U-boat

On 5 September 1914, 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 under the command of Otto Weddigen sank the obsolete British warships HMS Aboukir, HMS Cressy, and HMS Hogue (the "Live Bait Squadron") in a single hour.

In the Gallipoli Campaign in early 1915 in the eastern Mediterranean, German U-boats, notably the U-21, prevented close support of allied troops by 18 pre-dreadnought battleships by sinking two of them.[11]

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

For the first few months of the war, U-boat anticommerce actions observed the "prize rules" of the time, which governed the treatment of enemy civilian ships and their occupants. On 20 October 1914, SM U-17 sank the first merchant ship, SS Glitra, off Norway.[12] Surface commerce raiders were proving to be ineffective, and on 4 February 1915, the Kaiser assented to the declaration of a war zone in the waters around the British Isles. This was cited as a retaliation for British minefields and shipping blockades. Under the instructions given to U-boat captains, they could sink merchant ships, even potentially neutral ones, without warning.

On 7 May 1915, SM U-20 sank the liner RMS Lusitania. The sinking claimed 1,198 lives, 123 of them American civilians, and the attack of this unarmed civilian ship deeply shocked the Allies. According to the ship's manifest, Lusitania was carrying military cargo, though none of this information was relayed to the citizens of Britain and the United States, who thought that the ship contained no ammunition or military weaponry whatsoever and it was an act of brutal murder. Munitions that she carried were thousands of crates full of ammunition for rifles, 3-inch (76 mm) artillery shells, and various other standard ammunition used by infantry. The sinking of the Lusitania was widely used as propaganda against the German Empire and caused greater support for the war effort. A widespread reaction in the U.S. was not seen until the attack on the ferry SS Sussex, which carried many citizens of the United States.

The initial U.S. response was to threaten to sever diplomatic ties, which persuaded the Germans to issue the Sussex pledge that reimposed restrictions on U-boat activity. The U.S. reiterated its objections to German submarine warfare whenever U.S. civilians died as a result of German attacks, which prompted the Germans to fully reapply prize rules. This, however, removed the effectiveness of the U-boat fleet, and the Germans consequently sought a decisive surface action, a strategy that culminated in the Battle of Jutland.

Although the Germans claimed victory at Jutland, the British Grand Fleet remained in control at sea. Effective anticommerce warfare by U-boats resumed after Vice Admiral Reinhard Scheer, commander in chief of the High Seas Fleet, pressed for all-out U-boat war, convinced that a high rate of shipping losses would force Britain to seek an early peace before the United States could react effectively.

Sinking of the Linda Blanche out of Liverpool by SM U-21 (Willy Stöwer)

The renewed German campaign was effective, sinking 1.4 million tons of shipping between October 1916 and January 1917. Despite this, the political situation demanded even greater pressure, and on 31 January 1917, 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.S. declared war on Germany in April 1917.

Unrestricted submarine warfare in early 1917 was initially very successful, sinking a major part of Britain-bound shipping. 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 1918. Of the surviving German submarines, 14 U-boats were scuttled and 122 surrendered.[13] Under the terms of 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, then scrapped or given to Allied navies. Stephen King-Hall wrote a detailed eyewitness account of the surrender.[14]

Of the 373 German submarines that had been built, 178 were lost by enemy action. Of these, 40 were sunk by mines, 30 by depth charges, and 13 by Q-ships; 512 officers and 4894 enlisted men were killed. They sank 10 battleships, 18 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.[13] The Pour le Mérite, the highest decoration for gallantry for officers, was awarded to 29 U-boat commanders. Twelve U-boat crewmen were decorated with the Goldene Militär-Verdienst-Kreuz, the highest bravery award for noncommissioned officers and enlisted men.[15] 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

• 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

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.[16] In order to circumvent the restrictions of the treaty, a submarine design office called Ingenieurskantoor voor Scheepsbouw ( IVS ) was set up in the Netherlands[17] 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.[18] The IVS made designs for small 250 tons U-boats, medium 500 tons U-boats and large 750 tons U-boats.[19]

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.[20]

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.[21] 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 ).[22] Within the year, the Germans commissioned a total of 36 U-boats for a total of 12,500 tons:[23]

• 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:[23]

• 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 Word 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:[24]

• 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:[25]

• 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.[25]

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.[26]

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.[27]

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.[28] This torpedo was slower and had less range but it left no telltale bubble wake and was hence ideally suited for daylight attacks.[27] 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.[29][30]

