The Peak of German Unrestricted Submarine Warfare

The German blockade of the British Isles, the so-called Sperrgebiet, or “prohibited area,” might be described as a rectangle with cut corners. It ran from 20 miles from the Dutch coast to the Terschelling light vessel, then north to Utsire off the Norwegian coast, and then northwest to 62° N at its most northerly point, dipping to 3 miles south of the Danish-owned Faeroe Islands. It reached its most westerly point at 20° W before angling back to the Continent 20 miles off Cape Finisterre and then extending 20 miles off the neutral Spanish coast to the French frontier. There was also a prohibited zone in the Arctic Ocean, notably the approaches to Archangel and the Kola Peninsula. The Germans declared the waters in the Sperrgebiet closed to traffic, and that all neutral ships entering them would do so at their own risk. The Germans offered to permit one American steamer per week to proceed to Falmouth, provided its hull was marked with prominent red and white vertical stripes and it flew red-and-white-checked flags at each masthead. A daily Dutch paddle steamer with the same markings could also sail between Flushing and Harwich.

The entire Mediterranean was also a Sperrgebiet, except for the area west of a line running southeast from near the mouth of the Rhône to a point approximately 60 miles off the French North African coast. There was also a 20-mile-wide corridor running through the Mediterranean to Cape Matapan and Greek territorial waters. Unarmed neutral vessels were allowed in these waters, although subject to prize rules. The exceptions catered to the maritime needs of neutral Spain and then-neutral Greece. The Germans eliminated the corridor in November 1917.

The Germans soon paid the diplomatic price for their 1 February resumption of unrestricted submarine warfare. President Wilson of the United States felt mere diplomatic protests would no longer suffice, and on 3 February the United States severed relations with Germany. The president was still not convinced war was a foregone conclusion, but German action served to make it inevitable. At the end of February, the president learned of the Zimmermann telegram. This proposal by the German foreign secretary for a German-Mexican and possibly German-Japanese alliance in the event of war with the United States seemed to furnish further proof of Germany’s aggressive intentions. Its interception and disclosure were handled in a masterful fashion by British Intelligence. The inevitable sinkings by submarines also occurred. The Cunard liner Laconia (18,099 tons) was torpedoed and sunk by U.50 160 miles northwest of Fastnet on 25 February. The loss of life was relatively small among the 292 aboard, but there were three to four Americans among the twelve dead. There were also at least five American steamers sunk, including the Algonquin torpedoed without warning on 12 March. The German provocations were sufficient to bring the United States into the war. On 2 April Wilson asked Congress for a declaration of war. On 6 April the United States declared war on Germany—but not Austria-Hungary—and the same day seized German ships interned in American ports. The primary question now was whether the German naval and military leaders were correct in their assumption that it would not really matter and that the war would be over before American power could have any significant effect on events.

The priority given by the Germans to submarine construction in 1917 reflects the results of the unrestricted submarine campaign. At first it seemed all the Germans might have hoped for, even if by late spring it was evident the British might not succumb as fast as the Admiralstab’s U-boat enthusiasts had predicted. The losses inflicted by submarines rose from 328,391 tons in January to 520,412 tons in February, 564,497 tons in March, and a staggering 860,334 tons in April. April 1917 represented the peak of German success in the submarine campaign, for Allied losses fell to 616,316 tons in May. They went up somewhat to 696,725 tons in June, but would never again reach the April total. The “exchange rate” went from 53 in February to 74 in March to an astonishing 167 in April. In February, March, and April the Germans lost only nine submarines; two of them succumbed to their own mines rather than British countermeasures. Three months of unrestricted submarine warfare had reduced the world’s tonnage by more than two million tons, nearly 1.25 million tons British. The annual wastage of oceangoing tonnage was nearly 23 percent per year, rising to more than 50 percent per year in the last fortnight of April. The chance of a vessel safely completing a round voyage from the British Isles to a port beyond Gibraltar was now only one in four. The tonnage added through new construction or by transfer from foreign flags was simply insignificant in the face of these losses, and if they had continued at that rate, the British would have been compelled to make peace by November. As Henry Newbolt admitted in the official history, “Everything, indeed, combined to show that the Allies were really in sight of disaster.”

