[Illustration: SECTIONAL VIEW OF THE U.S. STEAM FRIGATE MERRIMAC.
_1857 inboard plan of screw frigate_ Merrimack. _Screw could be hoisted into a well to reduce drag when the vessel was under sail._]
[Illustration: Sidewheels: good maneuverability, but vulnerable above the water.]
[Illustration: The screw: more efficient, and protected below the waterline.]
Thus when naval officers learned of the conversion of _Merrimack_ into an armored blockade-breaker, they were understandably worried. In the debate over the type of vessel the Navy should develop to counter the Southern threat, John Ericsson’s proposal for a turreted, shallow-drafted coastal ironclad won out over larger, oceangoing designs with traditional broadsides. His ironclad, called U.S.S. _Monitor_, was the prototype of the turreted ironclad class named after it. (While monitors and _Virginia_-type ironclads continued to meet the needs of a mostly coastal and riverine naval war, two broadside ironclads were built by the Union. One of them, _New Ironsides_, was quite effective.)
_Monitor_ fought _Virginia_ to a standoff the day after the latter sank _Cumberland_. It was the first clash between steam-powered ironclads, and the world took notice. Captain John Dahlgren, creator of _Monitor’s_ two big guns, put it succinctly: “Now comes the reign of iron—and cased sloops are to take the place of wooden ships.”
It did not happen immediately: _Monitor_-class iron hulls were not very seaworthy. (_Monitor_ sank in late 1862 while being towed during a storm off Cape Hatteras.) Wooden ships under both sail and steam power continued to fight the Civil War’s deepwater battles, but Dahlgren’s words came true after the war. The evolving iron, and then steel, warship incorporated elements from both ironclads: the deeper hull and superstructure of _Virginia_ and, rather than multiple-gun broadsides, a few large guns in revolving _Monitor_-type turrets that allowed the guns to be trained without turning the entire vessel.
Only four monitors were built by navy shipyards, but officers considered them the best produced during the war. _Monadnock_, a double-turreted monitor built at Charlestown, was generally thought the best of the lot and the only one of this class to see action. After the war, it proved its unusual seaworthiness by voyaging around Cape Horn to San Francisco.