A convoy is a group of vehicles, typically motor vehicles or ships, traveling together for mutual support and protection. Often, a convoy is organized with armed defensive support, though it may also be used in a non-military sense, for example when driving through remote areas.
Age of sail 
Naval convoys have been used for years, and examples of merchant ships traveling under naval protection have been traced back to the 12th century. The use of organized naval convoys dates from when ships began to be separated into specialist classes and national navies were established.
By the French Revolutionary Wars of the late 18th century, effective naval convoy tactics had been developed to ward off pirates and privateers. Some convoys contained several hundred merchant ships. The most enduring system of convoys were the Spanish treasure fleets, that sailed from the 1520s until 1790.
When merchant ships sailed independently, a privateer could cruise a shipping lane and capture ships as they passed. Ships sailing in convoy presented a much smaller target: a convoy was as hard to find as 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. As a result of the convoy system's effectiveness, wartime insurance premiums were consistently lower for ships that sailed in convoys.
- The Battle of Portland (1653)
- The Battle of Ushant (1781)
- The Battle of Dogger Bank (1781)
- The Glorious First of June (1794)
- The Battle of Pulo Aura (1804)
By the end of the Napoleonic Wars the Royal Navy had in place a sophisticated convoy system to protect merchant ships. Losses of ships traveling out of convoy were so high that no merchant ship was allowed to sail unescorted.
World War I 
In the early 20th century, the dreadnought changed the balance of power in convoy battles. Steaming faster than merchant ships and firing at long ranges, a single battleship could destroy many ships in a convoy before the others could scatter over the horizon. To protect a convoy against a capital ship required providing it with an escort of another capital ship, at very high cost.
Battleships were the main reason that the British Admiralty did not adopt convoy tactics at the start of the first Battle of the Atlantic in World War I. But the German capital ships had been bottled up in the North Sea, 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 of their limited supply of torpedoes and shells. The Admiralty took a long time to respond to this change in the tactical position, and in April 1917 convoy was trialled, before being officially introduced in the Atlantic in September 1917.
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 spent 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 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. Ports could deal more easily with convoys because they tended to arrive on schedule and so loading and unloading could be planned.
In his book On the Psychology of Military Incompetence, Norman Dixon suggested that the hostility towards convoys in the naval establishment were in part caused by a (sub-conscious) perception of convoys as effeminating, due to warships having to care for civilian merchant ships. Convoy duty also exposes the escorting warships to the sometimes hazardous conditions of the North Atlantic, with only rare occurrences of visible achievement (i.e. fending off a submarine assault).
World War II 
The British adopted a convoy system, initially voluntary and later compulsory for almost all merchant ships, the moment that World War II was declared. Each convoy consisted of between 30 and 70 mostly unarmed merchant ships.  Canadian, and later 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 developed anti-convoy tactics and the British developed counter-tactics to thwart the Germans.
The power of a heavily armed warship against a convoy was dramatically illustrated by the fate of Convoy HX-84. On November 5, 1940, the German heavy cruiser 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 Armed Merchant Cruiser HMS Jervis Bay and failing light allowed the rest of the convoy to escape.
The power of a battleship in protecting a convoy was also dramatically illustrated when the German warships Scharnhorst and Gneisenau, carrying 11" guns, came upon an eastbound British convoy of 41 ships, HX-106 in the North Atlantic on February 8, 1941. When they noticed the presence in the escort of the old battleship, HMS Ramillies, they fled the scene, rather than risk damage from her 15" guns.
The enormous number of vessels involved and the frequency of engagements meant that statistical techniques could be applied to evaluate tactics: an early use of operational research in war.
Prior to overt participation in WWII, the US was actively engaged in convoys with the British in the North Atlantic Ocean, primarily supporting British activities in Iceland. This was discussed by John T. Flynn in his 1944 work "The Truth About Pearl Harbor" 
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. Fleet Admiral Ernest King ignored advice on this subject from the British as he had formed a poor opinion of the Royal Navy early in his career. The result was what the U-boat crews called their second happy time, which did not end until convoys were introduced.
The German anti-convoy tactics included:
- long-range surveillance aircraft to find convoys;
- strings of U-boats (wolfpacks) that could be directed onto a convoy by radio;
- breaking the British naval codes;
- improved anti-ship weapons, including magnetic detonators and sonic homing torpedoes.
The Allied responses included:
- air raids on the U-boat bases at Brest and La Rochelle;
- converted merchant ships, e.g., Merchant aircraft carriers, Catapult Aircraft Merchantman and armed merchant cruisers
- more convoy escorts, including cheaply produced corvettes, frigates, and escort carriers;
- improved anti-submarine weapons such as the hedgehog;
- long-range aircraft patrols to find U-boats;
- 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
- allocating vessels to convoys according to speed, so that faster ships were less exposed.
