In the 17th–19th centuries, a frigate was a three-masted, fully rigged sailing ship, often carrying 30–40 guns in all. Smaller and faster than ships of the line (the principal vessels of naval warfare), frigates served as scouts or as escorts protecting merchant convoys; they also cruised the seas as merchant raiders themselves. In World War II, Britain revived the term frigate using it to describe escort ships equipped with sonar and depth charges, and used these ships to guard convoys from submarines. In the postwar decades, the frigate also adopted an antiaircraft role, adding radar and surface-to-air missiles. Modern frigates can sail at a speed of 30 knots and carry a crew of 200.