Convoy

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. It may also be used in a non-military sense, for example when driving through remote areas. Arriving at the scene of a major emergency with a well-ordered unit and intact command structure can be another motivation.

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 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.

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).

...
Wikipedia