This article is about the general concept, particularly its use by the military. Convoy is also the name of a trucker's song by C.W. McCall and of a movie by Sam Peckinpah and Kris Kristofferson inspired by the song, as well as the Japanese name for the Transformers character Optimus Prime
A convoy is a group of vehicles or ships traveling together for mutual support. Often a convoy is organized with armed support for defensive support. In effect, it is a modification of a caravan.
For example, driving by car through a desert is safer in a convoy. If one car breaks down, others are available to help with repairs, and if it cannot be repaired, the people can be accommodated in the other cars.
In naval warfare, convoy tactics were developed during the age of sail to guard against pirates and privateers. By the French Revolutionary Wars of the late 18th century convoying was a well-established tactic, with some convoys containing several hundred merchant ships.
Convoying was a highly effective tactic against pirates and privateers. When merchant ships sailed independently, a privateer could simply cruise a shipping lane and capture ships as they passed. Ships sailing in convoy presented a much smaller target: a convoy was no more likely to be found than a single ship. Even if the privateer found a convoy and the wind was favourable for an attack, it could hope to capture only a handful of ships before the rest managed to escape, and a small escort of warships could easily thwart it.
Many naval battles in the age of sail were fought around convoys, including:
World War I
In the late 19th century, the dreadnought changed the balance of power in convoy battles. Steaming at several times the speed of merchant ships and firing accurately at ranges of several miles, a single battleship encountering a convoy could destroy dozens of ships before the remaining ships of the convoy could scatter over the horizon. To protect a convoy against the enemy's capital ships would mean providing it with its own escort of battleships: an unsupportable cost.
(The power of a battleship against a convoy is dramatically illustrated by the fate of convoy HX-84 in World War II. On November 5, 1940 the German pocket battleship Admiral Scheer encountered the convoy. Maiden, Trewellard, Kenbame Head, Beaverford, and Fresno were quickly sunk, and other ships were damaged. Only the sacrifice of the cruiser HMS Jervis Bay and failing light allowed the rest of the convoy to escape.)
The fear of battleships was the main reason for the British Admiralty not adopting convoy at the start of the first Battle of the Atlantic in World War I. But by the end of 1914, German capital ships had largely been cleared from the oceans and the main threat to shipping came from U-boats. From a tactical point of view, World War I-era submarines were similar to privateers in the age of sail: only a little faster than the merchant ships they were attacking, and capable of sinking only a small number of vessels in a convoy because they could only carry a few torpedoes. The Admiralty took a long time to respond to this change in the tactical position, and only in 1917, at the urging of the British Prime Minister, David Lloyd George, did they institute a convoy system. Losses to U-boats dropped to a small fraction of their former level.
Other arguments against convoy were raised. The primary issue was the loss of productivity, as merchant shipping in convoy has to travel at the speed of the slowest vessel in the convoy and spend a considerable amount of time in ports waiting for the next convoy to depart. Further, large convoys were thought to overload port resources.
Actual analysis of shipping losses in World War I conclusively disproved all these arguments, at least so far as they applied to transatlantic and other long-distance traffic. Ships sailing in convoys were far less likely to be sunk, even when not provided with any escort at all, the loss of productivity due to convoy delays was small compared with the loss of productivity due to ships being sunk, and ports could deal more easily with convoys because they tended to arrive on schedule and so loading and unloading could be planned.
World War II
The British adopted a convoy system the moment that World War II was declared. American supplies were vital for Britain to continue its war effort. The course of the second Battle of the Atlantic was a long struggle as the Germans found tactics that would work against convoys and the British developed counter-tactics to thwart the Germans. The enormous number of vessels involved and the frequency of engagements meant that statistical techniques could be applied to evaluate tactics: this was a significant early use of operational research in war. On the entry of the U.S. into World War II, the U.S Navy decided not to instigate convoys on eastern seaboard of the U.S. The result was what the U-boat crews called their second happy time, which did not come to an end until convoys were introduced. This was, unfortunately for the Allies, as near to a laboratory test as is ever seen in war time and it proved conclusively that convoys worked.
The German anti-convoy tactics included:
- long-range surveillance aircraft to find convoys;
- strings of U-boats (wolf packs) that could be directed onto a convoy by radio;
- breaking the British naval codes;
- improved anti-ship weapons, including magnetic and sonic homing torpedoes.
The Allied responses included:
- air raids on the U-boat bases at Brest and La Rochelle;
- more convoy escorts, including corvettes, frigates and escort carriers;
- improved sonar allowing escort vessel to track U-boats;
- improved anti-submarine weapons such as the hedgehog;
- long-range aircraft patrols to find U-boats;
- improved radar allowing planes to find and destroy U-boats;
- radio direction finding to find U-boats;
- breaking the German naval cipher (see Enigma machine article);
- larger convoys, allowing more escorts per convoy as well as the extraction of enough escorts to form support groups that operated in defence of convoys that faced an above-average risk of attack.
The Japanese never adopted a system of convoys in World War II and their merchant fleet was largely destroyed by Allied submarines.
Many naval battles of World War II were fought around convoys, including:
Prospects for convoys
Post-World War II, it seems likely that satellite surveillance, aircraft carriers, cruise missiles and modern submarines have turned the tactical advantage decidedly in favour of the attacker. See the Modern Naval tactics article for an idea of the problems facing the defender.
Operation Earnest Will, the U.S. Navy's 1987–88 escort of reflagged Kuwaiti tankers in the Persian Gulf during the Iran-Iraq War, was the largest convoy effort since World War II.