U.S.S. Cairo (1862)

The Cairo was an ironclad river gunboat, among the first commissioned in the Civil War. These 9 "City Class" vessels were assigned to James B. Eads and designed by Samuel M. Pook, and subsequently known in the service as the "Pook Turtles." Three were destroyed in the opening of the Mississippi; others were sunk and salvaged over and over, but the Turtles remained a crucial arm of the Union fleet in the river war on the Mississippi and its tributaries.

The "Pook Turtles" were an ironclad derivation of the river steamboat. They combined a casement battery on the main deck (inches above the flood), shallow draft, and internal paddle wheel. The paddle wheel was of the "reel" type seen in stern-wheelers. It turned in a race within the boat and benefited from armored protection; the top of the wheel's casing is visible on top of the casement, just forward of the stern flagstaff. Flat bottomed, the ship's hull had advanced watertight subdivision for the time; double rudders projected into the wake of the paddle wheel. The Cairo carried an armament of 14 heavy guns (see Specifications below). She was built at Mound City, IL and her engines were made in Pittsburgh; most of the remaining "Turtles" were constructed in Carondelet, MO (near St. Louis) at Eads' main yard. Cairo was ordered in September 1861 and commissioned January 15, 1862, becoming part of the Army's Western Gunboat Fleet, but commanded by Navy Lt. Pritchett. Byzantine chain-of-command was not unique to the Union forces; a parallel structure of overlapping jurisdiction existed in the Confederate river fleet, though as the Rebel force was removed from contention in 1862, it has not received as much attention from historians.

Specifications for the Cairo: 175'2" x 51'2" x 6'.  Displacement: 512 tons. Armament: (6) 32-pdr smoothbore; (3) 8" smoothbore; (4) 42-pdr rifle; (1) 12-pdr howitzer.  Armor: 2½" iron plate on casement, backed by 13¾" plank. Hull and conning tower: 2½" iron plate.   Top speed: 4 kts. Crew: 251.

During early 1862, the Cairo served in operations on the Ohio and Mississippi Rivers, assisting in the occupation of Clarksburg, TN, bombarding Fort Pillow, and fighting at the Battles of Plum Point and Memphis. After being transferred to the Vicksburg theatre, the veteran ironclad met a spectacular death, becoming the first Union ironclad sunk by a Confederate "torpedo," or mine. While leading a thrust up the Yazoo on December 12, 1862, she blundered unknowingly into a stretch of river being used as a proving ground for black-powder mines by the Confederacy's leading experts in the craft. Southern soldiers hiding out in nearby caves activated electric triggers to explode two mines near the Cairo's hull as she swept along near the bank, blowing huge holes in her hull. Though no lives were lost, the ironclad settled and sank in 30 feet of muddy river water, her twin 28' smokestacks and tall flagstaffs still poking above the surface.

Unlike several of her sisters in like circumstances, Cairo was not refloated because of ongoing campaigns in the area. With the passing of crewmen and witnesses, the location of the wreck eventually was forgotten. It was only through dedicated research in the 1960s that she was rediscovered--beautifully preserved by the cool river mud, surrounded by a treasure trove of historic artifacts. The wreck was raised in 1964, in the process being broken into three pieces. These were restored and put back together, and the ship is now on display at the Vicksburg National Military Park, her guns and engines magnificently restored, her casement mostly indicated only by the main beams left in place. This she is a sort of "visible ironclad" with see-through qualities for the public's instruction.

Of particular interest is the Cairo's propulsion system. It is the only known example of an original "western rivers" style steamboat engine. Technically, this is a 2-cylinder non-condensing inclined steam engine, with 22" bore and 6' stroke. The Cairo's paddle wheel was recessed in a race within the boat, protected by the vessel's outer casement armor. 22' in diameter and 15' in width, the paddle wheel was driven by cranks on either end of its iron axle, mounted at a 90-degree offset to each other. Power was delivered to the cranks by pitman arms made of wood and iron, their forward ends connected to the piston rods, their aft ends to the eccentric cranks on the axle. Further forward, the Cairo and her sisters carried five sausage-shaped, coal-fired fire-tube boilers mounted longitudinally, filling the hold from side to side. A transverse steam drum mounted above the aft end of the boilers acted as a manifold, distributing the steam to the two main engines and to the auxiliary engine (see diagram, below). The furnace gases were piped up a pair of lofty stacks aligned with the forward end of the boiler room. An auxiliary engine known as "The Doctor" ran various pumps, a silt filter, and other machinery around the boat; unfortunately this classic walking-beam engine was ruined during salvage, though drawings of it remain.

Plan of the CAIRO's Engines and Steam Plant

Cutaway Diagram of the CAIRO's interior

The salvaged engines were completely researched and restored by the American Society of Mechanical Engineers (ASME), which published a definitive appreciation of the ship's propulsion system, together with diagrams, photos, and full specifications, available online.

View inside the gun deck of a City class ironclad, circa 1862. This was probably the Carondelet or St. Louis, both of which were identical to the Cairo. The 17¼" thickness of the ship's casement is evident: look at the frames of those gunports.

One of the 32-pdr smoothbore cannon recovered from the Cairo's wreck. This cannon was identical to the models used on sailing warships of Nelson's day (circa 1800).