Naval convoys have been in use for centuries, with examples of merchant ships traveling under naval protection dating 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 still 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.
Many naval battles in the Age of Sail were fought around convoys, including:
By the end of the Napoleonic Wars the Royal Navy had in place a sophisticated convoy system to protect merchant ships. Losses of ships travelling out of convoy however 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 opportunity cost (i.e. potentially tying down multiple capital ships to defend different convoys against one opponent ship).
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. These submarines were 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 convoys were trialled, before being officially introduced in the Atlantic in September 1917.
Other arguments against convoys 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 an escort. 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
Convoy Routes in the Atlantic Ocean during 1941
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 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 capability 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 deterrence value of a battleship in protecting a convoy was also dramatically illustrated when the German light battleships (referred by some as battlecruisers) Scharnhorst and Gneisenau, mounting 11 in (28 cm) guns, came upon an eastbound British convoy (HX-106, with 41 ships) in the North Atlantic on February 8, 1941. When the Germans detected the slow but well-protected battleship HMS Ramillies escorting the convoy, they fled the scene rather than risk damage from her 15 in (38 cm) 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 World War II, 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" 
After Germany declared war on the US, the US Navy decided not to instigate convoys on American eastern seaboard. US 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.
In the Pacific Theater of World War II, Japanese merchant ships rarely traveled in convoys. Japanese destroyers were generally deficient in antisubmarine weaponry compared to their Allied counterparts, and the Japanese navy did not develop an inexpensive convoy escort like the Allies' destroyer escort/frigate until it was too late. In the early part of the conflict, American submarines in the Pacific were ineffective as they suffered from timid tactics, faulty torpedoes, and poor deployment, while there were only small numbers of British and Dutch boats. U.S. Admiral Charles A. Lockwood's efforts, coupled with strenuous complaints from his captains, rectified these problems and U.S. submarines became much more successful by war's end. As a result, the Japanese merchant fleet was largely destroyed by the end of the war. Japanese submarines, unlike their U.S. and German equivalents, focused on U.S. battle fleets rather than merchant convoys, and while they did manage some early successes, sinking two U.S. carriers, they failed to significantly inhibit the invasion convoys carrying troops and equipment in support of the U.S. island-hopping campaign.
Several notable battles in the South Pacific involved Allied bombers interdicting Japanese troopship convoys which were often defended by Japanese fighters, notable Guadalcanal (13 November 1942), Battle of Wau (5 January 1943), and the Battle of the Bismarck Sea (2–4 March 1943).
At the Battle off Samar, the effectiveness of the U.S. Navy's escorts was demonstrated when they managed to defend their troop convoy from a much larger and more powerful Japanese battle-fleet. The Japanese force comprised four battleships and numerous heavy cruisers, while the U.S. force consisted of escort carriers, destroyers, and destroyer escorts. Large numbers of American aircraft (albeit without much anti-ship ordnance other than torpedoes) and aggressive tactics of the destroyers (with their radar-directed gunfire) allowed the U.S. to sink three Japanese heavy cruisers at the cost of one escort carrier and three destroyers.
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
- Q-ships, submarine-hunters disguised as unarmed merchant ships to lure submarines into an attack
- more convoy escorts, including cheaply produced yet effective destroyer escorts/frigates (as corvettes were meant as a stopgap), and escort carriers;
- fighter aircraft (carried by escort carriers and merchant aircraft carriers) that would drive off German bombers and attack U-boats
- long-range aircraft patrols to find and attack U-boats;
- improved anti-submarine weapons such as the hedgehog;
- larger convoys, allowing more escorts per convoy as well as the extraction of enough escorts to form hunter-killer support groups that were not attached to a particular convoy
- allocating vessels to convoys according to speed, so that faster ships were less exposed.
They were also aided by
Many naval battles of World War II were fought around convoys, including:
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
US Navy warships escort the tanker Gas King
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 if they sailed alone.