U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. The term is an anglicised version of the German word U-Boot [ˈuːboːt] ⓘ, a shortening of Unterseeboot (under-sea boat), though the German term refers to any submarine. Austro-Hungarian Navy submarines were also known as U-boats.

U-boats are most known for their unrestricted submarine warfare in both world wars, trying to disrupt merchant traffic towards the UK and force the UK out of the war. In World War I, Germany intermittently waged unrestricted submarine warfare against the UK: 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 US in the First World War.[1] 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.[2] Ultimately the U-boats were defeated in May 1943.

Early U-boats (1850–1914)

The first German submarine, the SM U-1.

The first submarine built in Germany, the three-man Brandtaucher, sank to the bottom of Kiel Harbor on 1 February 1851 during a test dive.[3][4] Inventor and engineer Wilhelm Bauer had designed this vessel in 1850, and Schweffel and Howaldt constructed it in Kiel. Dredging operations in 1887 rediscovered Brandtaucher; she was later raised and put on historical display in Germany. The boats Nordenfelt I and Nordenfelt II, built to a Nordenfelt design, followed in 1890. In 1903, the Friedrich Krupp Germaniawerft dockyard in Kiel completed the first fully functional German-built submarine, Forelle,[5] which Krupp sold to Russia during the Russo-Japanese War in April 1904.[6]

At the beginning of the century, 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 Royal Navy. He focused on expensive battleships and there was no role for submarines in his fleet. Only when Krupp exported its submarines to Russia, Italy, Norway and Austria-Hungary did Tirpitz order one submarine.[7] The SM U-1 was a completely redesigned Karp-class submarine and when the Imperial German Navy commissioned it on 14 December 1906,[8] it was the last major navy to possess submarines.[7] 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 SM U-2 (commissioned in 1908) had two torpedo tubes.

The German submarine U-14, showing the kerosene vapour trail.

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 fuel engines, diving only when attacked or for torpedo strikes. The more ship-like hull design reflects the fact that these were primarily surface vessels that could submerge when necessary. 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. While U-boats were faster on the surface than submerged, the opposite is generally true of modern submarines.

Between 1908 and 1910 fourteen big boats with four torpedo tubes and two reload torpedoes were ordered. These boats used a kerosene engine which was more safe than gasoline and more powerful than steam, but the white exhausts 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 Germany was slow in developing a powerful and reliable diesel engine. Finally the U-19 class of 1912–13 had the first diesel engine installed in a German navy boat. Between 1910 and 1912 twenty-three diesel U-boats were ordered.[9] At the start of World War I in 1914, Germany had 48 submarines of 13 classes in service or under construction. During that war, the Imperial German Navy used SM U-1 for training. Retired in 1919, she remains on display at the Deutsches Museum in Munich.[10]

World War I (1914–1918)

Sea mines are loaded in a UC coastal submarine in the harbour of Zeebrugge

Operations

Main article: U-boat campaign

During 1914, the U-boats operated against the British fleet: on 5 September 1914, the light cruiser HMS Pathfinder was sunk by SM U-21, the first ship to have been sunk by a submarine using a self-propelled torpedo. On 22 September, U-9 sank the armoured cruisers HMS Aboukir, HMS Cressy, and HMS Hogue. As a result, the British Grand Fleet had to withdraw to safer waters in Northern Ireland. Against merchant ships, U-boats observed the "prize rules" which meant they had to stop and inspect the ship, and take the crew off the ship before they could sink it. On 20 October 1914, SM U-17 sank the first merchant ship, SS Glitra, off Norway. Only ten merchants were sunk in that way before policy was changed on 18 February 1915. 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 blockades. Under the instructions given to U-boat captains, they could sink merchant ships, even neutral ones, without warning.[11]

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 on the other hand three sinkings of liners with loss of American lives, outraged the US so that the Kaiser had to stop the campaign in September 1915: on 7 May 1915, SM U-20 sank RMS Lusitania, on 19 August, SM U-24 sank SS Arabic and on 9 September RMS Hesperian was sunk by SM U-20. Most of the U-boats were sent to the Mediterranean. Begin 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 25 Americans were killed in the torpedoing of the ferry SS Sussex, which was mistaken for a troopship by SM UB-29. The U.S. 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 1.4 million tons 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 campaing 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 U.S. declared war on Germany.[12]

German U-boat losses

Surface warships 55

Mines 48

Submarines 18

Q-ships 11

Merchant ships 7

Aircraft 1

Accidents 19

Unknown 19

Total 178

Unrestricted submarine warfare in 1917 was very successful, sinking more than 500,000 tons a month. With the introduction of convoys in August 1917 shipping losses declined to 300,000 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.[13] An armistice became effective on 11 November 1918. Under the terms of armistice, all U-boats were to immediately surrender. 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. Stephen King-Hall wrote a detailed eyewitness account of the surrender.[14]

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.[15] 512 officers and 4894 enlisted men were killed. Of the surviving German submarines, 14 U-boats were scuttled and 122 surrendered.[16] They sank 10 pre-dreadnought battleships, 18 heavy and light cruisers, and several smaller naval vessels. They further destroyed 5,708 merchant and fishing vessels for a total of 11,108,865 tons and the loss of about 15,000 sailors.[16]

The Pour le Mérite, the highest decoration for gallantry for officers, was awarded to 29 U-boat commanders.[citation needed] Twelve U-boat crewmen were decorated with the Goldene Militär-Verdienst-Kreuz, the highest bravery award for noncommissioned officers and enlisted men.[17] The most successful U-boat commanders of World War I were Lothar von Arnauld de la Perière (189 merchant vessels and two gunboats with 446,708 tons), followed by Walter Forstmann (149 ships with 391,607 tons), and Max Valentiner (144 ships with 299,482 tons). Their records have not been surpassed in any subsequent conflict.

