USS Cairo (1861) was a City class ironclad gunboat constructed for the United States Department of War by James B. Eads and Company in Mound City, Illinois during the American Civil War. She was the first vessel of the City class ironclads, also referred to as the Cairo class. Cairo started her duty with the Army and was commissioned as part of the Union Army's Western Gunboat Flotilla and served with the Army's Western Gunboat Fleet, commanded by Flag Officer Andrew Hull Foote. The U.S.S. Cairo was one of seven ironclad gunboats named in honor of towns along the major rivers. Cairo was transferred to the Navy on October 1, 1862. The ironclads were impressive vessels having thirteen cannon of various sizes on board. The ironclads were new naval technology in the 19th and early 20th century navies bridging between wood and iron construction and from sail to screw / paddle (steam) propulsion. They were initiative and creative in their design, some succeeded and some failed, however and for a full understanding of the evolution of the warship, the importance of these vessels must not be overlooked.
The need for iron armored ships like Cairo became necessary with the invention of the exploding shell in 1822 developed by Henri Paixhans. With this new technology naval warfare entered a new artillery era with exploding shells being fired on a horizontal trajectory so the exploding shell could break through the wooden ship and explode inside killing crew members and destroying the gun platforms. The rifled cannon was a relatively new development in naval artillery at the time of the Civil War. This new weapon model of an iron plated ship using exploding shells made the wooden ship of the line that had been prized for century's obsolete. Vessels like the U.S.S. Cairo became the premier naval fighting craft of the Civil War. Many navies of the world became second rate.
Gunnery tests conducted by both sides early in the war demonstrated that armor thicknesses of two inches or less were insufficient to shield against naval exploding gunfire. However the technology of the day only allowed the production of two inch iron plate, so most ironclads had multiple layers, called plate layering. Two layers of two-inch plate was generally the standard for ironclads with some having a third layer. Plates were usually rolled or cast in oblong blocks and rolled out to their desired dimensions but on some vessels the plates were actually hammered out from the forge. This hammering method was time consuming but accounted for the armors strength.
All forms of iron armor were new and untried and some unusual combinations were created. A rail and plate system was tried using a T- rail configuration with outer plating, assembled in an overlapping manner which proved inadequate. Slats of wood spaced with edge-on iron plates or bars was tried and failed. Some ironclads builders used a layer of rubber under the iron plates hoping for deflection but the rubber quickly rotted away. Another attempt to deflect had the armor coated with lard or grease, results were poor under the hot sun and the smell was a major problem for the ship's crew. Their weakest point was the hull. Not only was the hull easily penetrated, but once breached, there was no way to isolate the damage, such as by watertight compartments. This made them vulnerable to mines.
The best armor configuration during the Civil War was found to be the forerunner of composite armor being several layers of plate mounted over a solid wood base. The major factories were located in the North and if the South had possessed enough iron and forging factories to cover its ships in six or more inches of iron plate, their ironclads would have been more durable. The City class ironclads were designed with armor protecting the bow and front of the ship and armor surrounding the boilers and engines. The pilothouse or bridge was reinforced but armor was limited on other areas of the ship. As time went on more armor was added inside the vessel protecting the steam drum and engines. The overall armor was inadequate in two ways the deck and the stern were not covered and the lack of deck armor made them vulnerable to plunging fire.
The river ironclads engines very similar to steamboats and were cheap to build, easy to maintain and could run on firewood if necessary with some loss of power when coal, the primary fuel was not available. Steam engines of the era are generally described in terms of the number of boilers, the bore or diameter and the stroke or the distance traveled by the piston, and the horsepower needed to power the ship. Most ironclads were driven by screw propellers, but some bore internal wheels like Cairo. Her steam engine was powered by five fire-tubed boilers with a 22 inch cylinder with a stroke of 6 feet producing 140 psi (965 kPa). Wheels were obviously the poorer choice for propulsion than screws and in some ships like the Cairo the wheels were moved inboard for protection. Even though she did not use a screw she was the fastest of the class.
