Armed merchantman

An armed merchantman is a merchant ship equipped with guns, usually for defensive purposes, either by design or after the fact. In the days of sail, piracy and privateers, many merchantmen would be routinely armed, especially those engaging in long distance and high value trade. In more modern times, auxiliary cruisers were used offensively as merchant raiders to disrupt trade chiefly during both World War I and World War II, particularly by Germany.

While armed merchantmen are clearly inferior to purpose-built warships, sometimes they have scored successes in combat against them. Examples include East Indiamen mimicking ships of the line and chasing off regular French warships in the Battle of Pulo Aura in 1804, and the German auxiliary cruiser Kormoran sinking the Australian light cruiser HMAS Sydney in their battle in 1941, although Kormoran was also destroyed and had to be scuttled.

Pre-20th century

East Indiamen of various European countries were heavily armed for their long journeys to the Far East. In particularly dangerous times, such as when the home countries were at war, a convoy system would be used whereby the ships were escorted by a warship. However, many East Indiamen also travelled on their own, and therefore were heavily armed in order to defend themselves against pirates and privateers. They also defended themselves against warships, scoring signal victories at the Battle of Pulo Aura and the action of 4 August 1800. The British Royal Navy purchased several that it converted to ships of the line.