By 1917, Allied commanders had a number of fairly effective ASW weapons available to them. While active sonar (ASDIC) was still some way off, hydrophones, an early form of passive sonar, had been available since 1915. However, most of those in service in 1917 were useless in convoy as they could only be used when the ship was stationary, and were easily overwhelmed by the noise of the convoy. Depth charges, which gave escorts an effective way to attack submarines, had been in service since 1916. Even so, most escorts carried only a light depth charge armament, as they had no real way of detecting a submerged submarine.

The main protective value of convoy in WWI was that U-boats were much less likely to encounter it. A convoy was pretty much just as noticeable as a single merchant ship. However, it would take up much less space on the ocean than the same number of merchant ships sailing individually. This meant that a U-boat was much less likely to encounter a convoy than it was to encounter at least one merchant sailing individually. The other big advantage of the convoy was its speed. A submerged U-boat had a typical top speed of 8.5-9.5 knots, but this would exhaust the batteries in less than an hour. More usual sustained speeds were in the area of 3-4 knots, a speed which could be sustained for a day. Meanwhile, the typical convoy speed was 11-12 knots. By forcing a submarine to submerge, the convoy could easily outrun it. Even if a U-boat was able to attack, the convoy would have moved out of range by the time it could reload.

Given this, the typical role of the convoy escort in WWI was to force German U-boats to submerge before they could attack. In the standard formation, a convoy would have a line of escorts, spaced a mile apart, running in front of the convoy. This would, hopefully, spot U-boats before they could get into a firing position, allowing the convoy to manoeuvre out of the way. The line of escorts would force any approaching U-boat to submerge, thanks to the threat of gunfire. They also provided a physical barrier against attacks from the bow. Escorts would also be posted to the flanks of the convoy, to cover these approaches in a similar way. If escort numbers allowed, one or more ships would take up station astern of the convoy. This helped to dissuade shadowing U-boats, especially at dusk. These escorts were also usefully positioned to attack a U-boat that had made an attack on the convoy. Convoys might also have aerial escorts. Kite balloons were fairly common over open-ocean convoys, while those closer to land might have support from aircraft or blimps. These extended the range at which surfaced submarines could be sighted, and thus be forced to submerge.

If a convoy encountered a U-boat, either sighted ahead of the convoy, or through a torpedo impact on a merchant, the escorts would typically try to drive it away from the convoy. While tracking a U-boat was impossible, they would generally have an idea of the location of the submarine. This might come from the torpedo track, from sightings of the periscope, or from trails of oil left in the submarine's wake. The escorts would move to the position of the sighting, and make blind attacks using depth charges. Armed with some knowledge of how submariners might try to evade attacks, they would also drop charges along the sub's likely course. In 1917, these would typically be desultory attacks carried out by a single ship, aimed less at killing a submarine, and more at keeping it submerged until the convoy passed. However, moving into 1918, tactics shifted to encourage attempts at killing the submarine. This would involve attacks by up to half the escort, using up a significant proportion of their depth charge armament.