The steam engine fired by coal created constant health concerns for the ship's crew and the efficiency of the engines for ventilation and draft. Ventilators circulated fresh air through the ship, and draft systems pulled new air into the furnace. When this system worked the crew was not affected. To accomplish this was a system of machine-driven fans and blowers to forcing fresh air into the ship. To aid the air circulation were small wind sails placed over open deck portholes catching fresh air and improving ventilation below decks. In battle the portholes were covered with armor plates. Ironclads depended on their smokestacks for engine power. As smoke and hot gases were drawn out of the stack a low pressure was created in the firebox drawing in needed fresh air. During battle when the stacks were shot away the draft to the engine was reduced and the ship lost power. The internal cabin filled with smoke and the crew's ability to fight was also reduced. From the earliest days of steam, ironclads were constructed to operate in calm waters. This river use required a need for a shallow draft able to support the heaviest load possible, so a flat-bottomed design was best in the shallow river.
Steering was still accomplished by a wheel and transmitted by tiller ropes and tackles to the rudder, as it had been in the days of sail. The protection of these ropes was important when damaged steering ability was lost and normally grounding the vessel. Communication between the pilot and the engine room was still accomplished by means of bells and hollow pipes that orders were shouted into. This system of on board internal communication continued into ship construction thru WWII. If the tubes were damaged messages were sent using a runner. Signaling between ships was accomplished by use of flags, lanterns and rockets. Also at times Army signal personnel were assigned aboard ship to communicate between ships and land forces by semaphore code.
Weapons onboard were commonly rated in terms of inches of the bore of the weapon or pounds, the weight of a ball or shot. Rifled bored cannon had significant advantages over smoothbores in range, accuracy, and penetrating power. Cairo had 13 cannon of various sizes, 6 x 32 pounders, 3 x 8 inch, 3 x 42 pounders, and 1 x 30 pounder. Each cannon had a separate crew needed to man the gun. The 32 pounders had 10 men plus a boy, the powder monkey, the 8 inch and the 42 pounder needed 14 men and a boy, and the 30 pounder needed 12 men and a boy for each gun. The gun crews on the Cairo mirrored the gun crew's routine as on the wooden ships. The major types of ammunition in use were the explosive shell and the solid shot but still available was grapeshot. Explosive shells, equipped with variable-time fuses, were effective against wooden vessels and fortifications. Wooden ships would be set on fire to and the explosion of the shell would send deadly splinters in all directions intended to kill the crew. Also antipersonnel shot was the grapeshot, a shell that turned the cannon into enormous shotguns.
An engagement with Confederate gunboats at Plum Point Bend on May 11, 1862 the Cairo and her sister ships forced the Southerners to abandon the Fort by June 4th . Two days later on June 6th near Memphis Tennessee, Cairo along with seven Union ships fought a running battle against eight Confederate gunboats. Five of the Southern gunboats were sunk or forced aground two were damaged, boarded and taken, one gunboat escaped. This was a major victory helping to clear Southern control on the river. The Northern forces wanted the river ironclads to control the Mississippi and the Ohio which accomplished would cut the South in half. On December 12, 1862, was assigned to clear mines from the river preparing for an attack on Haines Bluff, Mississippi three days later. During this duty Cairo struck an electric mine detonated by Southern solders concealed on the river bank. Cairo sank in 12 minutes with all hands able to abandon ship without any casualties and became the first armored warship sunk by an electric mine.
Cairo sat on the river bottom forgotten till she was found in 1956 by Edwin Bearss, Warren Grabau and Don Jacks. They used maps and divers to locate the wreck covered by years of river silt. Years passed and when evidence mounted that the Cairo was in fact the wreck she was raised late in 1964. She was listed on the National Register of Historic Places on September 3, 1971. In 1972, the United States Congress enacted legislation authorizing the National Park Service to accept title to Cairo and restore the gunboat for display. In June 1977 the vessel was transported to the park and partially reconstructed on a concrete foundation near the Vicksburg National Cemetery. A museum was built and a shelter to cover the vessel was completed in October 1980. The original shelter was replaced by a fabric cover to provide better cover. The ship and artifacts were recovered from Cairo from her cannon to personal effects of the crew are on display at the Vicksburg National Military Park, Vicksburg Mississippi.