The USS Monitor heralded a new era in warship design. Facing off with the CSS Virginia at Hampton Roads, she fought the much larger (980 tons displaced versus approximately 4000, and two guns to ten) ironclad to a draw. The US Navy immediately saw the merits of the design, and despite the foundering of the original Monitor, several additional multiple-ship classes were built and put to great use during the American Civil War. Other navies worldwide began to emulate the design, producing their own monitor ironclads.
The development of the ironclad warship led down several paths, of which the USS Monitor was only one. Broadside ironclads such as the HMS Warrior and casemate ironclads such as the CSS Virginia, or the City-class gunboats of the US Navy, were others. The Monitor and her progeny were distinct from all earlier warship in that her main battery was contained in a rotating turret placed atop the hull, with very low freeboard. These ships were well armored, with six inches of armor plating on the turret being a starting point. While they displaced very little compared to a ship of the line, they had uncommonly thick armor coupled with disproportionately heavy armament for their size. Propelled solely by steam, they were nonetheless slow, usually doing less than ten knots at full speed.
The early form of the type was exemplified in the Passaic-class in US Naval service, which displaced only 1,335 tons. Despite the small size (approximately 1/5th of a ship of the line in displacement) they were armed with one XI-inch Dahlgren gun and one XV-inch Dahlgren each, contained in a single turret. The turret armor was eleven inches in thickness, with the belt armor running from three to five inches and the deck armor being one inch thick. The thick armor, heavy armament and low freeboard did combine to render these ships unsuitable for sailing on the open ocean, but they were primarily employed in coastal or riverine environments. This class was highly successful, excelling at the bombardment of fortified shore positions.
After the end of the American Civil War, naval development stalled in the United States for lack of funding. While the broadside ironclad would be further developed overseas, the monitor would see fewer improvements. The US Navy would order a new class of monitors in the 1870s, but they would not be commissioned until the 1890s, by which time they would be obsolete. They were similar to earlier double-ended (two turrets) monitors, with additional superstructure above the weather deck. This class would see service in the Spanish-American war, with mixed results. Poor ventilation had always troubled monitors, but the new ones were nearly crippled by it. Their poor seakeeping abilities and low speed made them spectacularly unsuited for war with a foreign foe. They would serve alongside newer battleships, which nonetheless owed elements of their designs to the USS Monitor.
Once the HMS Dreadnought burst onto the scene, most surviving monitors were converted into training ships, hulks, or other auxiliary vessels, though some would see action in WWI. Few survive in their original configuration. A developmental dead-end, the monitors were powerful, but showed their roots in a vessel intended for coastal and riverine operations, for a Navy not yet belonging to a world power. They were small ships, punching above their weight, but they were slow, unseaworthy and unpleasant to crew. Despite their defects, monitors were a neccessary step in the evolution of the warship.