The basic principle behind the convoy as practiced most assiduously in the Atlantic
during World War One
and World War Two
is this: noncombatants
which must travel dangerous waters will typically outnumber the combatants
available to escort
them - therefore, grouping them under the aegis
of a massed escort affords the most protection. The hundreds of cargo
ships crossing the Atlantic to supply the civilian economies and military units of Europe
from the industry and resources of the Americas were falling prey to the U-boat
by tens per month; with so many hull
s crossing what were well-established sea lanes
, the U-boats needed only to lurk in mid-ocean away from the reach of land-based air patrols and strike when ships came within reach.
Even under the restrictions of cruiser warfare, the slow speeds and cumbersome communications of the day meant that ships caught by a U-boat had essentially no chance. The main limitation that the rules of cruiser warfare placed on the U-boat was to hold it in place long enough to give its opponents a chance to determine its rough patrol area, forcing it to expend time shifting position before attacking again. Even so, the damage was horrific.
The convoy system changed that. It essentially limited transoceanic crossings to ships grouped by capability and destination. The ships would gather in staging areas such as New York Harbor, protected from attack during assembly; then, when the requisite escorts (typically destroyers or escort carriers) were ready, the convoy would steam out in formation at best speed, with the escorts patrolling its flanks and 'breaking trail' ahead of it. Even merchant ships were capable of maintaining speeds that U-boats couldn't maintain submerged, so speed and the 'zig-zag' were the convoy's best defense. Regular course changes to the left and right of the base course meant that an attacking U-boat would be hard-pressed to find a period long enough to fire on more than one or two targets before the convoy would 'zig' onto a different heading, typically one angled far enough 'away' from the U-boat to deny them a clear torpedo shot. If the convoy was fortunate, the U-boat would be unable to sail fast enough to set up a shot after sighting them without being seen, since making speeds faster than a convoy required it to travel on the surface - and the escorts were watching.
In May of 1941, the first convoy of World War Two's Battle of the Atlantic sailed across the Atlantic with continuous escort protection (Convoy HX-129). At the time, the U.S. Navy was unable to participate, as the U.S. was still a Neutral power; the British Royal Navy was providing escort from the U.S. 3-mile limit. Despite this, however, by August the U.S. Navy was patrolling convoy escort routes out to the middle of the North Atlantic under the reasoning that her own merchantmen were at risk carrying Lend-Lease material to Britain. The USN was tracking German U-boats for the Royal Navy as well - which was quite arguably 'over the line.'
On October 31st, the destroyer USS Reuben James was sunk in the Atlantic by U-552 while escorting convoy HX-156, becoming the first naval casualty for the United States in the Battle of the Atlantic - as yet undeclared by her. By the end of 1941, convoys were in use between Britain and Australia, as well as Britain and the USSR and the U.S. and the USSR, also carrying Lend-Lease.
There was another advantage to the convoy system as well - that of planning. It allowed those responsible for scheduling loads to better coordinate the shipment of needed materials across the pond, assigning critical stores to redundant convoys to ensure some got through. The shepherding of the escorts meant that even if ships were lost, better information on their sinkings offered some chance for long range patrols to pick up crew - or just to know where they went down, and what was lost. Flying boats would shadow convoys when possible for search and rescue; sometimes smaller patrol combatants (which could catch up) would detach for SAR if there were enough available.
Ships were still lost, and men still died, on both sides of the fight. Did convoying work? Certainly the loss numbers for Allied merchant shipping continued to rise until 1942, dropping only in 1943; however, the number of U-boats lost also began to rise sharply after the convoy system was put into place. The amount of shipping in use also rose dramatically, especially as the United States brought forward Lend-Lease and then entered the war, so it is difficult to claim that the loss numbers, which were nearly identical from 1940 to 1941, wouldn't have been much higher. One thing is certain: once long range air patrols became widely available on both sides of the Atlantic in 1943, coupled with the introduction of light aircraft carriers into convoy escorts in the same time period, convoy losses dropped dramatically with U-boat losses rising equally sharply. The Allies lost approximately 1,323 ships (7,047,000+ tons displacement) in 1942, and the Kriegsmarine lost 87 U-boats. In 1943, however, the Allied loss rate was back down to 588 ships (3,042,000+ tons) and the U-boat forces had lost 242 vessels. The air power available to the convoy escorts - and the convoy system itself, for forcing the U-boats to attack where those assets could be concentrated and directed - had begun to tell.