On 21 October 1918 Admiral Reinhard Scheer, the German Chief of the Admiralty Staff, ordered all U-boats to return to base. This ended the German campaign of unrestricted submarine warfare, which meant sinking merchant ships without warning, that had begun on 1 February.
Scheer’s orders were obeyed by almost all U-boat captains. UC 74 (Oberleutnant Hans Schüler) sank the 85 ton Greek sailing ship Aghios Gerasimos by gunfire in the Eastern Mediterranean on 23 October and a number of merchant ships were sunk by mines laid by UC 74 after 21 October off the Suez Canal. Other ships were sunk after 21 October by mines laid earlier.
Admiral Henning von Holtzendorff, Scheer’s predecessor, had argued that the Germans had to break the British economy in order to win the war and this could be achieved by destroying 600,000 tons of merchant shipping per month for five months, which would cut British trade by 39% within five months. The Germans were aware that this might bring the USA into the war against them but thought that it would not be able to replace the lost merchant shipping and that there would be insufficient transports to take US troops to Europe.
The U-boats’ best month before February 1917 was October 1916, when they sank 341,363 tons of merchant shipping according to German records. There are some differences between British and German figures but they are not large. German ones are slightly higher over the whole war but are lower in some months. V. E. Tarrant argues that the German numbers are the more accurate and they are quoted here.
As shown in the table below, the Germans achieved their target of sinking 600,000 tons of shipping in only three months: April, May and June 1917. However, the 860,334 tons sunk in April meant that the average for the first seven months of unrestricted submarine warfare was 612,310 per month. Von Holtzendorff’s belief that this level of loss for five months would break the British economy was proved to be wrong.
These figures are for ships from all countries, not just Britain sunk by U-boats, including by mines laid by U-boats, but excluding ships sunk by surface raiders and aircraft.
Source: Tarrant, V. E., The U-Boat Offensive, 1914-1945, (London: Arms and Armour, 1989), pp. 152-53. Originally from Der Krieg zur See, 1914-15, vol v: Der Handelskreig mit U-booten.
The losses were stemmed by the adoption of various counter-measures of which the introduction of convoys was the most important. Convoys had been used in previous wars and for troopships in this one. The Admiralty opposed them for a number of reasons:
Code breaking and radio direction finding could enable it to track U-boats. This had worked against surface raiders but was ineffective against U-boats because they could not be detected once submerged except by spotting their periscopes or torpedo tracks
An armed merchantman could avoid torpedo attack by zigzagging and fight off a gun attack by a surfaced U-boat.
Merchantmen would not be able to keep station, especially at night. In fact convoys were in areas where U-boat attack was unlikely for most of their journeys, giving them time to practice formation sailing.
Ships would be delayed in sailing. This was false as ships were already delayed by reports of U-boats near their ports.
Convoys would give U-boats an attractive target as they assembled in open seas. The US entry into the war meant that they could assemble in US ports. They could in any case have done so at Bermuda or Halifax, Nova Scotia.
Convoys would concentrate targets together, so would require to be escorted by as many as one destroyer per merchant ship. Allocating this number of destroyers to convoy escort would mean that the Grand Fleet would have to stay in port. Only eight to 10 escorts were actually needed for a 20 ship convoy and smaller ships than destroyers proved to be adequate convoy escorts.
It was actually almost as hard for a U-boat to spot a convoy as it was for it to find an independent ship. According to Vice Admiral Sir Arthur Hezlet, a British WWII submarine captain, a ship could be spotted from 10 miles away. A 20 ship convoy would be two miles wide so could be seen from 11 miles away. Convoys therefore meant that U-boats would find fewer targets, not more. Even if a U-boat spotted a convoy, it would probably have time to torpedo only one, at most two of its ships and would be counter attacked by the escorts. The British also used radio direction finding to avoid U-boats.
This was confirmed by Karl Dönitz, commander of Germany’s U-boats in WWII and a U-boat captain in WWI. He wrote that ‘[t]he oceans at once became bare and empty…for long periods…the U-boats…would see nothing at all.’ When a convoy did appear, the ‘U-boat might well sink one or two of the ships, or even several; but that was but a poor percentage of the whole.’ It would also be vulnerable to counter-attack by the convoy’s escort.
The British had been using convoys in the war. As well as troopships, ships sailing between England and the Netherlands had been convoyed since July 1916 because of the risk of destroyer attack. Ships carrying coal from Britain to France were convoyed from February 1917 at the request of the French. The Admiralty insisted on calling the convoys controlled sailings.
The French coal convoys had been organised by Commander Reginald Henderson. He obtained figures from the Ministry of Shipping that showed that the number of ocean going ships sailing to and from the UK was much lower than realised.
The Admiralty had included short voyages made by small, coastal ships in its reports of the number of ships calling at British ports in order to make the Germans think that the sea traffic to the UK was too great for the U-boats to destroy. The 2,500 voyages each way per week claimed was too many to convoy but the true number of ocean going ships arriving and leaving was 120 to 140 per week.
