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Convoys are groups of merchant ships that sail together to common destinations. In most cases, they are escorted by warships that provide protection from enemy naval, air, and submarine forces. Convoys are an ancient concept that proved vital in both world wars.
Convoys reduce merchant shipping losses in two ways. First, and most obvious, the escorting warships protect the merchant ships. Even if the escorts are unable to drive off enemy units, they may reduce the effectiveness of their attacks on the merchant ships. The escorts may also inflict losses on the attackers which, if they are heavy enough, can make continued raids on convoys unacceptably costly to the enemy. The number of escorts required to protect a convoy increases only slowly as the number of merchant ships increases: The number of merchant ships that can be safely packed into an imaginary circle on the ocean's surface increases as the square of the circle's radius, while the perimeter increases only in proportion to the circle's radius. Thus, large convoys economize the costs of escort protection.
None of the naval powers began the war with adequate numbers of
escort vessels. Britain began constructing
large numbers of
antisubmarine corvettes once war
broke out, and the United States
began constructing destroyer
escorts, many of
which went to Britain as Lend-Lease.
Japan did not complete its first
destroyer escort (Matsu) until April 1944. The
ideal escort vessel was cheap to construct, was at least slightly
faster than a surfaced submarine, had adequate range, and was equipped
with sonar and effective
antisubmarine weapons.
The second way convoys reduce shipping losses is so counterintuitive that most navies had to learn it by repeated hard experience. A lone ship, plane, or submarine must locate merchant ships before it can destroy them. If the merchant ships are sailing individually, the odds of a hunter finding its prey are relatively high. When the ships are grouped into large convoys, the odds of a successful hunt decrease in proportion to the size of the convoys. Of course, those hunters that do find a convoy have a rich selection of targets, so it mighty seem (and did to admirals as senior as Ernest King) that it is actually counterproductive to organize convoys unless they can be heavily escorted. This turns out not to be the case.
Because enemy units have a limited supply of munitions, even a
hunter that chances upon a completely unprotected convoy can usually
destroy only a fraction of its merchant ships. On the other hand,
hunters that chance upon single merchant ships are almost always able
to sink them. When the mathematics of the situation were worked out for
1940s weapons platforms, they showed that even a completely unescorted
convoy was better than individual sailing. The British had learned this by
late 1941; the Americans had
a very difficult time accepting this (admittedly surprising) result,
and American shipping losses off the East Coast were terrible in the
early months of 1942 and did not abate until the Americans began
organizing convoys regardless of the availability of escorts.
Ironically, it was an American officer, Admiral William S. Sims, who
had forcefully argued for the effectiveness of convoys to the British
Admiraly in the First World War; the roles were reversed in the Second
World War.
The appropriate response to the use of convoys by an enemy is to
find
ways to bring more munitions to bear on any convoy that is sighted. One
way of doing so is to increase the munitions loadout of the hunters or
to increase the hit probability of the munitions used. However, there
are technological limits to how much one can increase the damage a
single hunter can inflict, so both the Germans
in the Atlantic and the Americans in the western Pacific turned to wolf
pack tactics.
A wolf pack is a group of submarines that search in a pattern that is widely enough spaced to maximize search efficiency. Once one of the submarines sights a convoy, it calls in the other members of the wolf pack, increasing the numbers of weapons that can be brought to bear. This approach proved deadly in the Atlantic, until the British used signals intelligence to tap into the heavy radio traffic between the wolf packs and their controllers ashore. This allowed convoys to avoid some wolf packs, and as Allied air coverage and antisubmarine technology improved, it gave the Allies clues where to look for U-boats racing on the surface to join a wolf pack. U-boat losses became so severe that the Germans pulled their boats out of the Atlantic shipping lanes, conceding the Battle of the Atlantic.
The Japanese were reluctant to organize convoys early in the war.
The very poor performance of American torpedoes
early in the war meant that even ships sailing singly had a fair chance
of getting away
if they were discovered by an American submarine. Under these
conditions, the advantages of poorly escorted convoys are much reduced.
After the Americans worked most of the bugs out of their torpedoes,
sinkings of Japanese merchantmen increased sharply, and the Japanese
belatedly turned
to convoys. Regular convoys were introduced in late 1943, though some
merchant ships
continued to sail independently until March 1944. The Japanese Navy
showed the same reluctance to organize poorly escorted
convoys as the other naval powers: At one time, 32 ships waited for
three months at Palau for
lack of any escorts. This reluctance was compounded by the shortage of
suitable escort vessels.
The Americans responded to Japanese convoys by experimenting with wolf packs of about three submarines. Unlike the Germans (whose tactics had doubtless been analyzed by Allied operational researchers), the Americans did not attempt to coordinate large wolf packs from ashore. Instead, the wolf packs were locally controlled by a senior officer riding one of the submarines or (later on) the senior submarine commander, using short-range radio signals that were difficult for the Japanese to exploit. There is statistical evidence that these wolf packs were effective, but American submariners seem to have never really warmed to them. The romance of the lone hunter, free of external constraint, likely had a powerful hold.
The discussion so far, while describing general principles, has focused on convoys versus submarines. Convoys could also be attacked by surface raiders, but this proved a minor feature of the convoy wars in both the Atlantic and Pacific. Surface raiders were simply too vulnerable to detection by aircraft and so could not operate for long in the enemy's sea lanes. On the other hand, aircraft were ideal for wolf pack tactics, with their vastly greater speed than merchant ships. A convoy that was detected by enemy patrol aircraft within strike range of an enemy base was in for a very bad day, as the Japanese learned during the Guadalcanal campaign. The only defenses were to stay out of enemy aircraft range or to provide a strong combat air patrol of one's own fighters.
One way of providing air cover for a convoy was to base the aircraft on the ships of the convoy themselves. After some desperate experiments with modified merchantmen carrying sacrificial Hurricanes, the British developed the concept of the escort carrier. This was a small, relatively cheap aircraft carrier constructed on a merchant hull that could provide cost-effective air cover for a convoy. Though originally developed to defend against enemy air attack, the escort carriers proved highly effective against submarines, which in the 1940s had very limited submerged endurance and generally had to close a convoy on the surface. Submarines proved highly vulnerable to escort carrier aircraft during this surfaced approach.
Convoys do not come without a cost. The convoy can only advance at
the speed of the slowest merchant ship in the convoy, which negates the
speed advantage of the faster ships. Convoying also requires that ships
gather at the same departure point and leave at the same time, which
means some fully loaded ships will be sitting at their berths waiting
for a convoy to form. Shipping between minor ports will either have to
wait a long time or sail independently to a major port to join a
convoy. These difficulties can be reduced, but never entirely
eliminated, by careful planning and coordination. The Allies became
skillful at such coordination; the Japanese were hampered by serious
interservice rivalry.
References
The Pacific War Online Encyclopedia © 2008, 2010 by Kent G. Budge. Index
