SM U - 21 ( Austria - Hungary ) SM " U - 21 or U - XXI " was a or U - boat built for and operated by the Austro - Hungarian Navy ( ) during the First World War . The design for " U - 21 " was based on submarines of the Royal Danish Navy ' s " Havmanden " class ( three of which had been built in Austria - Hungary ) , and was largely obsolete by the beginning of the war . " U - 21 " was just over long and was armed with two bow torpedo tubes , a deck gun , and a machine gun . Construction on " U - 21 " began in mid 1915 and the boat was launched in September 1916 . After suffering damage during a diving trial in January 1917 , " U - 21 " underwent seven months of repairs before her commissioning in August 1917 . The U - boat conducted patrols off the Albanian coast in October 1917 , but experienced the failure of the seal on her main hatch . The repairs kept the boat out of action until June 1918 . But in July a piston in her diesel engine broke , knocking the submarine out of the rest of the war . At the end of World War I , " U - 21 " was ceded to Italy as a war reparation and scrapped in 1920 . " U - 21 " had no wartime successes . Design and construction . When it became apparent to the Austro - Hungarian Navy that the First World War would not be a short one , they 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 & Co . 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 " U - 21 " and her three sister boats on 27 March 1915 . " U - 21 " was one of two boats of the class to be built at the Pola Navy Yard . Due to demands by the Hungarian government , subcontracts for the class were divided between Hungarian and Austrian firms , and this politically expedient solution worsened technical problems with the design , resulting in numerous modifications and delays for the class in general . " U - 21 " was an ocean - going submarine that displaced surfaced and submerged and was designed for a complement of 18 . She was long with a beam of and a draft of . For propulsion , she featured a single shaft , a single diesel engine for surface running , and a single electric motor for submerged travel . She was capable of while surfaced and while submerged . Although there is no specific notation of a range for " U - 21 " , the " Havmanden " class , upon which the " U - 20 " class was based , had a range of at , surfaced , and at submerged . " U - 21 " was armed with two torpedo tubes located in the front and carried a complement of two torpedoes . She was also equipped with a 66 mm / 26 ( 2 . 6 in ) deck gun and an machine gun . Service career . " U - 21 " was launched on 15 August 1916 , the first of the four " U - 20 " - class boats . During a diving trial in January 1917 , the submarine was damaged when it sank too deep , requiring repairs that took place over the next seven months . " U - 21 " was commissioned on 15 August under the command of " Linienschiffsleutnant " Hugo von Seyffertitz . A 31 - year - old native of Brixen , von Seyffertitz was a first - time U - boat commander . Ten days after commissioning , " U - 21 " safely submerged to a depth of . However , her nose was dented when she hit bottom on another test dive in September , necessitating more repairs . On 29 September , von Seyffertitz steered the boat from the submarine base at Brioni to Cattaro , where she arrived on 1 October . On 4 October , " U - 21 " set out for a patrol off the coast of Albania , but had returned to Cattaro by mid October . On 15 October , von Seyffertitz and " U - 21 " departed from Cattaro to begin their first Mediterranean deployment . Slated to sail into the Ionian Sea , " U - 21 " instead had to turn back the following day when the main hatch seal on the conning tower leaked and could not be repaired . After her 18 October return to Cattaro , she sailed for Pola , arriving on 24 October . There , she would undergo another lengthy stay in port for repairs . While " U - 21 " was under repair , von Seyffertitz was transferred to . " Linenschiffleutnant " Robert Dürrial was assigned the new commander of " U - 21 " on 24 March 1918 . The 26 - year - old Galician had served as commander of for four months in 1917 . Dürrial led " U - 21 " out of Pola on 1 June for Cattaro , making stops en route at Arbe and Novigrad for repairs to the gyrocompass . On 16 July , while conducting patrols off the Albanian coast , a piston in " U - 21 " s diesel engine broke and Dürrial put in at Djenovic . On 25 July , " U - 21 " was towed to Pola , where she remained until the end of the war . She was ceded to Italy as a war reparation and scrapped in 1920 . Like all of her sister boats , " U - 21 " had no wartime successes .