Armed merchant ship
The term armed merchant ship may describe a number of similar ship modifications intended for significantly different missions. The term armed merchantman is generally used. East Indiaman describes late 18th and early 19th-century sailing ships engaged in trade while carrying guns similar to contemporary warships. Defensively Equipped Merchant Ships were civilian-crewed cargo ships carrying a small number of military personnel to operate an anti-submarine gun and anti-aircraft machine guns during the world wars of the early 20th century. Auxiliary cruisers were cargo ships commissioned as naval vessels with a military crew, converted to carry the guns of a light cruiser, and sometimes used as Merchant raiders. Armed merchant cruisers were fast passenger liners commissioned as naval vessels with a military crew and converted to carry the guns of a light cruiser. Naval trawlers were fishing trawlers commissioned as naval vessels with a military crew and equipped for minesweeping or anti-submarine escort. Q-ships were small civilian ships commissioned as naval vessels with a military crew, but retaining their original appearance while carrying concealed anti-submarine weapons. Armed boarding steamers were merchant steamers converted by the United Kingdom for boarding enemy vessels. Sources

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Auxiliary cruisers
Ship types