U - 20 - class submarine The U - 20 " class was a class of four submarine s or U - boats built for and operated by the Austro - Hungarian Navy ( ) during World War I . The class is sometimes referred to as the Havmanden " class because it was based upon the design of the Royal Danish Navy ' s 1911 " Havmanden " class submarines , three of which were built in Fiume . With a small fleet of six U - boats at the beginning of World War I , two of which were not operational , the Austro - Hungarian Navy acted to bolster its fleet . They reluctantly ordered four " U - 20 " boats in 1915 because construction could start immediately , even though the " Havmanden " - class design was largely obsolete by the beginning of the war . Political considerations caused the order to be split between Austrian and Hungarian firms , which contributed to construction problems and delays , keeping any of the boats from being operational until the middle of 1917 . The class boats were just over than long and were armed with two front torpedo tubes , a deck gun , and a machine gun . The engines for the boats were unreliable , which compounded handling problems with the design . The " U - 20 " class did not claim any wartime successes , yet lost two of the boats — " U - 20 " and " U - 23 " — to enemy action during the war . The remaining two were delivered as war reparations and broken up . The conning tower from " U - 20 " , which was raised and salvaged in 1962 , is on display in a military museum in Vienna . Background . The Austro - Hungarian Navy ' s U - boat fleet at the beginning of World War I consisted of six largely experimental submarines , two of which were not operational . When it became apparent that the war would not be a short one , Austria - Hungary moved to bolster their U - boat fleet by seizing the plans for the Danish " Havmanden " class submarines , three of which had been built at Whitehead ’ s in Fiume . Although the Austro - Hungarian Navy was not happy with the design , which was largely obsolete , it was the only design for which plans were available and which could be begun immediately in domestic shipyards . The Austro - Hungarian Navy unenthusiastically placed orders for four boats on 27 March 1915 . Design and construction . Before construction could begin , the Navy found itself in the midst of political situation because the Hungarian government demanded that a substantial portion of the submarine contracts be allocated to Hungarian firms . After intricate negotiations , the order was split into two orders of two submarines each , with " U - 20 " and " U - 21 " built at the Pola Navy Yard , and " U - 22 " and " U - 23 " built at the Hungarian yard at Fiume . The subcontracts were also divided between Hungarian and Austrian firms , with about two - thirds going to Hungarian companies and one - third to Austrian companies . This allocation of contracts , while politically expedient , exacerbated technical problems that resulted in numerous modifications and delays . The " U - 20 " - class boats were ocean - going submarines that displaced surfaced and submerged . The boats were long with a beam of and a draft of . For propulsion , they featured a single shaft , a single diesel engine for surface running , and a single electric motor for submerged travel . The boats were capable of while surfaced and while submerged . Although there is no specific notation of a range for the " U - 20 " class , the " Havmanden " class , upon which it was based , had a range of at , surfaced , and at submerged . The " U - 20 " class boats were designed for a crew of 18 . The " U - 20 " class boats were armed with two torpedo tubes located in the front and carried a complement of two torpedoes . They were also equipped with a 66 mm / 26 ( 2 . 6 in ) deck gun and an machine gun . The first of the class to be launched was " U - 21 " , when it slid down the ways on 15 August 1916 ; the final boat launched was " U - 22 " on 27 January 1917 . The boats entered service between August and November 1917 . Class members . During their active service , the boats of the " U - 20 " class suffered from unreliable engines which compounded the already poor handling characteristics of the boats . None of the class had any successes and two of their number were sunk during the war . " U - 23 " was the first sunk when she went down on 21 February 1918 from an explosive paravane in the Straits of Otranto . " U - 20 " , the lead boat of the class , sank in July 1918 when she was torpedoed by the Italian submarine " F - 12 " in the Tagliamento estuary . The remaining two boats were surrendered at war ' s end , with " U - 21 " being ceded to Italy in 1920 , and " U - 22 " to France ; both were broken up . SM " U - 20 " . SM " U - 20 " , the lead boat of the class , was laid down on 29 September 1915 at the Pola Navy Yard . She was launched on 18 September 1916 . During diving trials in March 1917 , " U - 20 " was accidentally rammed by the Austro - Hungarian Navy light cruiser , which required seven months of repairs for the U - boat . " U - 20 " was commissioned on 20 October , and operated out of Pola and Trieste . She was in the estuary of the Tagliamento river in early July 1918 when she was spotted by the surfaced Italian submarine " F - 12 " . " U - 20 " was hit by one torpedo at a range of and sank with all hands west of Trieste near position . The wreck of " U - 20 " was raised and salvaged in 1962 . Remains of her crewmen were interred on the grounds of the Theresian Military Academy at Wiener Neustadt . The conning tower and a midship section were donated to the Heeresgeschichtliches Museum in Vienna , where they are on display . " U - 20 " did not sink any ships during her service . SM " U - 21 " . SM " U - 21 " was built at the Pola Navy Yard and was launched on 15 August 1916 , and was the first of the class to be launched . " U - 21 " was damaged in a diving trial in January 1917 , requiring seven months for repairs . The U - boat was commissioned on 15 August 1917 . In early October , " U - 21 " conducted patrols off the Albanian coast , but a mid - October Mediterranean patrol was cut short by a leaky seal on her main hatch . After an additional eight months of repairs , " U - 21 " again conducted patrols off Albania . A broken piston in her diesel engine cut short her war career . At Pola at the war ' s end , " U - 21 " was ceded to Italy in 1920 as a war reparation and broken up . " U - 21 " did not have any successes during the war . SM " U - 22 " . SM " U - 22 " was built at the Hungarian yard and was launched on 27 January 1917 , the last of the class to be launched . The still - incomplete boat sank in the harbor at Fiume in June 1917 , but was raised , repaired , and relaunched in October . From her November commissioning , " U - 22 " conducted patrols off the Po River estuary and , later , northern Adriatic patrols from Trieste . After three months of repairs for her failed electric motor in mid 1918 , " U - 22 " resumed service , patrolling the Montenegrin coast out of Cattaro . In port at Cattaro at war ' s end , she was ceded to France as a war reparation and broken up . " U - 22 " sank no ships during the war . SM " U - 23 " . SM " U - 23 " was laid down on 8 December 1915 at the Hungarian yard and was launched on 5 January 1917 . She was commissioned on 1 September and initially patrolled off the Italian towns of Rimini and Ancona . After being transferred to Cattaro in late December , " U - 23 " departed on her final patrol on 20 February 1918 . The following day , " Linienschiffsleutnant " Klemens Ritter von Bézard , " U - 23 " s only commanding officer , guided the boat in an attack on the Italian transport in the Straits of Otranto . " U - 23 " then came under attack from the Italian destroyer " Airone " ; after first trying to ram the U - boat , the destroyer deployed an explosive paravane . When the paravane came into contact with the submerged " U - 23 " , it blew debris into the air , sinking the submarine with all hands . Like all of her sister boats , " U - 23 " had no wartime successes .