The motor launches were employed for anti-submarine work, fitted with hydrophones, and worked in company with drifters and torpedo-boat destroyers, or in minesweeping in areas in which their light draught rendered it advantageous and safer to employ them instead of heavier draught vessels to locate minefields, and in the Dover area they were largely used to work smoke screens for operations on the Belgian coast.
As the convoy system became more general, so the work of the small craft in certain areas altered from patrol and escort work to convoy duty.
These areas were those on the East Coast and north-west of Scotland through which the Scandinavian and East Coast trade passed, and those in the Channel frequented by the vessels employed in the French coal trade.
The majority of these ships were of comparatively slow speed, and trawlers possessed sufficient speed to accompany them, but a few destroyers of the older type formed a part of the escorting force, both for the purpose of protection and also for offensive action against submarines attacking the convoys, the slow speed of trawlers handicapping them greatly in this respect.
The difficulty of dealing with submarines may be gauged by the enormous number of small craft thus employed, but a consideration of the characteristics of a submarine and of the great volume of traffic passing up and down our coasts will assist in a realization of the varied and difficult problems set to the British Navy.
For instance, the total number of vessels passing Lowestoft during the month of April, 1917, was 1,837 British and Allied and 208 neutral, giving a _daily_ average of 62 British and Allied and 7 neutral ships; and as Admiral Sir Reginald Bacon has mentioned in his book, "The Dover Patrol, 1915-17" (page 51), an average of between 80 to 100 merchant vessels passed Dover daily during 1917.

A study of these figures gives some idea of the number of targets offered daily to ordinary submarines and minelaying submarines in two of the areas off our coasts.

When it is borne in mind that the Germans had similar chances of inflicting heavy losses on our mercantile marine all round the coasts of the United Kingdom, and that it was obviously impossible to tell where an underwater attack would take place, it will be realized that once submarines reached our coasts, nothing short of an immense number of small craft could deal satisfactorily with the situation, and afford any degree of protection to trade.

Minelaying by submarines was a particularly difficult problem with which to deal; the enemy frequently changed his methods, and such changes when discovered involved alterations in our own procedure.

Thus for some time after the commencement of minelaying by submarines, the whole of the mines of one submarine would be laid in a comparatively small area.