Sloops-of-war

In the 18th century and most of the 19th, a sloop-of-war in the Royal Navy was a warship with a single gun deck that carried up to eighteen guns. The rating system covered all vessels with 20 guns and above; thus, the term sloop-of-war encompassed all the unrated combat vessels, including the very small gun-brigs and cutters. In technical terms, even the more specialised bomb vessels and fireships were classed as sloops-of-war, and in practice these were employed in the sloop role when not carrying out their specialized functions.

In World War I and World War II, the Royal Navy reused the term "sloop" for specialized convoy-defence vessels, including the Flower class of World War I and the highly successful Black Swan class of World War II, with anti-aircraft and anti-submarine capability.

A sloop-of-war was quite different from a civilian or mercantile sloop, which was a general term for a single-masted vessel rigged in a way that would today be called a gaff cutter (but usually without the square topsails then carried by cutter-rigged vessels), though some sloops of that type did serve in the 18th century British Royal Navy, particularly on the Great Lakes of North America.

In the first half of the 18th century, most naval sloops were two-masted vessels, usually carrying a ketch or a snow rig. A ketch had main and mizzen masts but no foremast, while a snow had a foremast and a main mast but no mizzen.

The first three-masted (i.e. "ship rigged") sloops appeared during the 1740s, and from the mid-1750s most new sloops were built with a three-masted (ship) rig. The third sail afforded the sloop greater mobility and the ability to back sail.

...
Wikipedia