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.
Here is an example of the instructions for passing up Channel: _From Falmouth to Portland Bill._--Hug the coast, following round the bays, except when passing Torbay.

(Directions followed as to the procedure here.) _From Portland Bill to St.Catherines._--Pass close south of the Shambles and steer for Anvil Point, thence hug the coast, following round the bays.
And so on.
As it was not safe navigationally to follow round the bays during darkness, the instructions directed that ships were to leave the daylight route at dusk and to join the dark period route, showing dimmed bow lights whilst doing so.
Two "dark period routes" were laid down, one for vessels bound up Channel, and another for vessels bound down Channel, and these routes were some five miles apart in order to minimize the danger of collision, ships being directed not to use their navigation lights except for certain portions of the route, during which they crossed the route of transports and store ships bound between certain southern British ports (Portsmouth, Southampton and Devonport) and French ports.
Routes were similarly laid down for ships to follow when navigating to or from the Bristol Channel, and for ships navigating the Irish Sea.
Any system of convoy was at this time out of the question, as neither the cruisers to marshal the convoy to the submarine area, nor the destroyers to screen it when there, were available.
There was one very important factor in the situation, viz., the comparative rate at which the Germans could produce submarines and at which we could build vessels suitable for anti-submarine warfare and for defence of commerce.

The varying estimates gave cause for grave anxiety.
Our average output of _destroyers_ was four to five per month.