In the rating system of the Royal Navy used to categorise sailing warships, a sixth-rate was the designation for small warships mounting between 20 and 28 carriage-mounted guns on a single deck, sometimes with smaller guns on the upper works and sometimes without. It thus encompassed ships with up to 30 guns in all. In the first half of the 18th century the main battery guns were 6-pounders, but by mid-century these were supplanted by 9-pounders. 28-gun sixth rates were classed as frigates, those smaller as 'post ships', indicating that they were still commanded by a full ('post') captain, as opposed to sloops of 18 guns and less under commanders.
Sixth-rate ships typically had a crew of about 150–240 men, and measured between 450 and 550 tons. A 28-gun ship would have about 19 officers; commissioned officers would include the captain, and two lieutenants; warrant officers would include the master, ship's surgeon, and purser. The other quarterdeck officers were the chaplain and a Royal Marines lieutenant. The ship also carried the standing warrant officers, the gunner, the bosun and the carpenter, and two master's mates, four midshipmen, an assistant surgeon, and a captain's clerk.  The rest of the men were the crew, or the 'lower deck'. They slept in hammocks and ate their simple meals at tables, sitting on wooden benches. A sixth rate carried about 23 marines, while in a strong crew the bulk of the rest were experienced seamen rated 'able' or 'ordinary'. In a weaker crew there would be a large proportion of 'landsmen', adults who were unused to the sea.
The larger sixth rates were those of 28 guns (including four smaller guns mounted on the quarterdeck) and were classed as frigates. The smaller sixth rates with between 20 and 24 guns, still all ship-rigged and sometimes flush-decked vessels, were generally designated as post ships. These vessels could perhaps be considered comparable to the light cruisers and destroyers of more recent times, respectively.
Regardless of armament, sixth-rates were known as "post ships" because, being rated, they were still large enough to have a post-captain in command instead of a lieutenant or commander. 
During the Napoleonic Wars (1803–1815), the now elderly sixth-rate frigates were found to be too small for their expected duties, which were more easily performed by fifth-rate frigates. Most were phased out without replacement, although a few lasted in auxiliary roles until after 1815.
The Aubrey–Maturin series of novels by Patrick O'Brian features the sixth-rate ship HMS Surprise as the frigate captained by Jack Aubrey. It is based on the actual historical frigate of the same name, formerly the French Unité , which was captured and renamed by the Royal Navy in 1796. The Surprise was portrayed in the 2003 film Master and Commander which was adapted from the novels.
In the novel Mason and Dixon by Thomas Pynchon the title characters set sail for Sumatra in 1761 to view the Venus transit in the sixth-rate ship HMS Seahorse.
The novel The Watering Place of Good Peace by Geoffrey Jenkins includes a fictional sixth rate ship called HMS Plymouth Sound, which is described as being one of the fastest sailing ships in the Royal Navy.
In Hornblower and the Atropos by C.S. Forester, the titular character - Horatio Hornblower - commands a sixth-rate ship of 22 guns.
In the rating system of the Royal Navy, a third rate was a ship of the line which from the 1720s mounted between 64 and 80 guns, typically built with two gun decks. Years of experience proved that the third rate ships embodied the best compromise between sailing ability, firepower, and cost. So, while first-rates and second-rates were both larger and more powerful, the third-rate ships were in a real sense the optimal configuration.
In the rating system of the British Royal Navy used to categorise sailing warships, a first rate was the designation for the largest ships of the line, equivalent to the 'super-dreadnought' of more recent times. Originating in the Jacobean era with the designation of Ships Royal capable of carrying at least 400 men, the size and establishment of first-rates evolved over the following 250 years to eventually denote ships of the line carrying at least 80 guns across three gundecks. By the end of the eighteenth century, a first-rate carried no fewer than 100 guns and more than 850 crew, and had a measurement (burthen) tonnage of some 2,000 tons.
In the rating system of the Royal Navy used to categorize sailing warships in the 18th century, a fourth-rate was a ship of the line with 46 to 60 guns mounted. They were phased out of ship of the line service during the French Revolutionary and Napoleonic Wars, as their usefulness was declining; though they were still in service, especially on distant stations such as the East Indies. Fourth-rates took many forms, initially as small two decked warships, later as large frigates razéed from the initial two deck warships, and occasionally even heavily armed merchant ships such as HMS Calcutta.
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.
Steam frigates, also known as screw frigates and the smaller steam corvettes, steam sloops and steam schooners, were steam-powered warships that were not meant to stand in the line of battle. The first such ships were paddle steamers. Later on the invention of screw propulsion enabled construction of steam-powered versions of the traditional frigates, corvettes, and sloops.
