Definition:
The Sloop of War was a type of warship used during the Age of Sail. Smaller than a frigate, sloops of war were armed ships of various types that usually mounted fewer than 28 guns.
While many sloops of war were ship-rigged (three masts, square sails), the classification may be extended to encompass other type of armed ships such as brigs.
Examples:
USS Constellation (1854), USS Niagara
