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.