Early Convoys - Early convoys were small, comprising thirty to fifty ships, but they grew gradually larger as the war progressed. The largest, HXS.300 comprised 167 ships. It left New York for the U.K. on July 17, 1944 with an RCN mid-ocean escort of one frigate and six corvettes. "Fast" convoys maintained a speed of about nine knots and averaged 13-14 days between Halifax and the U.K., while "slow" convoys made about seven knots and averaged 16-17 days on the crossing.
Typical Convoy Structure - A typical convoy of forty ships might be ten columns wide, with four ships in each column. As a German U-boat generally attacked from ahead, or on the bow, the ships in the outside column were the most likely targets. These usually carried bulky, inert cargoes such as lumber or wheat, while ammunition ships, valuable cargo ships and tankers were positioned in the middle of the convoy. If the convoy had six escorts, one (the Senior Officer's ship, usually a destroyer) would be positioned ahead of the convoy by day and astern by night. The other escorts would be stationed one on each bow, one on each beam of the convoy and one astern.
Convoy Rescue Ships - In September 1940, the Germans introduced "wolf pack" tactics, attacking convoys at night with a number of U-boats. They would sink a ship in an outside column and then, when an escort fell out to rescue survivors, slip into the convoy between the columns. It might thus happen that half the escort would be carrying out rescue work and the other half hunting an attacker, leaving the convoy virtually unprotected. In January 1941, specially equipped merchant ships called convoy rescue ships were introduced, and one assigned where possible to each convoy. This left the escorts free to pursue their designed role.
Support Groups - As the war progressed and more escorts became available, the British Royal Navy (RN) and later the Royal Canadian Navy (RCN) were able to create support groups which could go to the aid of engangered convoys or convoys under heavy attack. This had become general practice by May 1943, the height of the Atlantic battle.