On 25 April 1917 the War Cabinet decided that the Prime Minister, David Lloyd George, should visit the Admiralty in order to find out what it was doing about U-boats. Lloyd George and his colleagues were in favour of convoys but the Admirals had resisted them. However, the next day Vice Admiral Alexander Duff, head of the Admiralty’s Anti-Submarine Division, who now had Henderson’s figures, submitted a paper to the Admiralty stating that ‘[i]t seems to me evident that the time has arrived when we must be ready to introduce a comprehensive scheme of convoy at any moment.’
Convoys would need 70 escorts. Only 30 destroyers were currently available but more would be built by the time that convoys could be fully organised and USN ships would soon be available.
The introduction of convoys did not eliminate shipping losses but it reduced them sharply, from 5% of sailings in the UK in April 1917 to 0.5% by the end of the war. Some ships continued to sail independently and suffered heavy losses.
A large number of aircraft were allocated to anti-submarine warfare in home waters: an average of 189 aeroplanes, 300 seaplanes and 75 airships in the last six months of the war, with an average of 310 being available each day. Their bombs were too small to sink U-boats but they had a significant deterrent effect. In 1918 U-boats attacked only six convoys with air escorts, sinking only three ships. Over the whole war only five ships were sunk in convoys with both air and surface escort.
The Americans and British were also able to build new ships more quickly than the Germans had forecast. The US entered the war with a relatively small shipbuilding industry but rapidly built one, including constructing accommodation for workers and public transport to take them to work. The British made the Admiralty responsible for both naval and merchant shipbuilding, so that it could make the necessary trade-offs. These including suspending construction of three of the four Hood class battlecruisers in order to build more merchantmen. Sir Eric Geddes, a businessman who had solved railway supply problems behind the Western Front was made Controller of the Admiralty, putting him in charge of the materiel side of the navy. This job was normally done by the Third Sea Lord, an admiral. Ships were built to standardised design.
Merchant shipping construction (tons)
Source: Friedman N., Fighting the Great War at Sea: Strategy, Tactics and Technology (Barnsley: Seaforth, 2014), pp. 284-86.
About 1,700,000 tons of Austrian and German merchant shipping interned in US ports were requisitioned by the US government. They included a number of liners, which transported US troops to France. Amongst them was the giant Vaterland, renamed the Leviathan in US service.
The U-boats failed to stop over 2,000,000 US soldiers sailing to Europe, 10% of them on board the Leviathan. Two eastbound troop transports were sunk by U-boats and another accidentally by collision. Three returning to the USA to collect more troops were also sunk. The total of US soldiers killed by U-boats was less than the 700 who died as a result of influenza caught on the voyage.
German U-boat construction in 1918 was only able to keep pace with losses. They had 224 [Sondhaus] or 226 [Gibson and Prendergast] under construction at the end of the war and completed 13 in October 1918. The hoped to complete 30 a month in 1919. However, they were struggling to man their new boats. U-boats needed experienced captains and fully trained crew: the six most successful U-boat captains in terms of tonnage sunk and 14 of the leading 18 survived the war. Consequently, the Germans refused to cut the training programme given to men before they were assigned to an active U-boat. About two dozen of those under construction at the end of the war could have been commissioned had trained personnel been available.
The U-boats inflicted severe damage to Allied shipping but their attempt to blockade the UK failed and brought the USA into the war against Germany.
 L. Sondhaus, German Submarine Warfare in World War I: The Onset of Total War at Sea (Boulder MD: Rowman & Littlefield, 2017). Kindle edition, location 4855.
 Ibid. Kindle locations 4861-68.
 D. Steffen, ‘The Holtzendorff Memorandum of 22 December 1916 and Germany’s Declaration of Unrestricted U-Boat Warfare’, Journal of Military History 68, no. 1 (2004).
 V. E. Tarrant, The U-Boat Offensive, 1914-1945 (London: Arms and Armour, 1989), pp. 152-53.
 N. Friedman, Fighting the Great War at Sea: Strategy, Tactics and Technology (Barnsley: Seaforth, 2014), pp. 274-78.
 Original source A. R. Hezlet, The Submarine and Sea Power (London,: 1967), pp. 94-95; Quoted in A. J. Marder, From the Dreadnought to Scapa Flow; the Royal Navy in the Fisher Era, 1904-1919, 5 vols (London: Oxford University Press, 1961-70). vol v, pp. 88-89.
 Original source K. Doenitz, Memoirs: Ten Years and Twenty Days (London,: 1959), p. 4; Quoted in Marder, From. vol. v, p. 89.
 Friedman, Fighting, pp. 276-77.
 C. E. Fayle, Seaborne Trade, 3 vols (London: HMSO, 1920). vol. iii, p. 100.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols (London: HMSO, 1938), p. vo. v. p. 18.
 Ibid. vol. v, p. 19.
 Ibid. vol. v, pp. 18-19.
 Marder, From. vol. v, p. 87.
 Ibid. vol. v, pp. 91-93.
 Friedman, Fighting, pp. 284-86.
 Ibid., p. 286.
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), p. 437.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918 (London: Constable, 1931), p. 363; Sondhaus, German. Kindle locations 4497-4513.