Early in 1917, when our mercantile losses were very heavy, some submarines were withdrawn from the Harwich and Humber districts and formed into a flotilla off the coast of Ireland for this form of operation.

Some risk had to be accepted in thus reducing our submarine strength in southern waters.

At the same time some Grand Fleet submarines were organized into a watching patrol in the area off the Shetland Islands, through which enemy submarines were expected to pass.

The watch off the Horn Reef and in the Heligoland Bight, which had previously been in force, was also maintained.
A little later the submarine flotilla off the Irish coast was strengthened, and a regular patrol instituted near the North Channel between Ireland and Scotland.

The next step was the withdrawal of some "C" Class submarines from coastal work on our east coast to work in the area between England and Holland near the North Hinder Lightship, a locality much frequented by enemy submarines on passage.