[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.  After a further period of five days the line would again change automatically.

It was necessary that Shipping Intelligence Officers should have in their possession the orders for directing traffic on to the various lines for some considerable time ahead, and the masters of ships which were likely to be for some time at sea were informed of the dates between which the various lines were to be used, up to a date sufficient to cover the end of their voyage.  There was, therefore, some danger of this information reaching the enemy if a vessel were captured by a submarine and the master failed to destroy his instructions in time.  There was also some danger in giving the information to neutrals.

However, the system, which was adopted, did result in a reduction of losses during the comparatively short time that it was in use, and the knowledge that patrol craft on the line would be much closer together than they would be in an approach area certainly gave confidence to the personnel of the merchant ships, and those who had been forced to abandon their ship by taking to the boats were afforded a better chance of being picked up.

Various arrangements were in existence for effecting rapidly a diversion of shipping from one route to another in the event of submarines being located in any particular position, and a continual change of the signals for this purpose was necessary to guard against the possibility of the code being compromised by having fallen into enemy hands, an event which, unfortunately, was not infrequent.

Elaborate orders were necessary to regulate coastal traffic, and fresh directions were continually being issued as danger, especially danger from mines, was located.  Generally speaking, the traffic in home waters was directed to hug the coast as closely as safe navigation permitted.  Two reasons existed for this, (a) in water of a depth of less than about eight fathoms German submarines did not care to operate, and (b) under the procedure indicated danger from submarine attack was only likely on the side remote from the coast.