They were also aided by
- improved sonar (ASDIC) allowing escort vessels to better track U-boats;
- breaking the German naval cipher;
- improved radar and radio direction finding allowing planes to find and destroy U-boats;
Many naval battles of World War II were fought around convoys, including:
- Convoy PQ-16, May 1942
- Convoy PQ-17, June–July 1942
- Convoy PQ-18, September 1942
- Operation Pedestal, August 1942
- The Naval Battle of Guadalcanal, November 1942
- The Battle of the Barents Sea, December 1942
- The Battle of the Bismarck Sea, March 1943
The convoy prefix indicates the route of the convoy. For example, 'PQ' would be Iceland to Northern Russia and 'QP' the return route.
The success of convoys as an anti-submarine tactic during the world wars can be ascribed to several reasons related to u-boat capabilities, the size of the ocean and convoy escorts.
In practice, Type VII and Type IX U-boats were limited in their capabilities. Submerged speed and endurance was limited and not suited for overhauling many ships. Even a surfaced U-boat could take several hours to gain an attack position. Torpedo capacity was also restricted to around fourteen (Type VII) or 24 (Type IX), thus limiting the number of attacks that could be made, particularly when multiple firings were necessary for a single target. There was a real problem for the U-boats and their adversaries in finding each other; with a tiny proportion of the ocean in sight, without intelligence or radar, warships and even aircraft would be fortunate in coming across a submarine. The Royal Navy and later the United States Navy each took time to learn this lesson. Conversely, a U-boat's radius of vision was even smaller and had to be supplemented by regular long-range reconnaissance flights.
For both major allied navies, it had been difficult to grasp that, however large a convoy, its "footprint" (the area within which it could be spotted) was far smaller than if the individual ships had traveled independently. In other words, a submarine had less chance of finding a single convoy than if it were scattered as single ships. Moreover, once an attack had been made, the submarine would need to regain an attack position on the convoy. If, however, an attack were thwarted by escorts, even if the submarine had escaped damage, it would have to remain submerged for its own safety and might only recover its position after many hours' hard work. U-boats patrolling areas with constant and predictable flows of sea traffic, such as the United States Atlantic coast in early 1942, could dismiss a missed opportunity in the certain knowledge that another would soon present itself.
The destruction of submarines required their discovery, an improbable occurrence on aggressive patrols, by chance alone. Convoys, however, presented irresistible targets and could not be ignored. For this reason, the U-boats presented themselves as targets to the escorts with increasing possibility of destruction. In this way, the Ubootwaffe suffered severe losses, for little gain, when pressing pack attacks on well-defended convoys.
Post-World War II 
The largest convoy effort since World War II was Operation Earnest Will, the U.S. Navy's 1987–88 escort of reflagged Kuwaiti tankers in the Persian Gulf during the Iran–Iraq War. In the present day, convoys are used as a tactic by navies to deter pirates off the coast of Somalia from capturing unarmed civilian freighters who would otherwise pose easy targets.
Humanitarian aid convoys 
In the 1990s these convoys became common traveling from Western Europe to countries of the former Yugoslavia, in particular Bosnia and Kosovo, to deal with the aftermath of the wars there. They also travel to countries where standards of care in institutions such as orphanages are considered low by Western European standards, such as Romania; and where other disasters have led to problems, such as around the Chernobyl disaster in Belarus and Ukraine.
The convoys are made possible partly by the relatively small geographic distances between the stable and affluent countries of Western Europe, and the areas of need in Eastern Europe and, in a few cases, North Africa and even Iraq. They are often justified because although less directly cost-effective than mass freight transport, they emphasise the support of large numbers of small groups, and are quite distinct from multinational organisations such as United Nations humanitarian efforts.
Truckers' convoys 
Truckers' convoys were created as a byproduct of the 55 mph speed limit and 18-wheelers becoming the prime targets of speed traps. Most truckers had difficult schedules to keep and as a result had to maintain a speed above the posted speed limit to reach their destinations on time. Convoys were started so that multiple trucks could run together at a high speed with the thinking being that if they passed a speed trap the police would only be able to pull over one of the trucks in the convoy. When driving on a highway, convoys are also useful to conserve fuel by drafting.
The truckers' convoy is more similar to a caravan than a military convoy.
See also 
- I.C.B. Dear and Peter Kemp, ed. (2007). "Convoy". The Oxford Companion to Ships and the Sea. Oxford Reference Online. Oxford: Oxford University Press. Retrieved 2008-12-07.
- Robb-Webb, Jon (2001). "Convoy". In Richard Holmes. The Oxford Companion to Military History. Oxford Reference Online. Oxford: Oxford University Press. Retrieved 2008-12-07.
- Dixon, Dr. Norman F. On the Psychology of Military Incompetence Jonathan Cape Ltd 1976 / Pimlico 1994 pp210–211
- Convoy from History Television.
- Aid Convoy (charitable organisation) information on partners
- Winton, John. Convoy, The Defense of Sea Trade 1890–1990, 1983. ISBN 0-7181-2163-5
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