During the Revolutionary War era and into the 19th century, naval vessels were grouped into three major classes: ships of the line, frigates, and sloops of war. This classification system, rooted in the Age of Sail, organized warships by size, firepower, and battlefield role. Each class served a distinct purpose, from dominating open-sea battles to running patrols along distant coastlines.
Ships of the Line
Ships of the line were the battleships of the sailing era. They were the largest warships afloat, carrying anywhere from 64 to over 100 guns arranged across two or three gun decks. Their name came from the tactic they were built for: line-of-battle warfare, where two opposing columns of warships sailed parallel to each other and fired coordinated broadsides. This formation style was developed by the British in the late 17th century and became the standard for most navies.
The Royal Navy formalized this with a rating system. First-, second-, and third-rate ships, those mounting roughly 60 to 110 guns, qualified as ships of the line. A first-rate ship carried over 100 guns, required a crew of 850 or more, and displaced around 2,500 tons. Third-rates, the most common type in active service, mounted 64 to 80 guns with crews of 500 to 650. During the Napoleonic Wars, Britain had just 8 first-rate ships in service but 103 third-rates, making the smaller ships of the line the true workhorses of fleet combat.
The early United States Navy didn’t actually build ships of the line until well after the Revolutionary War. The young nation simply couldn’t afford them. Instead, America relied heavily on the next class down.
Frigates
Frigates were the cruisers of the 18th century. Smaller and faster than a ship of the line, they typically carried 28 to 44 guns on a single gun deck. What they gave up in raw firepower, they gained in speed and versatility. Frigates scouted ahead of the main battle fleet, raided enemy merchant shipping, escorted convoys, and carried dispatches across oceans.
Under the Royal Navy’s rating system, frigates fell into the fifth and sixth rates. A fifth-rate frigate mounted 32 to 44 guns with a crew of 200 to 300, while a sixth-rate carried around 28 guns. Neither was ever included in the line of battle. Their hulls were lighter and their sail plans designed for pursuit, not slugging matches. By 1814, Britain had 134 fifth-rate frigates in commission, more than any other single rate, reflecting just how essential these versatile ships were to naval operations.
For the early American Navy, frigates were the backbone of the fleet. Ships like USS Constitution, a heavy frigate mounting 44 guns, punched well above their class. American frigates were deliberately built larger and more heavily armed than their British counterparts, which gave them a meaningful edge in single-ship engagements during the War of 1812.
Sloops of War
Sloops of war were the smallest of the three major classes, carrying 10 to 20 guns. They were unrated in the Royal Navy’s system, meaning they fell below the formal six-rate hierarchy. A commander, rather than a full captain, typically led them. Despite their modest armament, sloops of war were among the most numerous vessels in any navy and handled an enormous share of the day-to-day work: coastal patrols, anti-piracy missions, blockade duty, and long-range cruises to show the flag in distant waters.
The USS Marion offers a good picture of what sloop-of-war service looked like. Launched in 1839 from the Boston Navy Yard, she displaced 566 tons and mounted 16 guns. Over three decades of service, Marion cruised off Brazil, patrolled the West Coast of Africa to suppress the slave trade, operated in the Mediterranean and East Indies, and eventually joined the Gulf Blockading Squadron during the Civil War. That kind of globe-spanning, multi-mission career was typical for sloops. By 1814, Britain alone had 360 sloops of war in commission, dwarfing every other category.
How the Rating System Worked
The three-class system was a simplified version of a more detailed hierarchy. The Royal Navy rated ships from first rate (the most powerful) through sixth rate, with everything below sixth rate classified as “unrated.” Ships of the line covered the first three rates. Frigates covered the fifth and sixth rates. Sloops of war and smaller craft were unrated.
The fourth rate occupied an awkward middle ground. These ships mounted 50 to 60 guns on two decks, which sounds formidable, but by the mid-1700s they were considered too small to stand in the line of battle and too large and slow to serve as true frigates. From 1756 onward, 50-gun ships were no longer classified as ships of the line. They were often called “cruisers,” a catch-all term for escort and patrol vessels that weren’t powerful enough for fleet actions.
The Shift to Modern Classification
The three-class system of the Age of Sail eventually gave way as technology transformed naval warfare. Steam power, iron and steel hulls, explosive shells, and eventually aircraft made the old gun-count ratings irrelevant. New categories emerged to describe fundamentally different kinds of warships.
Modern navies organize their fleets around aircraft carriers, surface combatants, and submarines. Aircraft carriers replaced ships of the line as the centerpiece of naval power. They serve as mobile airfields capable of launching strikes against targets in the air, at sea, and on land, projecting force across entire oceans. Surface combatants, including guided-missile cruisers and destroyers, fill roles once handled by frigates: escort duty, air defense, and offensive missile strikes. Submarines split into their own specialized categories. Attack submarines hunt enemy ships and submarines, gather intelligence, and launch cruise missiles at land targets. Ballistic missile submarines carry nuclear warheads and serve as a hidden, survivable leg of nuclear deterrence, able to strike anywhere in the world even if other nuclear forces are destroyed.
The underlying logic, though, hasn’t changed much. Navies still need heavy hitters for major engagements, versatile mid-size combatants for a wide range of missions, and smaller or more specialized vessels for patrol, reconnaissance, and presence. The ships look nothing like they did in the 1700s, but the strategic roles those three classes filled remain the foundation of how fleets are built.