Consequently, with the comparatively small number of patrol craft available, the protection afforded was but slight, and losses were correspondingly heavy.

In the early spring of 1917, Captain H.W.Grant, of the Operations Division at the Admiralty, whose work in the Division was of great value, proposed a change in method by which the traffic should be brought along certain definite "lines" in each approach area.

Typical lines are shown in Chart B.
[Transcriber's note: Chart B is a navigational map of the waters southwest of Ireland, with approach routes marked.] The idea was that the traffic in, say, Approach Route B, should, commencing on a certain date, be ordered by the Routeing Officer to pass along the line Alpha.

Traffic would continue along the line for a certain period, which was fixed at five days, when it would be automatically diverted to another line, say Gamma, but the traffic along Gamma would not commence until a period of 24 hours had elapsed since discontinuance of the use of the line Alpha.

This was necessary in order to give time for the patrol craft to change from one line to the other.
During this period of 24 hours the arrangement for routeing at the ports of departure ensured that no traffic would reach the outer end of any of the approach lines, and consequently that traffic would cease on line Alpha 24 hours before it commenced on line Gamma.