The matter was kept constantly under review. In February, 1917, the Germans commenced unrestricted submarine warfare against merchant ships of all nationalities, and as a consequence our shipping losses, as well as those of Allied and neutral countries, began to mount steadily each succeeding month. The effect of this new phase of submarine warfare is best illustrated by a few figures.
During the last four months of 1916 the gross tonnage lost by submarine attack alone gave the following monthly average: British, 121,500; Allies, 59,500; neutrals, 87,500; total, 268,500.
In the first four months of 1917 the figures became, in round numbers:
British. Allies. Neutrals. Total.
January 104,000 62,000 116,000 282,000
February 256,000 77,000 131,000 464,000
March 283,000 74,000 149,000 506,000
April 513,000 133,000 185,000 831,000
(The United States entered the war on April 6, 1917.)
NOTE.—In neither case is the loss of fishing craft included.
It will be realized that, since the losses towards the end of 1916 were such as to give just cause for considerable anxiety, the later figures made it clear that some method of counteracting the submarines must be found and found quickly if the Allied cause was to be saved from disaster.
None of the anti-submarine measures that had been under consideration or trial since the formation of the Anti-Submarine Division of the Naval Staff in December, 1916, could by any possibility mature for some months, since time was necessary for the production of vessels and more or less complicated matériel, and in these circumstances the only step that could be taken was that of giving a trial to the convoy system for the ocean trade, although the time was by no means yet ripe for effective use of the system, by reason of the shortage of destroyers, sloops and cruisers, which was still most acute, although the situation was improving slowly month by month as new vessels were completed.
Prior to this date we had already had some experience of convoys as a protection against submarine attack. The coal trade of France had been brought under convoy in March, 1917. The trade between Scandinavia and North Sea ports was also organized in convoys in April of the same year, this trade having since December, 1916, been carried out on a system of "protected sailings." It is true that these convoys were always very much scattered, particularly the Scandinavian convoy, which was composed largely of neutral vessels and therefore presented exceptional difficulties in the matter of organization and handling. The number of destroyers which could be spared for screening the convoys was also very small. The protection afforded was therefore more apparent than real, but even so the results had been very good in reducing the losses by submarine attack. The protection of the vessels employed in the French coal trade was entrusted very largely to trawlers, as the ships composing the convoy were mostly slow, so that in this case more screening vessels were available, although they were not so efficient, being themselves of slow speed.
For the introduction of a system of convoy which would protect merchant ships as far as their port of discharge in the United Kingdom, there were two requirements: (a) A sufficient number of convoying cruisers or armed merchant ships, whose role would be that of bringing the ships comprising the convoy to some selected rendezvous outside the zone of submarine activity, where it would be met by the flotilla of small vessels which would protect the convoy through the submarine area. It was essential that the ships of the convoy should arrive at this rendezvous as an organized unit, well practised in station-keeping by day, and at night, with the ships darkened, and that the vessels should be capable also of zigzagging together and of carrying out such necessary movements as alterations of course, etc.; otherwise the convoy could not be safely escorted through the danger area. (b) The other essential was the presence of the escorting flotilla in sufficient strength.