A small Type UB I U-boat being transported from Antwerp to Zeebrugge

Classes

Körting kerosene-powered boats Type U 1, Type U 2, Type U 3, Type U 5, Type U 9, Type U 13, Type U 16, Type U 17

Mittel-U MAN diesel boats Type U 19, Type U 23, Type U 27, Type U 31, Type U 43, Type U 51, Type U 57, Type U 63, Type U 66, Type Mittel U

U-Cruisers and Merchant U-boats Type U 139, Type U 142, Type U 151, Type UD 1

UB coastal torpedo attack boats Type UB I, Type UB II, Type UB III, Type UF, Type UG

UC coastal minelayers Type UC I, Type UC II, Type UC III

UE ocean minelayers Type UE I, Type UE II

Interwar years (1919–1939)

The Finnish submarine Vetehinen in 1930 on the slipways at the Crichton-Vulcan shipyard in Turku, Finland

Construction

The Treaty of Versailles ending World War I signed at the Paris Peace Conference in 1919 limited the surface navy of Germany's new Weimar Republic to only six battleships, six cruisers, twelve destroyers and twelve torpedo boats. The treaty also restricted the independent tonnage of ships and forbade the construction of submarines.[18] In order to circumvent the restrictions of the treaty, a submarine design office called Ingenieurskantoor voor Scheepsbouw ( IVS ) was set up in the Netherlands[19] The IVS was run by Krupp and made it possible to maintain a lead in submarine technology by designing and constructing submarines in Holland for other nations.[20] The IVS made designs for small 250 tons U-boats, medium 500 tons U-boats and large 750 tons U-boats.[21]

The IVS constructed three 500 tons 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 tons submarine, the Vesikko. This submarine was nearly identical to the subsequent German Type II U-boat. A fifth very small 100 tons submarine, the Saukko was built in 1933 as well. In Spain a large 750 tons 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 by 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.[22]

The Spanish submarine E-1 in Cadiz

U-534, a type IX U-boat at Birkenhead Docks, Merseyside, England

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. For submarines the Germans obtained a parity in tonnage, but promised a 45 percent limit unless special circumstances arose.[23] This allowed 24,000 tons for U-boat building. Only one week after the signature of the agreement, the first of six Type II U-boats, U-1 was commissioned in the German Navy, which changed name from Reichsmarine ( State Navy ) to Kriegsmarine ( War Navy ).[24] Within the year, the Germans commissioned a total of 36 U-boats for a total of 12,500 tons:[25]

Twenty-four small 250 tons Type II U-boats

Ten medium 500 tons Type VII U-boats

Two large 750 tons Type I U-boats, based on the design of the Spanish submarine.

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. 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]

Eight small 250 tons improved type II U-boats

Seven medium 500 tons improved type VIIB U-boats

Eight large 750 tons of a new Type IX U-boats ( the type I was unsatisfactory )

Twenty-one of these twenty-three U-boats were commissioned before the start of 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 would be divided between medium and large U-boats:[26]

Seven medium 500 tons type VIIB U-boats

Five large 750 tons of the improved type IXB U-boats.

A type XB submarine sinking in the Atlantic. On the foredeck the vertical mineshafts are visible.

During 1938 Hitler changed his attitude towards Britain. Whilst he still hoped that Britain would not interfere in his foreign policy it became clear to him that he needed a Navy that could act as a deterrent. Hitler wanted to invoke the escape clause of the naval agreement and to have 70,000 tons of submarines. Between May 1938 and January 1939 Raeder ordered 52 more U-boats, to be completed by 1942:[27]

Twenty-one medium 500 tons type VIIB U-boats

Eleven large 750 tons type IXB U-boats

Three very large type XB minelaying U-Boats

Four huge type XI U-cruisers

In 1939, the ambitious Plan Z was launched. It called for the construction of a German Navy capable of challenging the Royal Navy. The plan included 249 U-boats for a total of 200,000 tons. But when World War II broke out only months after the plan was announced, only a handful of the planned U-boats ended up being built.[27]

When World War II started, Germany had 56 U-boats commissioned, of which 46 were operational and only 22 had enough range for Atlantic operations, the other 24 were limited to operations on the North Sea.[28]

Developments

A torpedo is loaded into a U-Boat through a torpedo hatch.