In the rating system of the Royal Navy used to categorise sailing warships, a second-rate was a ship of the line which by the start of the 18th century mounted 90 to 98 guns on three gun decks; earlier 17th-century second rates had fewer guns and were originally two-deckers or had only partially armed third gun decks.
In the rating system of the Royal Navy used to categorise sailing warships, a fifth rate was the second smallest class of warships in a hierarchical system of six "ratings" based on size and firepower.
The rating system of the Royal Navy and its predecessors was used by the Royal Navy between the beginning of the 17th century and the middle of the 19th century to categorise sailing warships, initially classing them according to their assigned complement of men, and later according to the number of their carriage-mounted guns. The rating system of the Royal Navy formally came to an end in the late 19th century by declaration of the Admiralty. The main cause behind this declaration focused on new types of gun, the introduction of steam propulsion and the use of iron and steel armour which made rating ships by the number of guns obsolete.
HMS Cornwall was a 74-gun third-rate Vengeur-class ship of the line built for the Royal Navy in the 1810s. She spent most of her service in reserve and was converted into a reformatory and a school ship in her later years. The ship was broken up in 1875.
HMS Unicorn was a 28-gun Lyme-class sixth-rate frigate of the Royal Navy. She was originally ordered as a 24-gun ship to the draft of the French privateer Tyger. The third vessel of the Royal Navy to bear the name, Unicorn, as well as HMS Lyme which was a near-sister, were the first true frigates built for the Royal Navy. They were actually completed with 28 guns including the four smaller weapons on the quarterdeck, but the latter were not included in the ship's official establishment until 22 September 1756. The two ships differed in detail, Unicorn having a beakhead bow, a unicorn figurehead, two-light quarter galleries and only five pairs of quarterdeck gunports, while Lyme had a round bow, a lion figurehead, three-light quarter galleries and six pairs of quarterdeck gunports.
HMS Tremendous was a 74-gun third rate ship of the line of the Royal Navy, built to the lines of HMS Ganges by William Barnard's yard at Deptford Green, and launched on 30 October 1784.
HMS Gloucester was a 74-gun, third rate Vengeur-class ship of the line built for the Royal Navy in the 1810s. She played a minor role in the Napoleonic Wars and was cut down into a 50-gun fourth rate frigate in 1831–32. The ship was converted into a receiving ship and broken up in 1884.
HMS Falmouth was a 50-gun fourth-rate ship of the line built for the Royal Navy in the first decade of the 18th century. The ship participated in several battles during the War of the Spanish Succession (1701–15) and the War of Jenkins' Ear (1739–48).
HMS Milan was a 38-gun fifth rate frigate of the Royal Navy. She had previously been Ville de Milan, a 40-gun frigate of the French Navy, but served for only a year before being chased down and engaged by the smaller 32-gun frigate HMS Cleopatra. Ville de Milan defeated and captured her opponent, but suffered so much damage that she was forced to surrender without a fight several days later when both ships encountered HMS Leander, a British fourth rate. Milan went on to serve with the Royal Navy for another ten years, before being broken up in 1815, after the conclusion of the Napoleonic Wars.
Post ship was a designation used in the Royal Navy during the second half of the 18th century and the Napoleonic Wars to describe a ship of the sixth rate that was smaller than a frigate, but by virtue of being a rated ship, had to have as its captain a post captain rather than a lieutenant or commander. Thus ships with 20 to 26 guns were post ships, though this situation changed after 1817.
HMS Venus was the name ship of the 36-gun Venus-class fifth-rate frigates of the Royal Navy. She was launched in 1758 and served for more than half a century until 1809. She was reduced from 36 to 32 guns in 1792. She was sold in 1822.
HMS Ariadne was a 20-gun Hermes-class sixth-rate post ship built for the Royal Navy during the 1810s. The vessel was completed in 1816, modified in the early 1820s and only entered service in 1823. Ariadne was assigned to the Cape of Good Hope Station, followed by a stint in the Mediterranean Sea. The post ship was taken out of service in 1828, turned into a coal hulk and sold for scrap in 1841.
HMS Thetis was a 46-gun Leda-class fifth-rate frigate built for the Royal Navy during the 1810s. She was first commissioned in 1823 and was assigned to the South America Station three years later. The ship was wrecked in 1830 off Cape Frio, Brazil, with the loss of 22 crewmen; most of her cargo of bullion was successfully salvaged.
HMS Arethusa was a 46-gun Leda-class fifth-rate frigate built for the Royal Navy during the 1810s. The ship was never commissioned and was converted into a lazarette in 1836. She was renamed HMS Bacchus in 1844 and was further converted into a coal hulk in 1851–52. The ship was sold for scrap in 1883.
HMS Talbot was a 28-gun Atholl-class sixth-rate frigate built for the Royal Navy during the 1820s.