John Ericsson was a Swedish-American engineer and inventor. He was active in England and the United States.
Ericsson collaborated on the design of the railroad steam locomotive Novelty, which competed in the Rainhill Trials on the Liverpool and Manchester Railway, which were won by inventor George Stephenson's (1781–1848), Rocket. Later in North America, he designed the United States Navy's first screw-propelled steam-frigate USS Princeton, in partnership with Captain (later Commodore) Robert F. Stockton (1795–1866) of the U.S. Navy, who unjustly blamed him for a fatal accident on the new vessel in 1844. A new partnership with Cornelius H. DeLamater (1821–1889), of the DeLamater Iron Works in New York City resulted in the first armoured ironclad warship equipped with a rotating gun turret, USS Monitor, which dramatically saved the U.S. (Union Navy) naval blockading squadron from destruction by an ironclad Confederate States naval vessel, CSS Virginia, at the famous Battle of Hampton Roads harbor at the southern mouth of the Chesapeake Bay (at the confluence of the James and Elizabeth Rivers) in March 1862, during the American Civil War (1861-1865).