Type XXIII submarine German Type XXIII submarines were the first so - called elektroboats to become operational . They were small coastal submarines designed to operate in the shallow waters of the North Sea , Black Sea and Mediterranean Sea , where larger Type XXI Elektro boats were at risk in World War II . They were so small they could carry only two torpedo es , which had to be loaded externally . As with their far larger brothers — the Type XXI — they were able to remain submerged almost all of the time and were faster than all previous designs worldwide , due to the improved streamlining of their shape , batteries with larger capacity and the snorkel , which allowed the diesel engines to be used while submerged . The Type XXI and XXIII U - boats revolutionized post - war submarine design . Background . When development began on the Type XXI U - boat in late 1942 , it was proposed to simultaneously develop a smaller version incorporating the same advanced technology to replace the Type II coastal submarine . Admiral Karl Dönitz added two requirements : as the boat would have to operate in the Mediterranean and the Black Sea , it had to be able to be transported by rail , and it had to use the standard 53 . 3 - cm torpedo tubes . The development of the Type XXIII was given a high priority , with an emphasis on using existing components as much as possible . To reduce development time , Hellmuth Walter designed the new submarine based on the previous Type XXII prototype . By 30 June 1943 the design was ready and construction began in parallel at several shipyards in Germany , France , Italy and German - occupied Russia . The lead contractor was Deutsche Werft in Hamburg . As with the Type XXI , the Type XXIII was intended to be constructed in sections , various modules being produced by different subcontractors . Some were to be assembled at foreign yards , including " U - 2446 " through " U - 2460 " at the Deutsche Werft yard at Mykolaiv . These were reassigned to the Linzner yard on 1 May 1944 and subsequently cancelled . In the end , circumstances meant that construction was concentrated at Germaniawerft in Kiel and Deutsche Werft in Hamburg , Germaniawerft building 51 and Deutsche Werft 49 . Of the 280 submarines ordered , only 61 entered service , and only 6 ever carried out a war patrol . Description . The Type XXIII had an all - welded single hull , the first submarine to use such a design . It had a fully streamlined outer casing and apart from the relatively small conning tower and a fairing which housed the Diesel exhaust silencer , it had an uncluttered upper deck . In line with Walter ' s design practice , there were no forward hydroplanes , although these were added later . The submarine was propelled by a single three - bladed propeller and steered by a single rudder . As with the Type XXI , the lower section of the figure - of - eight hull was used to house a large 62 - cell battery . In order to allow the boat to be transported by rail , the hull sections had to be limited in size to fit the standard loading gauge . For transportation , the hull was broken into four sections and the bridge was removed . Due to the space restrictions , the forward bow section had to be made as short as possible , which meant that only two torpedo tubes could be fitted and no reload torpedoes could be carried . The torpedoes were loaded by ballasting the submarine down at the stern so that the bow lifted clear of the water and the torpedoes could be loaded directly into their tubes from a barge . The Type XXIII proved to have excellent handling characteristics , and was highly maneuverable both on the surface and underwater . Its crash dive time was 9 seconds , and its maximum diving depth was . Speed submerged was , while surfaced speed was . A submerged speed of could be attained while snorkeling . Construction . The first Type XXIII , " U - 2321 " , was launched from Deutsche Werft in Hamburg on April 17 , 1944 . It was one of six XXIIIs that went on operational patrol around the British Isles in early 1945 . Forty - eight others followed from Deutsche Werft and 13 from Germaniawerft of Kiel . " U - 4712 " was the last one launched , on April 19 , 1945 . Service history . None of the six operational Type XXIIIs — " U - 2321 " , " U - 2322 " , " U - 2324 " , " U - 2326 " , " U - 2329 " and " U - 2336 " — were sunk by the Allies ships but they sank or damaged five ships for a total of 14 , 601 tons . The first war patrol of a Type XXIII began late in the war when " U - 2324 " sailed from Kiel on 18 January 1945 . Although she was to survive the war , she sank no enemy vessels . The first Type XXIII to achieve combat success was " U - 2322 " , commanded by Oberleutnant zur See Fridtjof Heckel . Sailing from a Norwegian base on 6 February 1945 , she encountered a convoy off Berwick , Northumberland , and sank the coaster " Egholm " on 25 February . " U - 2321 " , operating from the same base , sank the coaster " Gasray " on 5 April 1945 off St Abbs Head . " U - 2336 " , under the command of " Kapitänleutnant " Emil Klusmeier , later sank the last Allied ships lost in the European war on 7 May 1945 , when he torpedoed and sank the freighters " Sneland " and " SS Avondale Park " off the Isle of May inside the Firth of Forth . The " Sneland " and the " Avondale Park " were sunk around 23 : 03 , less than an hour before the official German surrender , and the " Avondale Park " was the last merchant ship to be sunk by a U - boat . At the time it was felt that Kapitänleutnant Klusmeier , who was on his first patrol , had deliberately ignored Donitz ' s ceasefire order , however Klusmeier claimed that he had never received the order . Losses . Eight Type XXIIIs were lost to various causes . In early May 1945 , 31 XXIIIs were scuttled by their crews . Twenty surrendered to the Allies and were sunk in Operation " Deadlight " . Only three — " U - 2326 " ( later British submarine " N35 " ) , " U - 2353 " ( later British submarine " N37 " ) , and " U - 4706 " ( later Norwegian submarine " Knerten " ) — survived the war . Postwar . One Type XXIII was allocated to the Soviet Union under the terms of the Potsdam Agreement , and a second unit was reportedly salvaged in 1948 . In 1956 , the German Federal Navy raised two Type XXIII boats , " U - 2365 " ( scuttled in the Kattegat in 1945 ) and " U - 2367 " ( which sank near Schleimünde following a collision with another U - boat ) , and recommissioned them as " U - Hai " ( Shark ) and " U - Hecht " ( Pike ) , with pennant numbers S 170 and S 171 respectively . " U - Hai " sank in a gale off the Dogger Bank in September 1966 , taking 19 of her 20 crewmen with her . Her loss is the greatest maritime disaster that both the Bundesmarine and the Deutsche Marine have suffered during the more than 50 years of their existence . The experience gained from the two recommissioned submarines led to the construction of the Type 206 submarine , which was in use until 2011 . See also . Video from the only located type XXIII - U2359 http : / / www . u2359 . com / ? p = 733