Following the Allied landings on Gallipoli on 25 April 1915 a force of British and French pre-dreadnought battleships and cruisers gave fire support to the troops ashore. It was known that German U-boats were on their way to the Dardanelles.
The first to arrive was U21, captained by Kapitänleutnant Otto Hersing. He had been in command of her on 5 September 1914 when she sank HMS Pathfinder, the first time that a submarine had sunk a ship with a motor torpedo.
U21 had sailed from Germany on 25 April 1915. She was supposed to re-fuel from a ship on the way, but its oil was unsuitable; her tanks were almost empty when she reached the Austro-Hungarian port of Cattaro in the Adriatic on 13 May. On 5 May she was spotted by HM Torpedo Boat 92 near the island of Alborán in the western Mediterranean, but attempts by U21 to torpedo 92 and by 92 to ram U21 failed. The Germans also sent ten smaller submarines of the UB type by rail in sections to the Adriatic. Three had arrived and were being assembled when U21 reached Cattaro, now Kotor.
Reports of the imminent arrival of U21 meant that on 17 May the number of battleships operating offshore was reduced from seven to four at Cape Helles and four to tow at Anzac Cove. Anti-submarine precautions were taken, including nets and destroyer patrols.
U21 appeared at the Dardanelles on 25 May. Around 7:30 am the Grimsby trawler Minoru spotted her and alerted the destroyers by giving blasts on her siren. HMS Harpy made for U21, which seemed to be heading for the French battleship St Louis, but did not attack. The British Official History speculates that U21 may have made off because of the presence of Harpy, but also notes that she had orders to attack the ships off Anzac.
Her periscope was spotted passed between the battleships HMS Swiftsure and Agamemnon; the former fired at her, but missed. Just after 10 am she fired a torpedo at the battleship HMS Vengeance, which evaded it. All available destroyers and trawlers were ordered to search for the submarine. Her periscope was spotted and fired on four times, and a destroyer passed over her, but she was too deep to be damaged. Since U21 was heading north the balloon ship HMS Manica and transports in that direction were ordered away. The battleship HMS Canopus, which was zigzagging and escorted by the destroyer HMS Ribble, spotted U21, but was not attacked.
About 12:25 the destroyer HMS Chelmer, which was patrolling round the battleship HMS Triumph at 15 knots off Anzac, spotted the wash of U21’s periscope 500 yards off Triumph. Chelmer made for it and Triumph opened fire, but Triumph was quickly hit by a torpedo that had passed through her anti-torpedo nets as if they had been ‘a spider’s web.’
Triumph’s watertight doors were all closed, but she capsized ten minutes after she had been hit. However, the order to abandon ship was given as soon as her list became dangerous, with the result that over 500 men were saved; only three officers and 70 men were lost, according to the Official History. The large number of survivors was thanks to the ‘skilful handling of the Chelmer‘ by her captain, Lieutenant-Commander Hugh England.
The sinking of Triumph resulted in the Allied battleships being withdrawn to Imbros, a protected harbour. However, the next day Vice Admiral Sir John de Robeck, the naval commander, decided that there should always be two off Helles, one of them in the Aegean and the other in the Dardanelles.
On 26 May the French battleship Jauréguiberry spotted a periscope 100 yards away whilst off the entrance to the Dardanelles. She was in the process of turning and ran over the submarine. Some of her crew thought that she had rammed the submarine, but this was not the case.
At 6:45 am on 27 May U21’s periscope appeared 400 yards from the battleship HMS Majestic. She fired on it, missing. Hersing fired a torpedo which went through a gap in the transports, passed through the nets and struck Majestic with what the British Official History called ‘a shot the best might envy.’ She capsized in nine minutes, but did not sink because the water was only nine fathoms deep. Instead, she rested on her foremast, with her keel exposed, leaving her ‘looking like a stranded whale. All but 43 of her crew were saved. The men lost on both battleships sunk by U21 are listed on naval-history.net.
The Allied battleships and cruisers were withdrawn to Imbros. They now provided fire support only when specifically requested by the army, with the day to day work being carried out by destroyers and monitors.
The Official History noted that U21’s feats had ‘grave moral effects’, both positively for the Ottomans and negatively for the Allies. There was also a considerable material impact, as supply arrangements were further complicated and ‘continuous battleship support for the army was no longer possible.
Tim Travers, however, contends that naval fire was not accurate enough to deal with ‘precise targets like trenches, machine guns or enemy batteries.’ Henceforth, the Allied troops were dependent on shore based artillery. Many officers subsequently described this as being inadequate, but Travers suggests that they are comparing Allied artillery support at Gallipoli with the volumes of fire that they later experienced on the Western Front. He notes that ‘comments at the time often praised the artillery.’
Hersing kept his boat in the area for two more days, before putting into an Ottoman coast station. He had difficulties entering the Straits because of a whirlpool, but reached Istanbul on 5 June with only half a ton of oil left to ‘scenes of great enthusiasm.’ He was later awarded Germany’s highest decoration, the Pour le Mérite. He and his boat survived the war.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. ii, p. 384 and footnote 1.
 N. Steel, P. Hart, Defeat at Gallipoli (London: Papermac, 1995), p. 184.
 Corbett, Newbolt, Naval. vol. iii, p. 28.
 Ibid. vol. iii, p. 29.
 Ibid. vol. iii, p. 29.
 Ibid. vol. iii, pp. 29-30.
 Steel, Hart, Defeat, p. 185.
 Corbett, Newbolt, Naval. vol. iii, p. 30.
 Ibid. vol. iii, p. 31.
 Steel, Hart, Defeat, p. 185.
 Ibid., p. 186.
 Corbett, Newbolt, Naval. vol. iii, p. 31.
 Ibid. vol. iii, p. 30.
 T. Travers, Gallipoli, 1915 (Stroud: Tempus, 2001), p. 226.
 Ibid., p. 227.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918. (London: Constable, 1931), p. 72.