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.[29]

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.[31][32]
• Crew quarters for officers and chief petty officers[33] with below decks a battery compartment.[31]
• Control room. 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.[31]
• Aft crew Quarters for petty officers with below decks a battery compartment. The galley and a toilet were located here.[31]
• 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.[34]
• Elektrical or Motor room. The elektrical motors were driven by the batteries. Alternatively, when driven by the Diesel engines, the motors acted as generators for the recharging the batteries.[34]
• Aft torpedo room. Only bigger type XI 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.[34]

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."[35] 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 Word 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 wee 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.[36]

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.[37] 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.[38] 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[39][40] 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.[41]

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 ant-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.[42] 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.[43] The renewed offensive against convoys reached its climax in March 1943, when two thirds of all ships sunk, were ships sailing in convoys.[44] 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.[45]

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.[46] In total 1131 U-boats entered service before the German surrender, of which 863 have executed war patrols, and 785 were lost.[47][48] 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.

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.[49][50]

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 m ) or long ( 1900 m ) 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.[51][52]

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 T5 "Zaunkönig", was designed to combat small and fast warships, and entered service in September 1943.[53] The Allies countered acoustic torpedoes with noisemaker decoys such as Foxer, FXR, CAT, and Fanfare.

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 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 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 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.[54][55] 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 was operational before the end of the war.[56]

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.[57] Based on the design of an Atlantic Walter U-boat, the Type XXI "Elektroboot" was designed to boost submerged performance. A smaller Type XXIII coastal Elektroboat was also taken into production. These Elektroboote were mass-produced, with prefabricated segments constructed at different sites and then assembled at the bigger shipyards.[58][59]

The Schnorkel mast and air flows

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.[60] 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 8 knots (15 km/h), lest the device snap from stress. Whilst running submerged with the Schnorchel, the Gruppenhorchgerät was useless because of interference with the noisy diesel engines. But the Schnorchel allowed the old Type VII and IX U-Boats to operate in waters which were previously denied to them.[61] Finally, Allied radar eventually became sufficiently advanced that the Schnorchel mast could be detected.

• Type I: first design for a large 750 tons U-boat. Only 2 built as the design was not very successful.
• 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.
• Type VII: the "workhorse" of the U-boats with 709 completed in World War II
• Type IX: these long-range U-boats operated as far as the Indian Ocean with the Japanese (Monsun Gruppe), and the South Atlantic
• Type X: long-range minelayers but mainly used to resupply other U-boats
• Type XIV: unarmed U-boat, used to resupply other U-boats; nicknamed the Milchkuh ("Milk Cow")
• Type XVII: small experimental coastal submarines powered by experimental hydrogen peroxide propulsion systems, not put into service
• Type XXI: known as the Elektroboot. The design was taken into mass-production, but only 2 set out for a war patrol before end of the war.
• Type XXIII: smaller version of the XXI used for coastal operations. operated on a small scale during 1945
• Midget submarines, including Biber, Hai, Molch, and Seehund
• Uncompleted U-boat projects

Throughout the war, an arms race evolved between the Allies and the Kriegsmarine. Sonar (ASDIC in Britain) allowed Allied warships to detect submerged U-boats, 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 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, as well as more antiaircraft guns, but by early to mid-1943, the Allies switched to centimetric radar (unknown to Germany), which rendered the radar detectors ineffective. U-boat radar systems were also developed, but many captains chose not to use them for fear of broadcasting their position to the enemy. Against ASDIC the Germans developed Bold, a chemical bubble-making decoy.

Advances in convoy tactics, high-frequency direction finding (referred to as ("Huff-Duff"), radar, sonar, depth charges, anti-submarine weapons such as "Hedgehog" and "FIDO", the intermittent cracking of the German Naval Enigma code, the introduction of the Leigh Light, long range patrol aircraft, escort carriers and the enormous US shipbuilding capacity, all turned the tide against the U-boats. At the same time, the Allies targeted the U-boat shipyards and their bases with strategic bombing.

Captured Type VII and Type IX U-boats outside their pen in Trondheim, Norway, 19 May 1945

In May 1941 code books, an Enigma machine and its settings were captured from the U-110 which could be boarded before she sank. 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 allowed rerouting convoys around U-boat patrol lines. In February 1942 the naval Enigma machines were altered and this advantage was lost until the new code was broken in October 1942, when U-559 was boarded as she was sinking, and crucial code books were salvaged.