but that was all; as soon as the U-boat saw the ship, it simply dived to security beneath the waves. Our destroyers had many chances to fire at the enemy but usually at very long ranges; some of them had lively scraps, which perhaps involved the destruction of U-boats, though this was always a difficult thing to prove. Yet the mere fact that submarines were seldom sunk by destroyers on patrol, either by ourselves or by the Allies, did not mean that the latter accomplished nothing. The work chiefly expected of destroyers on patrol was that they should keep the U-boats under the surface as much as possible
and protect commerce. Normally the submarine sails on top of the water, looking for its prey. As long as it is
beyond the merchantmen's range of vision, it uses its high surface speed of about 14 knots to attain a position ahead
of the advancing vessel; before the surface vessel reaches a point where its lookout can see the submarine, the U-boat
dives and awaits the favourable moment for firing its torpedo. It cannot take these preliminary steps if there is a destroyer anywhere in the neighbourhood; the mere presence of such a warship therefore constitutes a considerable protection to any merchant ship that is within sight. The submarine normally prefers to use its guns on merchant ships, for the torpedoes are expensive and comparatively few in number. Destroyers constantly interfered with these gunning operations. A long distance shot usually was sufficient to make the under-water vessel submerge and thus lose its power for doing harm. The
early experiences of our destroyers with submarines were of this kind; but the work of chasing U-boats under the
water, escorting a small proportion of the many cargo ships, and picking up survivors, important as it was, did
not really constitute effective anti-submarine warfare. It gave our men splendid training, it saved many a merchant
ship, it rescued many victims from the extreme dangers of German ruthlessness, it sank a small number of submarines, but it could never have won the war.
This patrol by destroyers and light surface vessels has been criticized as affording an altogether ineffective method of protecting shipping, especially when compared with the convoy system. This criticism is, of course, justified; still we must understand that it was the only possible method until we had enough anti-submarine craft
