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Convoys


Photograph of convoy WS-12

National Archives #80-G-464654

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, as was demonstrated by British operational researchers led by future Nobel Prize laureate Patrick Blackett.

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.

Karl Doenitz, later commander of the German Navy in the Second World War, wrote of the effects of convoy on German U-boat operations in the First World War (Massie 2003):

The oceans at once became bare and empty. For long periods at a time, the U-boats, operating individually, would see nothing at all; and then suddenly up would loom a huge concourse of ships, thirty or fifty or more of them, surrounded by a strong escort of warships of all types. The solitary U-boat, which most probably had sighted the convoy purely by chance, would then attack, thrusting again and again ... for perhaps several days and nights until the physical exhaustion of the command and crew called a halt. The lone U-boat might well sink one or two ships, or even several, but that was a poor percentage of the whole. The convoy would steam on. In most cases, no other German U-boat would catch sight of it and it would reach Britain, bringing a rich cargo of foodstuffs and raw materials safely to port.

The British Admiralty, for no reason anyone could clearly articulate, long clung to an ideal convoy size of 40 ships and an absolute maximum upper limit of 60 ships. It took some time for the operational researchers to persuade naval commanders that convoys should be as large as possible, but when this policy was finally adopted, sinkings decreased.

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 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. Furthermore, Japanese convoys also tended to be so small that a small wolfpack, or even a single American submarine, skillfully handled, could destroy most of the convoy.

Convoy escort tactics involved some important tradeoffs. If the escorts were positioned relatively far from the convoy, they would be spread along a long enough perimeter that a submarine might be able to slip between escorts and attack the ships in the convoy at close range. On the other hand, if the escorts were positioned close to the convoy to make a tighter perimeter, a submarine might attempt to attack the convoy ships from outside the perimeter with browning shots, which were long-range torpedo attacks aimed at the general mass of shipping in the convoy rather than individual ships. Browning shots had a much reduced chance of scoring a hit, but the attacking submarine was practically undetectable until it had already launched its torpedoes.

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 and the Americans would discover for themselves during the Mindoro invasion. The only defenses were to stay out of enemy aircraft range, to provide a strong combat air patrol of one's own fighters, or to carry out preemptive strikes on the enemy base.

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, reducing loss of efficiency to around 20% to 30%; the Japanese were hampered by serious interservice rivalry.

References

"ASW in World War II" (1946; accessed 2011-10-29)

Blair (1975)

Budiansky (2013)

Hastings (2007)

Massie (2003)

Miller (2007)

Roscoe (1953)

Sasgen (2010)

Venzon (2003)



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