Sloop - of - war In the 18th and most of the 19th centuries , a sloop - of - war in the British Navy was a warship ( also known as one of the escort types ) with a single gun deck that carried up to eighteen guns . As the rating system covered all vessels with 20 guns and above , this meant that the term " sloop - of - war " actually encompassed all the unrated combat vessels including the very small gun - brig s and cutters . In technical terms , even the more specialised bomb vessel s and fireship s were classed as sloops - of - war , and in practice these were actually employed in the sloop role when not carrying out their specialized functions . In later years the type evolved ; in World War II sloops were specialized convoy - defence vessels , with anti - aircraft and anti - submarine capability . Rigging . A sloop - of - war was quite different from a civilian or mercantile sloop , which was a general term for a single masted vessel rigged like what 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 . Brig sloop . 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 invariably only 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 carronade s ( 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 frigate s , 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 manoeuvered to exploit its superiority of range . The other limitation of brig sloops vis - à - vis post ships and frigates was their relatively restricted stowage for water and provisions , which made them less suitable for long - range cruising . On the other hand , their shallower draught made them excellent raiders against coastal shipping and shore installations . Bermuda sloop . 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 Eighteenth 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 . Classification . 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 ship s within the sixth rate of the British Navy ) . The name corvette was subsequently also applied to British vessels , but not until the 1830s . 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 gundeck ; 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 . " History . 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 1880s even the most powerful warships had fewer than a dozen large calibre guns . In the Royal Navy , the sloop evolved into an unrated vessel with a single gun deck and three masts , two square rig ged and the aftermost fore - and - aft rig ged ( 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 . Revival . 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 classes 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 to act during wartime as convoy escorts . As they were not intended to deploy with the fleet , sloops had a maximum speed of less than . 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 . World War II . During the Second World War , 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 ) .