Still later some submarines were attached to the Portsmouth Command, where, working under Sir Stanley Colville, they had some striking successes; others went to the Dover Command.

The latter were fitted with occulting lights on top of the conning-tower, and were moored at night to buoys in the Dover Net Barrage, in places where enemy submarines were likely to pass, in order that they might have a chance of torpedoing them.

A division of submarines was also sent to Gibraltar, to operate against enemy cruiser submarines working in that vicinity or near the Canaries.

Successes against enemy submarines were also obtained in the latter locality.
Finally, the arrival of some United States submarines enabled the areas in which this form of attack was in force to be still further extended, after the American personnel had been trained to this form of warfare.
There was a great increase in the number of enemy submarines sunk by this method of attack during 1917 as compared with previous years; the number of vessels sunk does not, however, convey a complete appreciation of the effect of this form of anti-submarine warfare.

The great value of it lay in the feeling of insecurity that it bred in the minds of the enemy submarine commanders.