Genesis of the Cairo
The USS Cairo was an ironclad Mississippi River gunboat, among the first ironclads commissioned in the Civil War. These seven City Class vessels were assigned by federal contract to James B. Eads and designed by Samuel M. Pook, and so were known in the service as the "Pook Turtles." All were formally named after river ports in the Mississippi basin: St. Louis, Cincinnati, Mound City, Louisville, Pittsburgh. Two more converted riverboats joined the new construction and were considered part of the class. Of these nine boats, three were destroyed in opening of the Mississippi; others were sunk and salvaged over and over. However, the Pook Turtles remained a critical arm of the Union fleet in the river war on the Mississippi and its tributaries.
The "turtles" were ironclad gunboats derived from the common American river steamer. They were flat-bottomed, with a "knuckle" bilge, three keels, and two rudders. They combined a sloped casement battery on the main deck (inches above the flood), shallow draft (6 feet, or 1.8 m), and recessed paddle wheel ("center wheel") for propulsion. The paddle wheel was of the reel type seen in stern-wheelers, but shorter. It turned in a well or race near the aft end of the boat, entirely inside the armored casement. The rounded top of the wheel's casing is visible on top of the casement, just forward of the stern flagstaff. The aftermost 60 feet (18.3 m) of the boat consisted of two pontoons ("fantails") on the sides of the paddle wheel wake, into which the twin rudders thrust to direct the course of the vessel. Between the casement and the stern extremity of the boat, the race was decked over, the planking supported principally by the fantails at either side. Abaft the paddle wheel, one gun on each side of the race aimed aft from the casement. Aft of the enclosed casement was a short deck used in navigation. Like all of Eads' gunboats, the Cairo was designed with advanced watertight subdivision: the 175' hull was divided into seven watertight compartments.
The Cairo carried an armament of 14 heavy guns (see Specifications below). She was built at Mound City, Illinois and her engines were made in Pittsburgh; most of the remaining "turtles" were constructed at Carondelet, Missouri,near St. Louis, in Eads' main yard. A wealthy entrepreneur with toes dipped in the shipping and foundry businesses, Eads became one of Uncle Sam's main contractors, building several classs of monitors for the navy after the success of the City class boats, which were instrumental in crushing Confederate naval ambitions on the Big Muddy. In keeping with Eads' excellent industrial contacts, charcoal iron plates arrived at the yards with clockwork regularity. Like her City class sisters building at Carondelet, Cairo's original protection sheme comprised 2½"-thick iron plates on the sides of the battery, but not extending all the way to the forward corner of the casement, and the same thickness on the forward panel of the casement; thus only 50% of the vessel was armored when she completed. The casement itself was robustly constructed of 24 - 26" wood. 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. The ship was transferred to U.S. Navy ownership on October 1, 1862, ten weeks before her demise.
Dimensions: 175' x 51'2" x 6'. Displacement: 888 tons full load. Armament: (6) 32-pdr 42-cwt smoothbores; (3) 8" 64-pdr smoothbores; (3) 7" 42-pdr Army rifles; (1) 12-pdr howitzer (boat gun for boarding/landing parties). Armor: 2½" iron plate on casement, backed by 26" white oak. Casement inclined by 35° on sides, 45° on bow and stern panels. Armor type: Charcoal iron, 13" wide sheets in lengths from 8'6" to 11'. Hull and conning tower: 2½" iron plate. Connning tower's forward panels backed by up to 19½" white oak. Full strength armor extended 55 in. below the bottom of the casement, well below the waterline. Additional plates of 1" railroad iron added by crew to bow casement panel and internally to protect conn, engines and wheel. Weight of hull and casement: ~350 tons. Weight of armor, including supplementary thickness added in the yard and by the crew: 122 tons. Propulsion: (5) coal-fired fire-tube boilers; 2-cyl inclined non-condensing Western Rivers type reciprocating engine. Engine inclined at 15°, 22" bore x 72" stroke, shafted to a 22'-diameter recessed paddle wheel. Boiler size: 36" dia x 24' length. Maximum pressure: 140 psi. Chimneys: 3'8" dia. x 28' tall, return flue type. Fuel consumption: 0.885 tons per hour. Maximum speed: 6 kts. Crew: 175. Cost: US $101,808 at 1861 valuation.
