In order to carry out the general policy mentioned, the eastern end of the Straits of Dover had been heavily mined at intervals during the war, and these mines had proved to be a sufficient deterrent against any attempt on the part of surface vessels larger than destroyers to pass through.  Owing to the rise of tide enemy destroyers could pass over the minefields at high water without risk of injury, and they frequently did so pass.  Many attempts had been made to prevent the passage of enemy submarines by means of obstructions, but without much success; and at the end of 1916 a “mine net barrage”—­i.e. a series of wire nets of wide mesh carrying mines—­was in process of being placed by us right across the Straits from the South Goodwin Buoy to the West Dyck Bank, a length of 28 miles, it being arranged that the French would continue the barrage from this position to the French coast.  The construction of the barrage was much delayed by the difficulty in procuring mooring buoys, and it was not completed until the late summer of 1917.  Even then it was not an effective barrier owing to the tidal effects, as submarines were able to pass over it during strong tides, or to dive under the nets as an alternative; it was not practicable to use nets more than 60 feet deep, whilst the depth of water in places exceeded 120 feet.

Deep mines were laid to guard the water below the net, but although these were moored at some considerable distance from the barrage, trouble was experienced owing to the mines dragging their moorings in the strong tide-way and fouling the nets.  One series had to be entirely swept up for this reason.  Many devices were tried with the object of improving this barrage, and many clever brains were at work on it. And all the time our drifters with their crews of gallant fishermen, with Captain Bird at their head, worked day after day at the task of keeping the nets efficient.

In spite of its deficiencies the barrage was believed to be responsible for the destruction of a few submarines, and it did certainly render the passage of the Straits more difficult, and therefore its moral effect was appreciable.  Towards the end of 1917, however, evidence came into our possession showing that more submarines were actually passing the Straits of Dover than had been believed to be the case, and it became a question whether a proportion of the drifters, etc., required for the maintenance of the nets of the barrage should be utilized instead for patrol work in the vicinity of the mine barrage then being laid between Folkestone and Cape Grisnez.  This action was taken, drifters being gradually moved to the new area.

In April, 1916, a net barrage, with lines of deep mines on the Belgian side of the nets, had also been laid along the Belgian coast covering the exits from the ports of Ostend and Zeebrugge as well as the coast between those ports.  These nets were laid at a distance of some 24,000 yards from the shore.  This plan had proved most successful in preventing minelaying by submarines in the Straits of Dover, and the barrage was maintained from May to October, but the weather conditions had prevented its continuance from that date.