The Germans also succeeded at first in their goal of terrorizing neutral shipping. British, Allied, and neutral ports were filled with neutral ships whose owners ordered them not to sail, and for a few weeks there was a general paralysis of neutral shipping. The British countered the crisis with ruthless measures of their own. They detained all neutral vessels in British ports and permitted them to sail for another Allied port only if they had received assurances they would not be laid up or diverted to a neutral port. Vessels trading with a neutral port were released only if they arranged to return with an approved cargo to a British or Allied port. Finally, in dealing with Dutch or Scandinavian ships, the British followed the so-called ship-for-ship policy in which vessels were allowed to sail only on the arrival in a British port of a similar vessel of the same flag.

The intense British pressure on neutral ships to continue trading with British or Allied ports was of little use if the ships were sunk. The German onslaught was now overwhelming the British system for the defense of trade, which was exposed as totally inadequate. Troopships had been specially escorted or convoyed since the beginning of the war. Commencing in early March 1917, ships carrying cargo termed “of national importance” were given special routes through one of three triangles that had their apexes at Falmouth, Queenstown, and Buncrana. The ships were ordered to enter the base line of the triangle at a designated degree of longitude and relied for protection within the triangle on patrolling destroyers, sloops, and trawlers. The method was far from perfect; there were only about 20 ships to patrol the approximately 10,000 square miles of each triangle. The loss rate was high; from March to June 1917,63, or 7 percent, of the 890 ships routed in this manner were sunk, and in June the loss rate was a disturbing 11 percent. For the great majority of their ordinary shipping the British relied on a system of dispersion and patrolled lanes along coastal routes, which they considered “had sufficed” in 1915 and 1916. Steamers left ports at dusk and made port at dawn, followed dispersed routes far from the main trade routes, and crossed dangerous points in the hours of darkness. Every steamer received its orders from a specially appointed naval officer, and when the number of patrol craft in service had increased to a sufficient point, were directed to follow certain well-defined and closely patrolled routes that, whenever possible, were close to the shore. The Admiralty would act on intelligence of U-boat activity, anticipate the U-boat’s future movements, and divert trade to alternate routes. When all routes appeared to be threatened, the Admiralty suspended all traffic until the submarine had been destroyed or changed its area of operations.

There were flaws in the system; for example, owing to the requirements of secrecy, local commands did not always have the latest intelligence available from intercepts. Ships could be diverted only as they left port, and there was no method of controlling them while they were at sea. Inbound ships on the approach routes would be acting on even older intelligence. Furthermore, while the suspension of traffic might have saved ships from being sunk, it also had the effect of enforcing the German blockade. The very detailed technical history produced by the Admiralty after the war made a significant point: “It is important to realize that the Routing System was not an alternative to direct protection, whether by patrols or convoy, but an auxiliary to such methods; when such methods were not available, owing to lack of ships, the Routing System could only hope to act as a palliative, and could never be a substitute for proper defensive methods.” Finally, there was another fatal flaw in any system of dispersion. However effective dispersion might have been, there were invariably certain focal points where approach and departure routes converged, and here submarines could count on finding attractive targets.

One of Jellicoe’s first actions after he became First Sea Lord at the beginning of December 1916 was to form the Anti-Submarine Division at the Admiralty. While still commander in chief of the Grand Fleet he had advocated that “a Flag Officer of authority” should preside at the Admiralty over a committee or department charged with the exclusive purpose of developing antisubmarine measures and empowered “to follow through suggestions with all speed and press their execution.” Rear Admiral Sir Alexander Duff was its first head, succeeded when he became assistant chief of the naval staff in May 1917 by Captain William W. Fisher.

