The introduction of the convoy system rendered the provision of anti-submarine protection at ports of assembly a matter of great importance, owing to the very large number of vessels that were collected in them. Some of the ports were already in possession of these defences, but amongst those for which net protection was prepared and laid during 1917 were Halifax, Sydney (Cape Breton), Falmouth, Lamlash, Rosslare (on the south-east coast of Ireland), Milford Haven, Sierra Leone and Dakar. This involved extensive work, and was undertaken and carried out with great rapidity by Captain F.C. Learmonth and his staff, whose work in the production of net defences during the war was of inestimable value, not only to ourselves, but to our Allies, for whom large supplies of net defences were also provided. The U.S.A. also adopted our system of net defence for their harbours on entry into the war. Many anxious months were passed at the Admiralty and at the ports named until the anti-submarine defences were completed. The escort of the convoys through the submarine zone imposed very heavy work upon the destroyers, sloops and other screening vessels. This was due partly to the fact that there were not sufficient vessels to admit of adequate time being spent in harbour to rest the crews and effect necessary repairs, and partly to the nature of the work itself and the weather conditions under which so much of it was carried out. It will be realized by those who have been at sea in these small craft that little rest was obtainable in the Atlantic between the west coast of Ireland and the mouth of the Channel and positions 800 to 400 miles to the westward, except in the finest weather. When to this is added the constant strain imposed by watching for the momentary appearance of a periscope or the track of a torpedo, and the vigilance needed, especially on dark and stormy nights, to keep touch with a large convoy of merchant ships showing no lights, with the inevitable whipping up of occasional stragglers from the convoy, some idea may be gathered of the arduous and unceasing work accomplished by the anti-submarine escorts. It had been my practice during 1917 to call for returns from all commands of the number of hours that vessels of the destroyer and light cruiser type were actually under way per month, and these returns showed how heavy was the strain on the destroyers, particularly those engaged in convoy work. For several months, for instance, the destroyers in the flotillas stationed at Devonport were under way on an average for just under 50 per cent. of the month. This meant that several destroyers in these flotillas averaged quite 60 per cent. or even 70 per cent. of their time under way, as other vessels of the flotilla were laid up during the periods under review for long refits due to collision or other damage, in addition to the necessary four-monthly refit.