U - boat U - boat is the anglicized version of the German word U - Boot , a shortening of Unterseeboot , which means " undersea boat " . While the German term refers to any submarine , the English one ( in common with several other languages ) refers specifically to military submarine s operated by Germany , particularly in World War I and World War II . Although at times they were efficient fleet weapons against enemy naval warships , they were most effectively used in an economic warfare role ( commerce raiding ) , enforcing a naval blockade against enemy shipping . The primary targets of the U - boat campaigns in both wars were the merchant convoys bringing supplies from Canada , the British Empire and the United States to the islands of the United Kingdom and ( during World War II ) to the Soviet Union and the Allied Countries in the Mediterranean . Austro - Hungarian submarines of World War I ( and before ) were also known as U - boats . Pre - War . The first submarine built in Germany was the three - man submarine " Brandtaucher " , which sank to the bottom of Kiel harbor during its first test dive . The vessel was designed in 1850 by the inventor and engineer Wilhelm Bauer and built by Schweffel & Howaldt in Kiel for the Imperial German Navy . " Brandtaucher " was later rediscovered during dredging operations in 1887 , and subsequently raised sixteen years later and placed in a museum in Germany , where it remains today . This was followed in 1890 by " W1 " and " W2 " , built to a Nordenfelt design . In 1903 , Germaniawerft dockyard in Kiel completed Germany ' s first fully functional submarine , " Forelle " which was sold to Russia during the Russo - Japanese War in April 1904 . The " SM U - 1 " was a completely redesigned Karp - class submarine and only one was built . It was commissioned by the Imperial German Navy on 14 December 1906 . It had a double hull , was powered by a Körting kerosene engine and was armed with a single torpedo tube . The fifty percent larger " SM U - 2 " had two torpedo tubes . A diesel engine was not installed in a German navy boat until the class of 1912 – 13 . At the start of World War I , Germany had 48 submarines of 13 classes in service or under construction . During the First World War the " SM U - 1 " was used for training and was retired in 1919 . It is currently on display at the Deutsches Museum in Munich . World War I . At the start of World War I , Germany had twenty - nine U - boats ; in the first ten weeks , five British cruisers had been lost to them . On 5 September 1914 , was sunk by , the first ship to have been sunk by a submarine using a self - propelled torpedo . On 22 September , sank the obsolete British warships , and ( 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 battleship s by sinking two of them . For the first few months of the war , U - boat anti - commerce actions observed the " prize rules " of the time which governed the treatment of enemy civilian ships and their occupants . On 20 October 1914 , sank the first merchant ship , the , off Norway . 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 blockade s . Under the instructions given to U - boat captains , they could sink merchant ships , even potentially neutral ones , without warning . In February 1915 , a submarine was rammed and sunk off Beachy Head by the collier SS Thordis commanded by Captain John Bell RNR after firing a torpedo . On 7 May 1915 , sank the liner RMS " Lusitania " with a single torpedo hit , although it is debated whether a second explosion was due to flammable cargo or another torpedo . The sinking claimed 1 , 198 lives , 128 of them American civilians , including noted theatrical producer Charles Frohman and Alfred Vanderbilt , a member of the prestigious Vanderbilt family . The sinking deeply shocked the Allies and their sympathizers because an unarmed civilian merchant and passenger vessel was attacked . According to the ship ' s manifest , " Lusitania " was carrying military cargo . After further investigations , it has been confirmed that the " Lusitania " was in fact carrying ammunition for the allies to use against the Germans . However , this was not known at the time and the " Lusitania " was mistaken for a troopship . It was not until the sinking of the ferry that there was a widespread reaction in the USA . The initial U . S . response was to threaten to sever diplomatic ties , which persuaded the Germans to issue the Sussex pledge that re - imposed 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 re - apply prize rules . This , however , removed the effectiveness of the U - boat fleet , and the Germans consequently sought a decisive surface action , a strategy which culminated in the Battle of Jutland . Although the Germans claimed victory at Jutland , the British Grand Fleet remained in control at sea . It was necessary to return to effective anti - commerce warfare by U - boats . 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 . 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 the spring of 1917 was initially very successful , sinking a major part of Britain - bound shipping . Nevertheless 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 and all surviving German submarines were surrendered . Of the 360 submarines that had been built , 178 were lost but more than 11 million tons of shipping had been destroyed . Surrender of the Fleet . Under the terms of the 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 , scrapped , or given to Allied navies . Stephen King - Hall wrote a detailed eyewitness account of the surrender . Inter - war . At the end of World War I , as part of the Paris Peace Conference , 1919 , the Treaty of Versailles restricted the total tonnage of the German surface fleet . The treaty also restricted the independent tonnage of ships and forbade the construction of submarines . However , a submarine design office was set up in the Netherlands and a torpedo research program was started in Sweden . Before the start of World War II , Germany started building U - boats and training crews , labeling these activities as " research " or concealing them using other covers . When this became known , the Anglo - German Naval Agreement limited Germany to parity with Britain in submarines . When World War II started , Germany already had 65 U - boats , with 21 of those at sea , ready for war . World War II . 