm | -- | -- | |||
| U-3-class | 2 | 1909 | 2 x 45 cm | -- | -- | |||
| Kobben | 1 | 1910-11 | 2 x 45 cm | 1 x 45 cm | -- | |||
| Atropo | 1 | 1912 | 2 x 45 cm | -- | -- |
| + German U-boat losses by cause | |
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On the continent German hopes for a quick victory were dashed and a stalemate had settled on the front. The Germans hoped to break the deadlock by starting an unrestricted submarine campaign against shipping in the waters around the British Isles. This was also cited as a retaliation for British minefields and shipping . Under the instructions given to U-boat captains, they could sink merchant ships, even neutral ones, without warning. Only 29 U-boats were available for the campaign, and not more than seven were active around the British Isles at any time. The U-boats failed to enforce a blockade but the sinking of three liners including the RMS Lusitania, with loss of American lives, outraged the US so that the Kaiser had to stop the campaign on 19 September 1915. After the stop of the campaign, most of the U-boats were sent to the Mediterranean.
At the beginning of 1916, 54 U-boats were available, and the Kaiser allowed again operations around the British Isles, but with strict rules: no attacks on liners, and outside the war zone around the British Isles attacks were only allowed on armed merchant ships. But on 24 March, after 25 Americans were killed in the torpedoing of the ferry the US threatened to sever diplomatic ties, which persuaded the Germans to fully reapply prize rules.
In September 1916, 120 U-boats were in service, and again some were sent to the Mediterranean. Whilst around British Isles prize rules were observed, in the Mediterranean a new unrestricted campaign was started. The renewed German campaign was effective, sinking of shipping between October 1916 and January 1917. 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. Germany took the gamble that the U-boat campaign would force the UK out of the war before the US could effectively enter. On 3 February, the US severed diplomatic relations with Germany, and on 6 April, the US declared war on Germany. Unrestricted submarine warfare in 1917 was very successful, sinking more than a month. With the introduction of convoys in August 1917, shipping losses declined to a month on average, which was not sufficient to force the UK out of the war. With deteriorating conditions on the continent, all U-boats were recalled on 31 October 1918. Under the terms of armistice of 11 November 1918, U-boats were to immediately surrender. All U-boats were either scrapped or given to Allied navies.
Of the 373 German U-boats that had been built, 179 were operational or nearly operational at the end of the war. 178 were lost by enemy action. 512 officers and 4894 enlisted men were killed. Of the surviving German submarines, 14 U-boats were Scuttling and 122 surrendered. They sank 10 pre-dreadnought battleships, 18 cruisers, and several smaller naval vessels. They further destroyed 5,708 merchant and fishing vessels for a total of and the loss of about 15,000 sailors. The Pour le Mérite, the highest decoration for gallantry for officers, was awarded to 29 U-boat commanders. The three most successful U-boat commanders were Lothar von Arnauld de la Perière (195 ships with ), followed by Walter Forstmann (149 ships with ), and Max Valentiner (144 ships with ).
In the spring of 1915 it became clear that the war would continue well into 1916 and more coastal U-boats were ordered. The Type UB I and UC I had fullfilled the expectations but were underpowered. Thirty U-boats of the successing Type UB II were ordered, which were with around double in size. As with the cessation of unrestricted U-boat warfare in September 1915, mine warfare became more prominent, a record number of 64 Type UC II minelaying U-boats were ordered, which allowed for the first time for the mass-production of a U-boat. The Type UC II proved to be an ideal combination of mine and torpedo armament: on the same patrol it could lay minefields and attack shipping on the way to its target.
