The chances of the success of the hunting flotillas had been promising in the early experiments, and the fitting out of patrol craft and organizing and drilling them, proceeded as rapidly as the vessels could be obtained, but largely owing to the slow production of trawlers it was not until November that the first hunting flotilla fitted with the "fish" hydrophone was actually at work.

The progress made after this date is illustrated by the fact that in December, 1917, a division of drifters, with a "P" boat, fitted with this "fish" hydrophone hunted an enemy submarine for seven hours during darkness, covering a distance of fifty miles, kept touch with her by sound throughout this period, and finished by dropping depth charges in apparently the correct position, since a strong smell of oil fuel resulted and nothing further could be heard of the submarine, although the drifters listened for several hours.

On another occasion in the same month a division of drifters hunted a submarine for five hours.

The number of hydrophones was increased as rapidly as possible until by the end of the year the system was in full operation within a limited area, and only required expansion to work, as was intended, on a large scale in the North Sea and the English Channel.
Meanwhile during 1917 _directional_ hydrophones, which had been successfully produced both by Captain Ryan and by the Board of Invention and Research, had been fitted to patrol craft in large numbers, and "hunting flotillas" were operating in many areas.

A good example of the working of one of these flotillas occurred off Dartmouth in the summer of 1918, when a division of motor launches fitted with the Mark II hydrophone, under the general guidance of a destroyer, carried out a successful attack on a German submarine.