SM U - 21 ( Germany ) SM U - 21 was one of the most famous U - boat s to serve in the Imperial German Navy in World War I . She was the first submarine to sink a ship with a self - propelled torpedo . She also sank the British battleships and . U - 21 survived the war and sank while under tow by a British warship in 1919 . Outbreak of War . Ordered in 1911 and built by the Imperial Shipyard in Danzig , U - 21 was commissioned in 1913 . In August 1914 at the outbreak of war , she was stationed at Heligoland , as part of the Third Half Flotilla ( Second Flotilla ) . She was commanded by Otto Hersing , who stayed with her until the end of the war . On her third war patrol , she sighted the Royal Navy scout cruiser , off the coast of St Abbs Head in Scotland , on 5 September 1914 . A single torpedo hit the cruiser ' s hull causing a boiler explosion which in turn ignited the ship ' s magazine . " Pathfinder " sank within four minutes and only 18 of her crew of 268 survived . This was the first ship to be sunk by a submarine - launched self - propelled torpedo - the had been sunk in 1864 with a spar torpedo , a simple charge fixed to a pole . U - 21 was also the first U - boat to enter the Irish Sea . On 29 January she shelled the airship shed s on Walney Island off Barrow - in - Furness , but was driven off by shore batteries . In the Mediterranean 1915 - 17 . On 5 June 1915 , U - 21 was transferred to the Mediterranean Sea to support Germany ' s ally , Turkey . During this voyage , she evaded air attack in the Strait of Gibraltar and became the first U - boat to enter the Mediterranean and the first submarine to refuel at sea . Arriving off Galipoli on 25 May , she sighted the pre - Dreadnought battleship , HMS " Triumph " , which was at anchor bombarding Turkish coastal artillery at Gaba Tepe . Approaching to within 300 metres , Hersing fired a single torpedo , which penetrated the " Triumph ' s " torpedo nets and then her hull . She began to list and capsized within 10 minutes ; 73 of her crew of nearly 600 were lost . Most of the British and French battleships were withdrawn , but two days later , it was felt that it would be safe for HMS " Majestic " to resume the bombardment , close inshore and surrounded by transport ships and torpedo nets . " U - 21 " fired two torpedoes through a gap in the barricade and the " Majestic " quickly sank ; 40 of her crew were lost . The U - boat had been spotted by an RNAS aircraft flown by Wing Commander Samson which made several bombing runs . Hersing escaped by diving under the French battleship " Henri IV " ; when the U - boat reappeared later , Samson had run out of bombs and could only fire his rifle at her . " U - 21 " retired to İstanbul . On 4 July she sank the French transport ship " Carthage " and then made her way into the Adriatic Sea , where she joined the " Deutsche U - Flotilla Pola " , based at Cattaro . From 21 September 1915 until 16 October 1916 she was flying the flag of the Austro - Hungarian Navy , which assigned her the number " U - 36 " . On the 8th February 1916 , she sank the French armored cruiser " Amiral Charner " off the Syrian coast . End of the War . In March 1917 she joined the High Seas Fleet in Kiel . The " U - 21 " survived the war and still under Hersing ' s command , accidentally sank on 22 February 1919 in the North Sea while under tow by a British warship . Some accounts maintain that she was scuttle d by Hersing , who took up potato farming after the war . During the war , " U - 21 " sank a total of 40 Allied ships ( 113 , 580 tons ) and damaged two further ships ( 8 , 918 tons ) .