The question of what should be done to counter the submarines became the major issue of the naval war by the spring of 1917. For a long time the majority of naval officers, and certainly the prevailing opinion at the Admiralty, was in favor of the system of hunting patrols as opposed to escort or convoy work. The latter was considered “defensive,” as opposed to “offensive” hunting patrols in areas where submarines were known to be operating. Hunting patrols were generally considered the proper role for men-of-war and naval officers. The traffic lanes close inshore were patrolled by the auxiliary patrol, converted vessels that entered service in large numbers during the war. Farther out, the approach routes were patrolled by sloops or Q-ships. The general idea was that no merchant vessel attacked by gunfire ought to have far to steam before a patrol vessel arrived to assist. The fitting of merchantmen with defensive armament had also offered hope earlier in the war when statistics indicated they had less chance of being sunk and a greater chance of escape if attacked. The German switch to ruthless underwater attack without warning canceled that advantage. The initial effectiveness of Q-ships also declined once the surprise factor had been lost and the Germans routinely attacked without warning. There is some evidence the Germans made a deliberate effort to destroy Q-ships in 1917, sinking those that were recognized before they had the slightest chance of defending themselves. U-boat commanders became much more proficient at recognizing through periscopes characteristics such as seams for collapsible plates, which betrayed the nature of the ship. No fewer than sixteen Q-ships were lost to submarine attack in 1917.

The idea of hunting patrols with destroyers or sloops patrolling areas where submarines were known to be operating was also attractive, but the results were disappointing. Naval officers who rode to the hounds ashore sometimes even used the metaphors of fox hunting to describe their goals. But they lacked the “hounds” or tools to pick up the “scent.” Science and technology raised some hopes for defeating the submarine when hydrophones of various sorts were introduced. The hydrophones were first developed by Commander C. P. Ryan, who founded the Admiralty Experimental Station at Hawkcraig, which remained the most important hydrophone research center throughout the war. It was not the only one; there were ultimately no fewer than twenty-nine antisubmarine research centers of various sorts in the British Isles and another two run by the British in the Mediterranean. The British established hydrophone stations on shore and eventually fitted with various types of listening devices all sorts of craft, ranging from motor launches to P-boats, trawlers, and destroyers. Special hydrophone hunting units were formed to try to trap a submarine by triangulation. The listening devices generally failed to fulfill the great hopes placed in them. Without entering into the technical details, they were on the whole too primitive to be a serious menace to the submarine. The hydrophone hunting groups might also necessitate all vessels in the area stopping their engines so as to avoid masking the sound of the submarine. Stopping a ship in waters where submarines were known to be operating was hardly an attractive activity for most skippers. After they entered the war, the Americans also lavished a great deal of effort on hydrophones. The results were equally disappointing. Success in the effort to render the oceans transparent was as elusive then as it remains today. The real counter to the submarine offensive was the system of convoys to which the British belatedly turned. Before discussing this, however, it would be well to examine methods on which the British lavished considerable effort with only limited success.

U-boat War in World War One

Published sources
K.Adm. Arno Spindler, Der Handelskrieg mit U-Booten
Erich Gröner, German Warships 1815-1945, vol 2
Robert M. Grant, U-boats Destroyed
Robert M. Grant, U-boat Intelligence
V.E. Tarrant, The U-Boat Offensive 1914-1945
Termote Tomas, Verdwenen in de Noordzee, De Krijger, Erpe-Mere, 1999
Humphreys Ron, The Dover Patrol, Sutton, Stroud, 1998
Eberhard Rössler, Die Unterseeboote der Kaiserlichen Marine
Ryheul Johan, Marinekorps Flandern, Mittler, Hamburg 1997
Gibson & Prendergast, The German submarine war 1914-18
K.Adm Stoelzel, Ehrenrangliste der Deutschen Kaiserlichemarine
Bendert, Harald, Die UB-Boote der kaiserlichen Marine 1914 – 1918
Bendert, Harald, Die UC- Boote der Kaiserlichen Marine 1914-1918