A specially designed armed vessel, Floating Batteries are thought to be an invention of Spain (and possibly Germany). Seen around the 18th Century, they were large floating rafts reinforced with wooden and steel plated sides (resembling a floating Fort). Early documents show a length of 50 feet with a 13 foot girth. 24 artillery pieces (of varying calibers) were installed. Three small Masts (for sail/wind power) were added. Some versions included a semi-enclosed slanted roofing. Floating Batteries were quite possibly the forerunner of the later Ironclad warships. Some variations were steam powered as well (around the 19th Century). In theory, the Floating Battery could be deployed off an island or peninsula (at a hard to reach firing position). Guns could be fired for effect.
Obviously, the Floating Battery's “enemy” was Coastal Defense Artillery (which could easily destroy the relatively light armor of the vessel). During the start of The American Civil War, both North and South attempted and deployed Floating Artillery Batteries (with some modest success). With the development of “true” Ironclad ships and better Coastal Defense systems, The Floating Battery would cease to be used.