Q-ship
Q-ships, also known as Q-boats, decoy vessels, special service ships, or mystery ships, were heavily armed merchant ships with concealed weaponry, designed to lure submarines into making surface attacks. This gave Q-ships the chance to open fire and sink them. The basic ethos of every Q-ship was to be a wolf in sheep's clothing.
They were used by the British Royal Navy (RN) during the First World War and by both the RN and the United States Navy during the Second World War (1939–1945), as a countermeasure against German U-boats and Japanese submarines.
Read more about Q-ship: First World War, Second World War, Use Against Modern Pirates, Q-ships in Fiction