Compared to its WWI equivalent the U-boat design was greatly improved. By using a new steel alloy and by welding instead of riveting they had stronger hulls and could dive deeper. The diving time was decreased to thirty seconds for a medium U-boat. The power of diesel engines was increased so U-boats had a greater surface speed. Range was increased by installing fuel saddle tanks, which were on the bottom open to the sea in order to balance pressure, and the diesel was floating freely on the seawater in the saddle tank. Also a technique was developed for economical cruising where only one of the two diesel engines was running and driving the two propeller shaft through a coupling with the two electro engines.[29]

Another vast improvement was the introduction of new torpedo types for the U-boats : the classic G7a torpedo propulsed with compressed air had a much larger warhead than its WWI equivalent, but more important was the introduction of the electric G7e torpedo.[30] This torpedo was slower and had less range but it left no telltale bubble wake and was hence ideally suited for daylight attacks.[29] During WWI the Germans had briefly experimented with magnetic pistols and these were further developed now as the standard pistol for torpedoes. The classic contact pistol required a torpedo to detonate against the ship side hull, whilst a magnetic torpedo could detonate below a ship, resulting in a much more damaging explosion. It was hoped that one torpedo would hence suffice to break the back of a ship and a U-boat could sink many more ships with its supply of torpedoes.[31][32]

All U-boats were now also equipped with long- and short wave transmitters, which enabled to communicate with bases ashore and fellow U-boats at sea. This allowed for better operational information and guidance.[31]

U-Boat design and layout

Cross-section of a Type VII U-boat

From bow to stern, A typical U-boat design comprised these sections:

Bow torpedo room. The torpedo tubes were loaded but torpedoes needed maintenance so there was space to unload the tubes. Below the floor plates four spare torpedoes were stored. Two more spares were stored above the floorplates where they occupied much of the available space. The crew responsible for the torpedo maintenance and launching had their sleeping bunks in this compartment, along with the lowest ratings on board. As long as the two spare torpedoes above the floorplates were not launched, living conditions were very cramped here. Once launched, space for extra bunks became available but anyway there were not enough sleeping bunks for all the crew, these were 'hot bunks' which switched occupants as they went on or off duty.[33][34]

Crew quarters for officers and chief petty officers[35] with below decks a battery compartment.[33] The captain had a curtained bunk which faced 2 small rooms: the wireless room and the hydrophone room.[35]

Control room. The big periscope for general use was located here. The rudder, diving planes, ballast and trim tanks were operated here with valves and buttons. Below decks there was space to retract the periscope and ammunition for the deck gun. A cilindrical tube with a ladder lead to the conning tower.[33]

Conning tower. This space protruded from the cilindrical hull but was still within the pressure hull. Here the angle and depth settings for the torpedoes were calculated with a analogue data solver. During submerged attacks the captain was on station here, operating the second, smaller attack periscope which generated less wake at the surface. Above the conning tower was the bridge.[33]

Aft crew Quarters for petty officers with below decks a battery compartment. The galley and a toilet were located here.[33]

Engine ( Diesel ) room. The Diesel engines needed air, which was supplied through a pipe outside the pressure hull from the bridge, as high as possible from sea level. There was no exhaust pipe, in order to reduce smoke the exhaust was mixed with sea water. The Diesel engine could drive an air compressor in order to feed air tanks needed for venting the ballast tanks.[36]

Electrical or Motor room. The electric motors were driven by the batteries. Alternatively, when driven by the Diesel engines, the motors acted as generators for the recharging the batteries.[36]

Aft torpedo room. Only bigger type IX U-boats had such compartment. smaller U-boats did not have aft torpedo tubes at all or had a single torpedo tube installed in the Motor room, with a spare torpedo stored below decks between the engine.[36]

World War II (1939–1945)

U-boat pens in Saint-Nazaire, France

Operations

Main article: 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."[37] Cross-Atlantic trade in war supplies and food was extensive and critical for Britain's survival. The continuous action surrounding Allied shipping became known as the Battle of the Atlantic.

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 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 less 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 next months the U-boats put their 'wolfpack' tactic against convoy in practise with spectacular results. 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.[38]

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 rerout convoys around U-boat concentrations.[39] 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.[40] 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[41][42] and next he ordered a substantial force of U-boats to operate in the Mediterranean in order to support the Italians and Rommels Afrika Korps.[43]

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.[44] 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.[45] The renewed offensive against convoys reached its climax in March 1943, when two thirds of all ships sunk, were ships sailing in convoys.[46] 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.[47]

U-boats operated also off the southern African coasts and even as far east as the Arabian Sea and Indian Ocean.

By the end of the war, almost 3,000 Allied ships (175 warships; 2,825 merchant ships) had been sunk by U-boat torpedoes.[48] In total 1131 U-boats entered service before the German surrender, of which 863 have executed war patrols, and 785 were lost.[49][50] 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.

Torpedo developments

The U-boats' main weapon was the torpedo, though mines and deck guns (while surfaced) were also used. Early German World War II torpedoes were fitted with one of two types of pistol triggers – 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. 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 – 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.[51][52]

The pattern running of a FAT torpedo

In order to give U-boats better opportunities against well-defended convoys, several types of "patt