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.
