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.
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.
A warship or combatant ship is a naval ship that is used for naval warfare. Usually they belong to the navy branch of the armed forces of a nation, though they have also been operated by individuals, cooperatives and corporations. As well as being armed, warships are designed to withstand damage and are typically faster and more maneuverable than merchant ships. Unlike a merchant ship, which carries cargo, a warship typically carries only weapons, ammunition and supplies for its crew.
In wartime, the distinction between warships and merchant ships is often blurred. Until the 17th century it was common for merchant ships to be pressed into naval service, and not unusual for more than half of a fleet to be composed of merchant ships—there was not a large difference in construction, unlike the difference between a heavily armoured battleship and an ocean liner. Until the threat of piracy subsided in the 19th century, it was normal practice to arm larger merchant ships such as galleons. Warships have also often been used as troop carriers or supply ships, such as by the French Navy in the 18th century or the Imperial Japanese Navy during the Second World War. In war since the early 20th century, merchant ships were often armed and used as auxiliary warships, such as the Q-ships of the First World War and the armed merchant cruisers of the Second World War.
Ship camouflage is a form of military deception in which a ship is painted in one or more colors in order to obscure or confuse an enemy's visual observation. Several types of marine camouflage have been used or prototyped: blending or crypsis, in which a paint scheme attempts to hide a ship from view; deception, in which a ship is made to look smaller or, as with the Q-ships, to mimic merchantmen; and dazzle, a chaotic paint scheme which tries to confuse any estimate of distance, direction, or heading. Counterillumination, to hide a darkened ship against the slightly brighter night sky, was trialled by the Royal Canadian Navy in diffused lighting camouflage.
Ships were sometimes camouflaged in classical times. Mediterranean pirate ships were sometimes painted blue-gray for concealment. Vegetius records that Julius Caesar's scout ships were painted bluish-green when gathering intelligence along the coast of Britain during the Gallic Wars. Ships were sometimes painted deceptively during the Age of Sail, while both sides in the American Civil War camouflaged their ships, whether to run blockades or for night reconnaissance.