The ironclad warship, USS Cairo and her sister ships reflect rapid advances in industrial and naval technology in mid-19th century America, a pivotal period in naval warfare history.

When the Civil War began in 1861, most ships were constructed of wood and powered by sail. Design and construction hadn’t changed much in 200 years. The launch of the U.S.S. Carondelet in 1861, the first ironclad built in the Western Hemisphere, signaled a new era of naval warfare.  She combined old and new technologies. New developments included the steam driven engine, armor plating, and shell guns.  These vessels were perfect for operating in the shallow coastal waters and rivers of the South.

Steam Engines
Iron Plating
Explosive Shells
USS Cairo

Steam Engines
The steam engine was not a new invention, and was described by an Egyptian scientist in 120 B.C.  In the 1600s and 1700s, steam engines were developed for use in pumping water out of mines.  By the 1800s, the steam engine had developed sufficiently to use in boats and ships.  Steam became a primary source of power and sail secondary.  In the early1800s steamboats became popular and revolutionized river transportation, particularly on the Mississippi and other Western Rivers.  Development continued and by the time James Buchanan Eads built the Cairo and her six sisters, the steam engine design had been standardized.  Cairo used two single-cylinder, non-condensing, reciprocating main steam engines and five coal-fired flue boilers.  The engines were simple and robust, so that they were easy to maintain.  Steam from the boilers was used once in the engines and then exhausted up the smokestacks to help improve the draft of the boilers. The five long, cylindrical boilers each used large pipes through their length, called flues, to heat the water and produce steam for the main engines and various steam pumps.

Iron  Plating
Vulnerable parts of the Cairo’s superstructure were covered in iron plating for protection from enemy gunfire.  Unlike wooden sides, which could turn into huge splinters when hit, armor plating was less likely to shatter when hit by a projectile. Wooden backing worked as an effective “shock absorber” when vessels were hit. The iron plating of the gunboat’s wooden pilothouse, forward casemate, and the casemates of the port and starboard sides over the engines and boilers consisted of 122 tons of 2-1/2" iron plating.  Unfortunately for several sister ships, the weight of armor plating prevented its use over the entire superstructure.  Dozens of crewmen were scalded to death when the boilers were hit by enemy gunfire that penetrated gaps in the armor. 

Explosive Shells
Explosive shells were more destructive to wooden ships than solid shot. Wood could absorb iron shot, limiting damage to small holes and large splinters, but explosive shells tore large holes in wood and produced many more splinters. For centuries, solid shot was used exclusively against ships until the introduction of Henri-Joseph Paixhans’ shell gun. The shell gun allowed for horizontal firing of shells and improved accuracy.  Before, shells could only be fired from mortars at high angles making ships quite difficult targets. The introduction of rifled ordnance at about the same time as the shell gun made both more accurate and deadly.

U.S.S. Cairo
Cairo was armed with 13 guns capable of firing shells. Her armament included three 42-PDR Army rifles, three 64-PDR Navy Smoothbores (also called 8”), six 32-PDR Navy Smoothbores, and one 30-PDR Parrott rifle.  The mix of rifles and smoothbore shell guns offered capabilities against a range of targets.

During the Vicksburg Campaign, ironclads were used extensively to provide additional firepower during bombardments, and patrol the Mississippi and its tributaries for enemy activity.  They were also used as shields when the Navy convoyed fragile troop transports past the Confederate guns defending Vicksburg.