Kapitanleutnant Kurt Tebbenjohanns steered his mine laying submarine UC 44 cautiously up the Irish Sea near Waterford on the South Coast of Ireland. His orders were to lay a minefield in the path of convoys heading for the Liverpool docks.
Disaster struck his boat on 4th August 1917 when it hit one of its own mines and sank. Later the British Navy raised it from the seabed and recovered documents that confirmed how easy it was for U-Boats to pass the existing anti-submarine barriers in the Dover Straits. Many officers had suspected this, although the Admiralty and Admiral Bacon who commanded the Dover patrol in the Straits had rejected it as unlikely. The crew of UC 44 died not knowing that this incident was to lead eventually to sackings at the top of the Admiralty, and a change in anti-submarine tactics in the English Channel.
Soon after February 1917 Germany announced the start of a campaign of unrestricted submarine warfare against merchant shipping. In April Admiral Jellicoe, the First Sea Lord, told his American counterpart that the Germans would win the war unless the sinking of shipping could be stopped. There was panic and inertia in the Admiralty when during August and September shipping losses continued to exceed the rate of replacement. The Admiralty, after two years of argument, then reluctantly agreed to try out a convoy system.
Part of the problem in the Dover Straits was the failure of existing measures to stop U-Boats using it. The main U-Boat bases were at Bruges and Ostend on the Belgium coast. With a maximum of 25-30 days at sea German submarines needed to pass through the Straits to attack shipping heading for British ports along the four main convoy routes from the Western Approaches. Troop ships also crossed the Straits regularly with reinforcements for armies fighting on the Western Front in France and these made a tempting target.
Many different measures were tried to limit losses from U-Boat attacks in the Channel. An early attempt at laying down an anti-submarine net barrage was made in 1915. Panels of 51mm thick steel wire 91 metres long and 18 metres deep were suspended from heavy balks of timber. This barrier was never completed and the attempt was abandoned because the tidal movement was too strong.
A year later another attempt was made to sink a line of indicator nets with lights suspended from buoys 251 metres apart. These lit up if a U-Boat became entangled, enabling patrolling vessels to see and attack it. This attempt was not successful as the nets quickly deteriorated or were torn apart by floating debris. It also relied upon patrols being nearby when the submarine surfaced. From German records it was estimated that from February 1917 until the end of the war the ineffective net barrage in the Dover straits sank only two U-Boats, UC 21 and UB 33.
A deep minefield was laid down across the Straits between Folkestone and Gris Nez and by 1918 mines at varying depths of 40, 60, 80 and 100 feet had been put in place. Patrol boats, drifters and destroyers watched over the minefields, but it was suspected submarines surfaced to cross it and others took their chance below. British mines were spherical with mechanical firing devices and faulty mooring systems, and until the advent of newer H2 and H4 mines many naval officers regarded them as unreliable. A critical German assessment of the British mine was that technically it was not of the highest standard, The success rate of U-Boats being sunk by mines was poor. The Straits were navigated 334 times in 1917 and mines sank only three U-Boats. A competent U-Boat captain found he could easily steer his boat through a minefield at night.
Other measures used to find and destroy U-Boats did not have much success. Kite balloons were flown in the Channel with observers seeking to track the course of submarines under the shallow sea. This method proved costly in the use of ships, was made difficult by the weather and was deemed a total failure. Merchant ships were armed and used as decoys. Known as Q ships their aim was to entice U-Boats to surface and then to sink them by gunfire. After some initial successes U-Boat captains got wise to the deception, stayed under the sea, and torpedoed the ship whether it was a suspected armed vessel or not. After August 1917 and until the end of the war no Q ship sank any submarines.