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 , since the Treaty of Versailles had limited the surface navy of Germany to six battleships ( of less than 10 , 000 tons each ) , six cruisers and 12 destroyers . Prime Minister Winston Churchill wrote " The only thing that really frightened me during the war was the U - boat peril . " In the early stages of the war , the U - boats were extremely effective in destroying Allied shipping , initially in the mid - Atlantic , where there was a large gap in air cover . There was an extensive trade in war supplies and food across the Atlantic , which was critical for Britain ' s survival . This continuous action became known as the Battle of the Atlantic , as the British developed technical defences such as ASDIC and radar , and the German U - boats responded by hunting in what were called " wolfpacks " where multiple submarines would stay close together , making it easier for them to sink a specific target . Later , when the United States entered the war , the U - boats ranged 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 . The U . S . military engaged in various tactics against German incursions in the Americas ; these included military surveillance of foreign nations in Latin America , particularly in the Caribbean , in order to deter any local governments from supplying German U - boats . 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 more ship - like hull design reflects the fact that these were primarily surface vessels which had the ability to submerge when necessary . This contrasts with the cylindrical profile of modern nuclear submarine s , which are more hydrodynamic underwater ( where they spend the majority of their time ) but less stable on the surface . Indeed , while U - boats were faster on the surface than submerged , the opposite is generally true of modern subs . The most common U - boat attack during the early years of the war was conducted on the surface and at night , see submarine warfare . This period , before the Allied forces developed truly effective antisubmarine warfare ( ASW ) tactics , was referred to by German submariners as " die glückliche Zeit " or " the happy time . " Torpedoes . The U - boats ' main weapon was the torpedo , though mine s and deck gun s ( while surfaced ) were also used . By the end of the war , almost 3 , 000 Allied ships ( 175 warships ; 2 , 825 merchant ships ) were sunk by U - boat torpedoes . Early German World War II torpedoes were straight runners , as opposed to the homing and pattern - running torpedoes which were fielded later in the war . They were fitted with one of two types of pistol trigger : " 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 . One of the most effective uses of magnetic pistols would be to set the torpedo ' s depth to just beneath the keel of the target . The explosion under the target ' s keel would create a shock wave , and the ship could break in two . In this way , even large or heavily armored ships could be sunk or disabled with a single well - placed hit . In practice , however , the depth - keeping equipment and magnetic and contact exploders were notoriously unreliable in the first eight months of the war . Torpedoes would often run at an improper depth , detonate prematurely , or fail 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 at high latitudes fluctuations in the Earth ' s magnetic field . These were eventually phased out , and the depth - keeping problem was solved by early 1942 . Later in the war , Germany developed an acoustic homing torpedo , the G7 / T5 . It was primarily designed to combat convoy escorts . The acoustic torpedo was designed to run straight to an arming distance of 400 meters and then turn toward the loudest noise detected . This sometimes ended up being the U - boat itself ; at least two submarines may have been sunk by their own homing torpedoes . Additionally , it was found these torpedoes were only effective against ships moving at greater than . At any rate , the Allies countered acoustic torpedoes with noisemaker decoys such as " Foxer " , " FXR " , " CAT " and " Fanfare " . The Germans , in - turn , countered this by introducing newer and upgraded versions of the acoustic torpedoes , like the late war G7es , and the T11 torpedo . However , the T11 torpedoes did not see active service . U - boats also adopted several types of " pattern - running " torpedoes which ran straight out 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 if the weapon missed its primary target . U - boat developments . 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 , some traveling as far as Japan and the east coast of the United States . With the Type XXI " Elektroboot " , German designers realized the U - boat depended on submerged ability both for combat effectiveness and survival ; this was the first submarine whose design favored submerged performance . The Type XXI featured a revolutionary streamlined hull design , which was the basis for the later nuclear submarine . Its propulsion system featured a large battery capacity , which allowed it to cruise submerged for long periods and reach unprecedented submerged speeds . A larger battery was possible because the space it occupied was originally intended to store hydrogen peroxide for a Walter turbine , which was unsuccessful on the Type XVII . Throughout the war an arms race evolved between the Allies and the " Kriegsmarine " , especially in 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 warning receivers , to give them ample time to dive before the enemy closed in . However , at some point the Allies switched to centimetric radar ( unbeknownst to Germany ) which rendered