The Fourth Rate was the point in the Royal Navy's Rating System where ships of the line met Frigates. In other words, it contained the largest Frigates and the smallest ships of the line. HMS Leopard was a ship of the line of 50 guns, which carried her guns on two complete gundecks, one of the smallest ships of the line and a type of ship which was verging on obsolete even at the time she was built. HMS Vernon/Actaeon was a large, spar-decked Frigate, similar to the ones made famous by the Americans during the War of 1812. Spar-decked means that although they carried their guns on two decks, the upper deck guns were actually out in the open and that the ships were built with a single dedicated enclosed gun deck. The Spar Deck was a single continuous deck running from bows to stern and made for a stronger hull than the more traditional forecastle and quarterdeck arrangement used on earlier Frigates. The large, 56-gun spar-decked Frigates were actually larger than a 50-gun ship of the line, but were more lightly built. HMS Vernon/Actaeon would have actually been similar in size to a 74-gun Third Rate ship of the line and would have weighed in around the 1600 - 1800 ton mark.
A Fourth Rate ship carried between 46 and 60 guns.
HMS Vernon/Actaeon would have carried 24pdr long guns on her gundeck and heavy carronades (usually 42pdrs) on her spar deck. At the time she was built, weapons technology was advancing rapidly. The advent of the explosive shell made wooden-hulled warships obsolete and within thirty years of the ship's launch, the Royal Navy was building iron-clad and then iron-hulled warships.
HMS Excellent, the ship to which HMS Vernon/Actaeon became a tender was originally built as the 98-gun Second Rate ship of the line HMS Boyne at Portsmouth. She was built to the same design as the previous HMS Boyne, which had caught fire and blown up in Portsmouth Harbour in 1795. That particular design was even older and was actually a slight modification of the design of HMS Victory, originally floated out of the No2 Dry Dock at Chatham in 1765. The second HMS Boyne had been launched in 1810, renamed to HMS Excellent and converted into a gunnery training ship in 1834, renamed again to HMS Queen Charlotte in 1859 before being broken up in Portsmouth in 1861. As tender to HMS Excellent, HMS Vernon/Actaeon would have just acted as an overspill accommodation hulk.
A print of the HMS Excellent Gunnery School circa 1872. The ships are HMS Excellent (foreground), HMS Calcutta (84 - middle) and HMS Vernon (background).
A photo of HMS Vernon in 1896 (by now renamed to HMS Actaeon)
HMS Actaeon in 1904
HMS Actaeon on her way to the breakers in 1923