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 specialised functions.
In World War I and World War II, the Royal Navy reused the term "sloop" for specialised 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, a main mast and a mizzen immediately abaft the lower main mast.
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 mast afforded the sloop greater mobility and the ability to back sail.
In the 1770s, the two-masted sloop re-appeared in a new guise as the brig sloop, the successor to the former snow sloops. Brig sloops had two masts, while ship sloops continued to have three (since a brig is a two-masted, square-rigged vessel, and a ship is a square-rigger with three or more masts, though never more than three in that period).
In the Napoleonic period, Britain built huge numbers of brig sloops of the Cruizer class (18 guns) and the Cherokee class (10 guns). The brig rig was economical of manpower (important given Britain's chronic shortfall in trained seamen relative to the demands of the wartime fleet) and, when armed with carronades (32-pounders in the Cruizers, 18-pounders in the Cherokees), they had the highest ratio of firepower to tonnage of any ships in the Royal Navy (albeit within the short range of the carronade). The carronades also used much less manpower than the long guns normally used to arm frigates. Consequently, the Cruizer class were often used as cheaper and more economical substitutes for frigates, in situations where the frigates' high cruising endurance was not essential. A carronade-armed brig, however, would be at the mercy of a frigate armed with long guns, so long as the frigate manoeuvred to exploit its superiority of range. The other limitation of brig sloops as opposed to post ships and frigates was their relatively restricted stowage for water and provisions, which made them less suitable for long-range cruising. However, their shallower draught made them excellent raiders against coastal shipping and shore installations.
The Royal Navy also made extensive use of the Bermuda sloop, both as a cruiser against French privateers, slavers, and smugglers, and also as its standard advice vessels, carrying communications, vital persons and materials, and performing reconnaissance duties for the fleets.
Bermuda sloops were found with gaff rig, mixtures of gaff and square rig, or a Bermuda rig. They were built with up to three masts. The single masted ships, with their huge sails, and the tremendous wind energy they harnessed, were demanding to sail, and required large, experienced crews. The Royal Navy favoured multi-masted versions as it was perennially short of sailors, at the end of the 18th century, and such personnel as it had, particularly in the Western Atlantic (priority being given to the continuing wars with France for control of Europe), received insufficient training. The longer decks of the multi-masted vessels also had the advantage of allowing more guns to be carried.
Originally a sloop-of-war was smaller than a sailing frigate and was (by virtue of having too few guns) outside the rating system. In general, a sloop-of-war would be under the command of a master and commander rather than a post captain, although in day-to-day use at sea the commanding officer of any naval vessels would be addressed as "captain".
A ship sloop was generally the equivalent of the smaller corvette of the French Navy (although the French term also covered ships up to 24 guns, which were classed as post ships within the sixth rate of the British Navy). The name corvette was subsequently also applied to British vessels, but not until the 1830's.
American usage, while similar to British terminology into the beginning of the 19th century, gradually diverged. By about 1825 the United States Navy used "sloop-of-war" to designate a flush-deck ship-rigged warship with all armament on the gun deck; these could be rated as high as 26 guns and thus overlapped "third-class frigates," the equivalent of British post-ships. The Americans also occasionally used the French term corvette.
In the Royal Navy, the sloop evolved into an unrated vessel with a single gun deck and three masts, two square rigged and the aft-most fore-and-aft rigged (corvettes had three masts, all of which were square-rigged). Steam sloops had a transverse division of their lateral coal bunkers in order that the lower division could be emptied first, to maintain a level of protection afforded by the coal in the upper bunker division along the waterline.
During the War of 1812 sloops of war in the service of the United States Navy performed well against their Royal Navy equivalents. The American ships had the advantage of being ship-rigged rather than brig-rigged, a distinction that increased their manoeuvrability. They were also larger and better armed. Cruzier-class brig-sloops in particular were vulnerable in one-on-one engagements with American sloops-of-war.
In the second half of the 19th century, successive generations of naval guns became larger and with the advent of steam-powered sloops, both paddle and screw, by the 1880's even the most powerful warships had fewer than a dozen large calibre guns, and were therefore technically sloops. Since the rating system was no longer a reliable indicator of a ship's combat power, it was abolished together and with it the classifications of sloops, corvettes and frigates. Instead a classification based on the intended role of the ship became common, such as cruiser and battleship.
During the First World War, the sloop rating was revived by the British Royal Navy for small warships not intended for fleet deployments. Examples include the Flower classes of "convoy sloops", those designed for convoy escort, and the Hunt class of "minesweeping sloops", those intended for minesweeping duty.
The Royal Navy continued to build vessels rated as sloops during the interwar years. These sloops were small warships intended for colonial "gunboat diplomacy" deployments, surveying duties, and acting during wartime as convoy escorts. As they were not intended to deploy with the fleet, sloops had a maximum speed of less than 20 knots (37 km/h). A number of such sloops, for example the Grimsby and Kingfisher classes, were built in the interwar years. Fleet minesweepers such as the Algerine class were rated as "minesweeping sloops". The Royal Navy officially dropped the term "sloop" in 1937, although the term remained in widespread and general use.
During World War II, 37 ships of the Black Swan class were built for convoy escort duties. However, the warship-standards construction and sophisticated armaments of the sloop of that time did not lend themselves to mass production, and the sloop was supplanted by the corvette, and later the frigate, as the primary escort vessel of the Royal Navy. Built to mercantile standards and with (initially) simple armaments, these vessels, notably the Flower and River classes, were produced in large numbers for the Battle of the Atlantic. In 1948 the Royal Navy reclassified its remaining sloops and corvettes as frigates, even though the term sloop had been officially defunct for nine years.