Previous entries in this blog have dealt with the several sinkings of British cruisers by German U-boats: HMS Pathfinder by U21 on 5 September 1914, HMS Aboukir, Cressy and Hogue by U9 on 22 September and HMS Hawke on 15 October 1914, also by U9.
British submarines also scored successes in the early stages of the war, with E9 sinking the German cruiser SMS Hela on 13 September and B11 the Ottoman pre-dreadnought battleship Mesudiye on 13 December 1914. The first loss of a submarine to a warship had come as early as 9 August, when HMS Birmingham rammed and sunk U15.
The main impact of submarines in the rest of the war and in the Second World War was against merchant shipping, although they continued to sink warships. In the early stages of the First World War, however, they were used mainly against warships.
The rules of cruiser warfare required raiders to stop and search enemy merchant ships. They could be sunk or captured only if their cargoes contained either war materials (absolute contraband) or items such as food or fuel that had peaceful uses but were intended for the enemy’s military (conditional contraband conditional contraband). The safety of the crews of merchant ships that were sunk had to be ensured, making it difficult for submarines to conduct war against merchant shipping within the rules. They had no space for prisoners and no spare men to act as prize crews for captured ships.
In 1914 Germany possessed only 24 operational boats. Four were used for training and 16 were under construction. Before the war Kapitänleutnant Blum of the German Navy had estimated that 222 U-boats would be needed to successfully carry out a war against British commerce under international law. On 8 October 1914 the commander of Germany’s U-boats, Korvettenkapitän Hermann Bauer, urged that German U-boats should attack British commerce on the grounds that the British had already violated international law at sea, but his advice was not acted upon until 1915.
The first sinking of a merchant ship by a submarine came on 20 October, when Oberleutnant Johannes Feldkirchner’s U17 stopped the steamship Glitra, which was carrying a cargo of coal, coke, oil and general goods from Grangemouth to Stavanger, 14 miles off the Norwegian coast. The crew were ordered to take to their boats and their ship was sunk. U17 towed the boats towards the coast, before a pilot boat then took over. A Norwegian torpedo boat appeared shortly afterwards.
The Naval Staff Monograph, written by Royal Navy officer after the war for internal use, notes that there was a significant difference in the British and German Prize Regulations. Both gave officers significant leeway in deciding whether or not to sink enemy merchantmen. However, the British one warned that naval officers who sank merchant ships ‘without good cause’ might find themselves liable for the compensation due to the owners, while the German one allowed the destruction of a ship ‘if it seems inexpedient or unsafe to bring her in.’
There were few sinkings of merchant ships in 1914. On 26 October, Kapitänleutnant Rudolf Schneider’s U24 torpedoed the 4,590 ton French steamer Amiral Ganteaume, but she did not sink. She was carrying 2,500 Belgian refugees, 40 of whom were killed. This was described as ‘barbarous’, an ‘outrage’ and an ‘atrocity’ by British post war authors. However, Schneider may have thought that she was a troop ship. Whether he did or not, the incident showed the potential political and diplomatic implications of submarine warfare against merchant shipping.  The Naval Staff Monograph argues that he fired without ascertaining whether the people crowding her decks were civilians or soldiers.
Only two more Allied merchant ships were sunk by U-boats before the end of the year. Oberleutnant Otto Hersing’s U21 stopped the steamers Malachite (718 tons) on 21 October and Primo (1366 tons) five days later. Both were sunk by gunfire after the crews had been given time to abandon ship.
The most important actions of U-boats continued to be against warships. Both sides had submarines patrolling outside enemy bases and their own boats hunting the enemy ones. On 18 October Kapitänleutnant Bernd Wegener’s U27 spotted the British submarine E3 on the surface in the Helgoland Bight.
The submerged German boat approached to within 300 yards of the British one, using the rays of the sun to make it hard for the British to spot her periscope. The personnel on the conning tower were apparently all looking the other way. U27 then fired a torpedo into E3, which sank immediately. Four survivors were spotted in the water, but Wegener did not surface, fearing that there might be other British submarines nearby, and all 28 men on board E3 were lost. She was the first submarine ever to be sunk by another submarine.
U27 achieved another first 13 days later, when she sank the elderly cruiser HMS Hermes, which had been converted to a seaplane carrier before the war, and was being used as a aircraft transport. This was the first sinking of an aviation ship by a submarine. The quick arrival of other British ships meant that only 21 men were lost.
Apart from E3, the British Empire lost two submarines in 1914: the Australian AE2, probably to an accident, off Rabaul on 14 September; and D3 to a mine, probably British, during the Yarmouth Raid on 3 November.
The German lost four boats in addition to U15 in 1914: U18 was rammed by first the trawler Dorothy Gray and then the destroyer Garry inside Scapa Flow on 23 November and was so badly damaged that she had to be scuttled; U13 in August and U5 and U11 in December failed to return from patrols, presumably having struck mines.
The ships sunk by submarines in the North Sea were mostly old. The only dreadnought to be torpedoed by a submarine in 1914 was the French Jean Bart, which was damaged, but not sunk, by the Austro-Hungarian boat U12 in the Adriatic on 21 December.
 V. E. Tarrant, The U-Boat Offensive, 1914-1945 (London: Arms and Armour, 1989), p. 7.
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), p. 291.
 Ibid., p. 292.
 Naval Staff Monograph (Historical) vol. xi, Home Waters part ii, September and October 1914. p. 113. gives Glitra’s displacement as being 526 tons but most others sources follow the official history in saying 866 tons; C. E. Fayle, Seaborne Trade., 3 vols. (London: HMSO, 1920). vol. i, p. 285.
 A. S. Hurd, The Merchant Navy, 3 vols. (London: HMSO, 1921). vol. i, p. 268.
 Fayle, Seaborne. vol. i, p. 285.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918. (London: Constable, 1931), p. 15.
 Halpern, Naval, p. 292.
 Naval Staff vol. Xi. p. 144.
 Gibson, Prendergast, German, p. 17.