Germany’s decision, under pressure from the USA, to end unrestricted submarine warfare in September 1915 did not end Allied shipping losses to U-boats. In the final three months of the war they sank 140 ships of 361,326 tons.
Most of the losses in waters around the British Isles were from mines. The small coastal minelaying U-boats UC1, 3, 5, 6, 7 and 9 , operating from Zeebrugge, laid mines in 16 locations. Seventeen vessels struck mines around Dover and the Nore alone. Mines were laid off Portsmouth in an attempt to disrupt transports to France, one of which sank the destroyer HMS Velox on 25 October.
The minelayers did not escape unscathed. UC6 was damaged in early October: the Germans claimed that this was the result of her being rammed by a destroyer but no British destroyer reported such an incident that month. UC9 left port on 20 October and never returned. Her fate is unknown. UC8 ran aground in Dutch waters on 4 November and was interned.
Mines were laid in 13 different places in November. Their victims included the hospital ship Anglia, which was sunk on 17 November with the loss of about 80 staff and wounded soldiers. Mines closed Boulogne to shipping on 10, 12-14 and 29 November. Lord Kitchener, the Secretary of State for War, was due to travel home from France on the last of those dates and had to go from Dunkirk instead of the more usual Boulogne to Folkestone route.
One raid was carried out by a U-boat in the North Sea in December 1914, with the objective of keeping British escorts that might otherwise have been sent to the Mediterranean in home waters. U24 sank one Belgian and three British steamers during it.
A number of U-boats were transferred to the Mediterranean. U21 arrived at the Austro-Hungarian port of Cattaro, now Kotor, on 5 May. As described here, she sank the British battleships HMS Majestic and Triumph off Gallipoli in late May. The coastal boats UB1, 3, 7, 8, 14 and 15 and the coastal minelayers UC12, 13, 14 and 15 were sent partially assembled by rail and completed at Pola.
In early August U34 and 35 sailed to the Mediterranean, followed by U33 and 39 at the end of the month and later by U38: these boats were all of the U31 class. U21, UB7 and 8 and UC14 and 15 were based at Istanbul, with the others operating from Austro-Hungarian ports. During 1915 U-boats sank 54 British and 38 Allied and neutral ships in the Mediterranean. As well as merchant ships, they sank a number of troopship, starting with the British Royal Edward, torpedoed by UB14 (Oberleutnant Heino von Heimburg) on 13 August whilst bound from Alexandria to Mudros with the loss of 866 lives. UB14 also ambushed and sank the British submarine E20 in the Sea of Marmara as a result of information obtained when the French submarine Turquoise was captured by the Ottomans. Other warship losses to U-boats in the Mediterranean included the Italian submarine Medusa on 10 June and armoured cruiser Amalfi on 7 July, both sunk by UB15, then captained by von Heimburg. The Italian armoured cruiser Guiseppe Garibaldi was sunk by the Austro-Hungarian U-IV on 18 July and the submarine Nereide by the Austro-Hungarian U-V on 5 August.
The Austro-Hungarian navy had begun the war with seven small submarines, named by Roman numerals here to differentiate them from German boats. Their early actions were confined to attacks on French warships. U-IV fired at but missed the armoured cruiser Waldeck-Rousseau on 17 October 1914. On 21 December 1914 U-XII torpedoed and damaged the dreadnought Jean Bart, which was sailing at 9 knots with no destroyer screen despite it being three months since U9 had sunk three British armoured cruisers in a single action. As late as 26 April 1915 the armoured cruiser Leon Gambetta was making only 6.5 knots and had no destroyer screen when she was torpedoed and sunk with the loss of 650 men by U-V, captained by Linenschiffleutnant Georg Ritter von Trapp, whose family were the subject of The Sound of Music. During the war the Austro-Hungarians added only the captured French Curie and a number of German UB boats to their fleet.
One reason to switch U-boats to the Mediterranean was to prevent them damaging German relations with the United States of America by killing Americans. On 7 November, however, the German U38 (Kapitänleutnant Max Valentiner), flying Austro-Hungarian colours because Germany and Italy were not yet at war, sank the Italian liner Ancona, killing over 200 people, including about 20 Americans. The US protested to Austria-Hungary, which promised to pay an indemnity and to punish the boat’s captain, who was not named.
The Germans, who did not want further incidents with the USA, ordered their U-boat captains to observe prize laws in the Mediterranean, meaning that they had to allow the passengers and crew time to evacuate a merchant ship before sinking her. This was mostly complied with until 1917. Valentiner’s U38, however, sank five British and several Allied merchant steamers between 27 December 1915 and 4 January 1916 with the loss of over 500 lives, 334 of them on the liner Persia, sunk on 30 December 1915.
 V. E. Tarrant, The U-Boat Offensive, 1914-1945, pp. 152-53.
 Naval Staff Monograph (Historical) 1926 vol. xv, Home Waters part vi, October 1915 to May 1916. p. 15.
 Ibid., p. 19.
 Ibid., p. 21. and note 2.
 Ibid., p. 22.
 Ibid., p. 24.
 Ibid., p. 23.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918., p. 61.
 Naval Staff vol. Xv. p. 24.
 Gibson, Prendergast, German, p. 61.
 Ibid., p. 71.
 Ibid., pp. 73-79.
 P. G. Halpern, A Naval History of World War I, p. 385.
 Gibson, Prendergast, German, pp. 78-79.