Completion of the barrage in the Strait of Dover

The British chronology of the war records under the date of 3 April 1915 the completion of the barrage in the Strait of Dover, the shortest route from the North Sea to the English Channel. It is hard to point to a single event that would have concluded the work on that day. The barrage had been built in stages since the winter. At the beginning of April its first part, namely the minefields and a belt of indicator nets watched by drifters and destroyers of the Dover Patrol, was already fully in operation. The most ambitious element, a heavy net barrage across the whole strait, was still under construction.

Mines and nets

The first British mines in this area were laid in October 1914. At the insistence of the First Sea Lord, John Fisher, the Admiralty decided on a much larger minefield at the eastern entrance to the strait, in the path of ships sailing along the Belgian coast. Between 4 and 16 February 1915 minelayers laid 3,390 mines there in 17 lines. They were, however, intended above all to stop ships travelling on the surface, and many of them broke from their moorings or exploded of their own accord.

The main weapon against submarines was to be indicator nets, light nets of steel wire which did not stop a U-boat but betrayed its presence. The commander of the Dover Patrol, Rear-Admiral Horace Hood, presented a plan for a defence based on them on 7 February. Between the coasts of England and France, from the shoals in the east of the strait to a line from the Varne buoy to Cap Gris-Nez, an area was marked out in which fishing boats, the so-called drifters, shot their nets. On 23 February ships were forbidden to pass through this area. Shipping had to keep to narrow passages along both shores, where the shallow water was dangerous for submarines. At the same time similar nets were being laid in the North Channel.

The beginnings were very difficult. The nets tore in storms, and the clips joining their sections proved too weak. Between 19 and 21 February the drifters lost 61 nets, and Hood was able to keep only about 9 miles of nets in the water. The First Lord of the Admiralty, Winston Churchill, accused him of poor organisation of the work. The first success came on 4 March, when the submarine U-8 was destroyed in the area of the nets. As more drifters, nets and buoys arrived, the barrage grew. According to the British Naval Staff Monograph, by the middle of April 15–16 miles of nets were being maintained by day, attached to ropes towed by the drifters.

The barrage from Folkestone to Gris-Nez

The Admiralty also wanted to place a much more solid obstacle in the path of the U-boats. On 23 February, at a conference chaired by Churchill, it was decided to bar the whole width of the strait with an anti-submarine net. On 1 March the British Treasury approved £400,000 for the purpose. The barrage was to run from Folkestone across the Varne and Ridge shoals to Cap Gris-Nez, over a length of more than 19 miles, with two passages for ships along both shores. Work began simultaneously from the English and the French side. On 2 April Hood sent a patrol detachment to protect the section under construction off Folkestone.

It was estimated that construction would take at least four months. This proved too optimistic. The heavy wooden booms, weighing several tons each and stretched between floats, offered such great resistance to the strong tidal currents that the mooring chains wore through within a few weeks. Hood’s successor, Admiral Reginald Bacon, recalled that the damage kept pace with the new work, and that remnants of the barrage drifted as far as the coasts of Essex and Belgium. In the end he judged the whole undertaking impracticable, and the Admiralty agreed to abandon it.

The Germans choose the longer route

For all its shortcomings, the lighter barrage of nets and mines did its job. On 6 April the periscope of a submerged submarine tore away two nets of the drifter Hyacinth. The indicator buoy failed and the destroyers were unable to find the U-boat. It was U-33, whose commander reported that on the line between Dover and Gris-Nez he had twice heard a grating noise, as if the boat were sliding over steel cables, and had seen drifters with their nets out. He concluded that the strait was barred by a fixed net. On 10 April the German Admiralstab forbade the submarines of the High Seas Fleet to pass through the strait. From then on they had to sail to the Atlantic around Scotland, which greatly lengthened the voyage and shortened the time for hunting. On 17 April U-32 reported that for several hours it had dragged a piece of net behind it while being chased by destroyers, and it returned home by the northern route.

The British did not know of the German ban. The Admiralty decided that someone else would manage the barrage better, and on 13 April Hood handed over command of the Dover Patrol to Bacon and himself took over a cruiser squadron off the coast of Ireland. The British Naval Staff Monograph later judged that he had left undeservedly, since the Dover net was the only anti-submarine measure of that period to have achieved an undoubted success. In June the Admiralty noted that no ship had been sunk in the English Channel since 15 April.

The closure of the strait was not, however, watertight. Smaller submarines from the bases in Flanders continued to try to get through it. In 1916 the Germans found that it could be crossed on the surface at night, and began to use it again. It was effectively closed only by the deep minefield from Folkestone to Gris-Nez, lit at night by searchlights, laid at the turn of 1917 and 1918.