HMS Leander was a Portland-class 50-gun fourth rate of the Royal Navy, launched at Chatham on 1 July 1780. She served on the West Coast of Africa, West Indies, and the Halifax station. During the French Revolutionary Wars she participated in the Battle of the Nile before a French ship captured her. The Russians and Turks recaptured her and returned her to the Royal Navy in 1799. On 23 February 1805, while on the Halifax station, Leander captured the French frigate Ville de Milan and recaptured her prize, HMS Cleopatra. On 25 April 1805, cannon fire from Leander killed an American seaman while Leander was trying to search an American vessel off the US coast for contraband. The resulting "Leander affair" contributed to the worsening of relations between the United States and Great Britain. In 1813, the Admiralty converted Leander to a hospital ship under the name Hygeia. Hygeia was sold in 1817.

In 1603 all English warships with a complement of fewer than 160 men were known as 'small ships'. In 1625/26 to establish pay rates for officers, a six-tier naval ship rating system was introduced. These small ships were divided into three tiers: fourth-, fifth- and sixth-rates. Up to the end of the 17th century, the number of guns and the complement size were adjusted until the rating system was actually clarified. A 'fourth-rate' was nominally a ship of over thirty guns with a complement of 140 men.