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 use of Q-ships contributed to the abandonment of cruiser rules restricting attacks on unarmed merchant ships and to the shift to unrestricted submarine warfare in the 20th century.

They were used by the British Royal Navy and the German Kaiserliche Marine during the First World War and by the Royal Navy, the Kriegsmarine, the Imperial Japanese Navy, and the United States Navy during the Second World War (1939–45).