On the resumption of the U-boat campaign in 1916, it was realized that not enough large U-boats could be built in time, and as a compromise it was decided to build an enlargened coastal U-boat capable of patrolling in the Western Approaches. The hull of the Type UC II was taken as a basis, the forward mineshaft room was replaced with a torpedo room, more powerfull diesel engines provided better surface speed and larger fuel tanks extended the range. On 2 May 1916 the first twenty-four Type UB III were ordered and as the war prolonged, more than 200 were ordered but only 96 were commissioned before the armistice.
| standard U-boat types | Type U 51, Type U 57, Type U 63, Type U 66, Type U 81, Type U 87, Type U 93 |
| U-cruisers and merchant U-boat types | Type U 139, Type U 142, Type U 151, Type UD 1 |
| UB coastal U-boat types | Type UB I, Type UB II, Type UB III, Type UF, Type UG |
| UC coastal minelaying U-boat types | Type UC I, Type UC II, Type UC III |
| UE ocean minelaying U-bot types | Type UE I, Type UE II |
The IVS constructed three 500-ton medium submarines in Finland between 1927 and 1931, known as the Vetehinen-class. These ships were the prototypes for the subsequent German Type VII U-boat. In 1933 a small 250-ton submarine, the Vesikko was built. This submarine was nearly identical to the subsequent German Type II U-boat. A fifth very small 100-ton submarine, the Saukko was built in 1933 as well. In Spain a large 750-ton boat was built between 1929 and 1930. After the Spanish lost interest in the U-boat, they sold it to Turkey where it entered service as Gür. German sailors assisted in the trials for these submarines. These secret programs were exposed in the Lohmann Affair and as a result the Head of the Reichsmarine, Hans Zenker, had to resign. His successor Erich Raeder continued the policy of secretly breaching the Versailles treaty. On 15 November 1932, a plan was approved for an expansion of the German navy which included U-boats.
, a type IX U-boat at Birkenhead Docks, Merseyside, England]]In 1935, Britain sought to limit Germany's increasingly apparent breaches of the Versailles treaty, negotiating the Anglo-German Naval Agreement. Under the agreement, Germany was relieved of some Treaty of Versailles restrictions and permitted to build ships in a 100:35 tonnage ratio to the British fleet. For submarines the Germans obtained parity in tonnage, but promised a 45 percent limit unless special circumstances arose. This allowed 24,000 tons for U-boat building. Only one week after the agreement was signed, the first of six Type II U-boats, was commissioned in the German Navy, which changed name from Reichsmarine (Imperial Navy) to Kriegsmarine (War Navy). Within the year, the Germans commissioned a total of 36 U-boats for a total of 12,500 tons:
Karl Dönitz was appointed as head of the submarine section of the Kriegsmarine. He believed firmly that in spite of the Anglo-German Naval agreement and Hitler's policy of avoiding conflict with Britain, the next war would be with Britain. He requested the remaining 11,500 tons be used to build twenty-three medium submarines, which he considered ideal for a commerce war against British convoys. Raeder disagreed, and opted for a more balanced expansion of the submarine fleet:
Twenty-one of these twenty-three U-boats were commissioned before World War II. In 1937, Britain announced it would expand its submarine fleet from 52,700 to 70,000 tons. Again, Raeder decided that the extra 7,785 tons would be divided between medium and large U-boats:
When World War II started, Germany had 56 U-boats commissioned, of which 46 were operational and 22 having sufficient range for Atlantic operations; the other 24 were limited to North Sea operations.
Another vast improvement was the introduction of new torpedo types for the U-boats: the classic G7a torpedo propelled by compressed air had a much larger warhead than its WWI equivalent, but more important was the introduction of the electric G7e torpedo. Though slow with range limitations, it left no telltale bubble wake, making it ideal for daylight attacks. During WWI, the Germans had briefly experimented with and these were further developed now as the standard pistol for torpedoes. The classic contact pistol required a torpedo to detonate against the ship's hull, whilst a magnetic torpedo could detonate below a ship, resulting in a much more damaging explosion. Thus, it was hoped that one torpedo would suffice to break the back of a ship, and a U-boat could sink many more ships with its supply of torpedoes.
All U-boats were now also equipped with long- and short-wave transmitters, enabling communications with bases ashore and with other U-boats. This allowed for better operational information and guidance.
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 invasion of Norway in April 1940 temporarily halted 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 tactics, was referred to by German submariners as the happy time (die glückliche Zeit).
