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 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, the by now elderly sixth-rate frigates was found to be too small for the duties expected of a fifth-rate frigate, and was phased out without replacement, although a few lasted in auxiliary roles until after 1815.
In later years sixth-rate frigates were sometimes called "jackass frigates", as they were really not quite big enough for proper frigate duties.
HMS Surprise, a fictional frigate captained by Jack Aubrey (based on the actual historical frigate, formerly the French L'Unité, which was captured and renamed by the Royal Navy in 1796) plays a prominent role in many of Patrick O'Brian's popular novels, collectively known as the Aubrey-Maturin series. It was portrayed in the 2004 film Master and Commander.
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