The rating system in the Royal Navy as originally devised had just four rates, but early in the reign of Charles I the original fourth rate was divided into new classifications of fourth, fifth, and sixth rates. While a fourth-rate was defined as a ship of the line, fifth and the smaller sixth-rates were never included among ships-of-the-line. Nevertheless, during the Anglo-Dutch Wars of the 17th century, fifth rates often found themselves involved among the battle fleet in major actions. Structurally, these were two-deckers with a complete battery on the lower deck, and fewer guns on the upper deck. The fifth rates at the start of the 18th century were small two-deckers, generally either 40-gun ships with a full battery on two decks, or "demi-batterie" ships, carrying a few heavy guns on their lower deck and a full battery of lesser guns on the upper deck. However, the latter were gradually phased out, as the low freeboard meant that it was often impossible to open the lower deck gunports in rough weather. The 40-gun fifth rates continued to be built until the later half of the 18th century. From mid-century, a new fifth-rate type was introduced: the classic frigate, with no gun ports on the lower deck, and the main battery of from 26 to 30 guns disposed solely on the upper deck, although smaller guns were mounted on the quarterdeck and forecastle. Fifth-rate ships served as fast scouts or independent cruisers and included a variety of gun arrangements. The fifth rates of the 1750s generally carried a main battery of twenty-six 12-pounders on the upper deck, with six 6-pounders on the quarterdeck and forecastle to give a total rating of 32-guns. Larger fifth rates introduced during the late 1770s carried a main battery of twenty-six or twenty-eight 18-pounders, also with smaller guns on the quarterdeck and forecastle. Tonnage ranged from 700 to 1450 tons, with crews of 215 to 294 men. To be posted aboard a fifth-rate ship was considered an attractive assignment. Fifth rates were often assigned to interdict enemy shipping, offering the prospect of prize money for the crew. Fifth-rate frigates were considered useful for their combination of manoeuvrability and firepower, which, in theory, would allow them to outmanoeuvre an enemy of greater force and run down one of lesser force. It was for this reason that frigates of this sort were commonly used in patrol and to disrupt enemy shipping lanes much as heavy cruisers would later in history.