the radar detectors ineffective . U - boat radar systems were also developed , but many captains chose not to utilize them 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 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 . Waste disposal was a problem when the U - boats spent extended periods without surfacing . Speed was limited to , lest the device snap from stress . The " schnorchel " also had the effect of making the boat essentially noisy and deaf in sonar terms . Finally , Allied radar eventually became sufficiently advanced that the Schnorchel mast itself could be detected beyond visual range . The later U - boats were covered in a sound - absorbent rubber coating to make them less of an ASDIC target . They also had the facility to release a chemical bubble - making decoy , known as " Bold " , after the mythical kobold . Counter - measures . Advances in convoy tactics , high frequency direction finding ( referred to as " Huff - Duff " ) , radar , active sonar ( called ASDIC in Britain ) , depth charge s , ASW spigot mortars ( also known as " hedgehog " ) , the intermittent cracking of the German Naval Enigma code , the introduction of the Leigh Light , the range of escort aircraft ( especially with the use of escort carrier s ) , the use of mystery ships , and the full entry of the U . S . into the war with its enormous shipbuilding capacity , all turned the tide against the U - boats . In the end , the U - boat fleet suffered extremely heavy casualties , losing 793 U - boats and about 28 , 000 submariners ( a 75 % casualty rate , the highest of all German forces during the war ) . At the same time , the Allies targeted the U - boat shipyards and their bases with strategic bombing . Enigma machine . The British had a major advantage in their ability to read some German naval Enigma codes . An understanding of the German coding methods had been brought to Britain via France from Polish code - breakers . Thereafter , code - books and equipment were captured by raids on German weather ships and from captured U - boats . A team including Alan Turing used special purpose " Bombe s " and early computers to break new German codes as they were introduced . The speedy decoding of messages was vital in directing convoys away from wolf - packs and allowing interception and destruction of U - boats . This was demonstrated when the Naval Enigma machines were altered in February 1942 and wolf - pack effectiveness greatly increased until the new code was broken . The , a Type IXB , was captured in 1941 by the Royal Navy , and its Enigma machine and documents were removed . was also captured by the British in October 1942 , three sailors boarded her as she was sinking , and desperately threw all the code books out of the submarine . Two of them , Able Seaman Colin Grazier and Lieutenant Francis Anthony Blair Fasson continued to throw code books out of the ship as it went under water , and went down with it . Further code books were captured by raids on weather ships . was boarded by crew from the Canadian ship on March 6 , 1944 , and codes were taken from her , but by this time in the war most of the information was known . The , a Type IXC , was captured by the United States Navy in June 1944 . It is presently a museum ship in Chicago at the Museum of Science and Industry . Battle of Bell Island . Two events in the battle took place in 1942 when German U - boats attacked four allied ore carriers at Bell Island , Newfoundland . The carriers and the were sunk by on September 5 , 1942 , while the and " PLM 27 " were sunk by on November 2 with the loss of 69 lives . When the submarine launched a torpedo at the loading pier , Bell Island became the only location in North America to be subject to direct attack by German forces in World War II . Operation Deadlight . Operation Deadlight was the code name for the scuttling of U - boats surrendered to the Allies after the defeat of Germany near the end of the war . 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 . Post World War II . From 1955 , the West German " Bundesmarine " was allowed to have a small navy . Initially two sunken Type XXIIIs and a Type XXI were raised and repaired . In the 1960s , West Germany re - entered the submarine business . Because Germany was initially restricted to a 450 tonne displacement limit , the Bundesmarine focused on small coastal submarines to protect against the Soviet threat in the Baltic Sea . The Germans sought to use advanced technologies to offset the small displacement , such as amagnetic steel to protect against naval mines and magnetic anomaly detector s . The initial Type 201 was a failure because of hull cracking ; the subsequent Type 205 , first commissioned in 1967 , was a success , and 12 were built for the German navy . To continue the U - boat tradition , the new boats received the classic U designation starting with the " U - 1 " . With the Danish government ' s purchase of two Type 205 boats , the German government realized the potential for the submarine as an export . Three of the improved Type 206 boats were sold to the Israeli Navy becoming the " Gal " - class . The German Type 209 diesel - electric submarine was the most popular export - sales submarine in the world from the late 1960s into the first years of the 21st century . With a larger 1 , 000 - 1 , 500 tonne displacement , the class was very customizable and has seen service with 14 navies with 51 examples being built as of 2006 . Germany has brought the U - boat name into the 21st century with the new Type 212 . The 212 features an air - independent propulsion system using hydrogen fuel cell s . This system is safer than previous closed - cycle diesel engines and steam turbines , cheaper than a nuclear reactor and quieter than both . While the Type 212 is also being purchased by Italy , the Type 214 has been designed as the follow - on export model and has been sold to Greece , South Korea and Turkey . In July 2006 , Germany commissioned its newest U - boat , the " U - 34 " , a Type 212 .