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 machine communications and could henceforth reroute convoys around U-boat concentrations. When American warships started to escort Atlantic convoys, the U-boats were restricted in their operations as Hitler wanted to avoid conflict with the US. 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, and next, he ordered a substantial force of U-boats to operate in the Mediterranean in order to support the Italians and Erwin Rommel Afrika Korps. and Type IX U-boats outside their pen in Trondheim, Norway, 19 May 1945]]When the US entered the 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. By mid-1942 an adequate defense was organized in these regions; U-boats returned to their original and crucial hunting grounds on the North Atlantic convoy lanes. The renewed offensive against convoys reached its climax in March 1943, when two-thirds of all ships sunk had been sailing in convoys. 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.
By the end of the war, almost 3,000 Allied ships (175 warships; 2,825 merchant ships) had been sunk by U-boats. In total, 1181 U-boats entered service before the German surrender, of which 863 executed war patrols, and 785 were lost. 222 U-boats were scuttled by their crews and 174 surrendered to Allied forces. 121 U-boats were scuttled in 1945-46 during Operation Deadlight.
The Walter U-boats had very large hulls to store 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 massively increased battery power, U-boats enjoyed much greater speeds and endurance when submerged. Based on the design of an Atlantic Walter U-boat, the Type XXI "Elektroboot" was designed to boost submerged performance. Smaller Type XXIII coastal Elektroboote were also taken into production. These Elektroboote were mass-produced, with prefabricated segments constructed at different sites and then assembled at the bigger shipyards. After the German invasion of the Netherlands in 1940, the Germans captured some Dutch submarines equipped with a Schnorchel (snorkel) but saw no need for them until 1943. The 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. 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, often causing painful ear injuries. Speed was limited to , lest the device snap from stress. Whilst running submerged with the Schnorchel, the Gruppenhorchgerät was rendered useless by diesel engine noise. The Schnorchel nonetheless allowed old Type VII and IX U-boats to operate in waters that previously denied to them.
The first three Type 201 U-boats were commissioned in 1962. To continue the U-boat tradition, the new boats received the classic "U" designation starting with U-1. In 1962, a redefinition of the tonnage calculation to include solid ballast meant the Type 201 would displace 395 tons. Consequently, tonnage limits established in the London and Paris Conferences were increased to 450 tons for current submarines and future U-boats limited to 1,000 tons. During construction of the first U-boats, it became apparent that an extra sonar needed to be installed. Since the Type 201 was constructed in sections, it was easy to enlarge the hull with an extra 1.8-meter section to accommodate the sonar. This extra section increased displacement to 420 tons, remaining well-within conference limits. Deemed the Type 205, five of these enlarged vessels would be completed by the end of 1963.
Having secured the permission to build larger submarines, the Type 209 diesel-electric submarine was designed as a much larger Type 205, with the same characteristics and armament, but with much larger battery capacity, the possibility for torpedo reloads and extra sensors. It was proposed as a very customizable export-sales submarine, available in five variants with a displacement between 1,000–1,500 tons. The first customer was the Hellenic Navy, which received four Type 209's in 1971-72. , fifty-one boats had been built for thirteen navies, with another twelve still ordered. In 1974, three 540 tons U-boats were built for the Israeli Navy. Based both on the type 206 and as a smaller Type 209, these three U-boats were classified as the Type 540. For political reasons these U-boats were not built in Germany but by Vickers Limited in England. In 1982, the Norwegian Navy ordered six 1,000 tons U-boats, to replace half of the Kobben-class submarines, these were classified as the Type 210. After receiving two Type 209 U-boats in 1974, the Argentine Navy ordered six larger 2,000 tons TR-1700 U-boats in 1977. Two of these were built in Germany and delivered after the Falklands War in 1984-85, the remaining four were to be built in Argentina, but these were never completed. In the early 1990s, Israel ordered three 1,565 tons submarines which were enlarged versions of the Type 209 but resembled more the later Type 212. The first three submarines were delivered in 1999-2000 as the Dolphin-class or Type 800.
The Type 216 was a design for a 4,000 tons U-boat with a much larger endurance. It was proposed to but was not bought by Australia. , no Type 216s have been ordered. Between 2022 and 2024, Germany built four 2,000 tons Type 218 U-boats for the Republic of Singapore Navy, which are based on the Type 212 and Type 214, with the same propulsion method and 'X' rudder, but armed with eight torpedo tubes.
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