Dimensions: 53.3m x 15.6m x 1.83m. Displacement: 888 tons full load. Armament: (6) 32-pdr 42-cwt smoothbores; (3) 203 mm 64-pdr smoothbores; (3) 178 mm 42-pdr Army rifles; (1) 12-pdr howitzer (boat gun for boarding/landing parties). Armor: 64 mm iron plate on casement, backed by 66 cm white oak. Casement inclined by 35° on sides, 45° on bow and stern panels. Armor type: Charcoal iron, 33-cm-wide sheets in lengths from 2.59 m to 3.35 m. Hull and conning tower: 64 mm iron plate. Connning tower's 3 forward panels backed by 502 mm white oak, after 5 panels: 305 mm. Full strength hull armor extended 1.4 m below the bottom of the casement, well below the waterline. Additional plates of 25 mm railroad iron added by crew to bow casement panel and internally to protect conn, engines and wheel. Weight of hull and casement: ~350 tons. Weight of armor, including supplementary thickness added in the yard and by the crew: 122 tons. Propulsion: (5) coal-fired fire-tube boilers; 2-cyl inclined non-condensing Western Rivers type reciprocating engine. Engine inclined at 15°, 559 mm bore x 1.83-m stroke, shafted to a 6.71-m diameter recessed paddle wheel. Boiler size: .9-m diameter x 7.32 m length. Maximum pressure: 0.97 MPa. Chimneys: 1.12 m dia. x 8.53 m tall, return flue type. Fuel consumption: 0.885 tons per hour. Maximum speed: 11.2 km/hr Crew: 175. Cost: US $101,808 at 1861 valuation.
Deck plan of the similar gunboat Benton shows center wheel and paddle race (blue rectangle near stern).
During early 1862, the Cairo served in operations on the Ohio and Mississippi Rivers, assisting in the occupation of Clarksburg, Tennessee, bombarding and occupying Fort Pillow, and fighting in the crucial Battles of Plum Point and Memphis. In the latter encounter, the Federal flotilla routed a Southern force of eight unarmored rams, sinking or capturing all but one of them, on June 5, 1862. After experience in action, captains of the gunboat fleet concurred that their 2½" armor was not enough. For example, 4½" was common in contemporary European ironclads, while the Monitor approached invulnerability with 8" of laminated iron on her turret. Some supplementary armor was added to Cairo in the shipyard, but the crew was kept busy adding thicknesses of railroad iron as time allowed, almost until the day she was sunk.
When Memphis had been captured and the Union's hold consolidated, the ironclad squadron was transferred to the Vicksburg theatre, where Gen. Ulysses S. Grant had launched his famous offensive to encircle the stubborn Rebel fortress (it surrendered July 4, 1863 after a long and bitter siege). During mid-March, five of the Pook ironclads took part in the hazardous Black Bayou expedition, under fire and confronting Confederate obstructions every yard of a 40-mile inland voyage behind enemy lines. None of the ironclads was lost, but their attempt to flank Vicksburg was a failure. Arriving above Vicksburg at Helena, Arkansas, Cairo's crew at first knew only the workaday side of war: suppressing smuggling across the river; salvaging coal from a sunken barge to alleviate a temporary fuel shortage in the fleet; supervising river traffic, including steady flows of Confederate prisoners bound to Vicksburg for exchange, and liberated Union prisoners returning upstream.
It was in this campaign that the veteran ironclad met her doom, becoming the first warship ever sunk by a "torpedo," or underwater mine -- the IED of its day. Her commander, LTC Thomas Selfridge, was designated to lead a thrust up the Yazoo, a tributary of the Mississippi that empties in to the main river just NW of Vicksburg. The purpose of the mission was to clear the river of torpedoes (right) and make it safe for navigation. Selfridge was specifically instructed to let trained men in small boats disable the treacherous devices. On December 12, 1862, the flotilla entered the mine-infested portion of the Yazoo, some 16 miles from the confluence with the Mississippi. Unbeknownst to the Yanks, the stretch of river they were about to invade was the principal proving ground for black-powder mines in the Confederacy, staffed by its leading experts in the craft. As the bluejackets nosed into their territory, Southern soldiers watched from dugouts along the levee, ready to explode the mines manually. LTC Selfridge, impatient with the slow pace of operations, ordered the Cairo forward. As she swept along near the bank, the Rebel watchers activated electric triggers to explode two mines under her hull, blowing huge holes in the bow. Selfridge turned her bow toward the bank, but it was clear the Cairo was in trouble. The remainder of the squadron hastened to assist, taking off the men in a rushed evacuation. The Cairo rapidly settled and sank in 36 feet of muddy river water, her bow nudging the east bank of the river. In 7 minutes, all that could be seen was the tops of the twin 28-foot smokestacks and some tall flagstaffs still poking above the surface. Mercifully, no one was killed in the incident.
No attempt was made to refloat the Cairo. The Yanks pulled down the telltale chimneys and flagstaffs of their flagship before departing the scene, to hide the wreck and discourage any effort to salvage the guns. 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 Cairo was located -- beautifully preserved by the cool blue-clay river mud, filled with a treasure trove of historic artifacts. The wreck was raised in 1964, accidentally being broken into three pieces in the process. These were stabilized and reassembled at the Ingalls Shipyard in Pascagoula, on the Gulf coast. Title was passed to the National Park Service, and the partially reconstructed ship is now on display at the Vicksburg National Military Park, her guns and engines magnificently restored, her forward armored bulkhead in position, but parts of her casement structure indicated only by the burly white-oak beams left in place. Thus Cairo is a sort of "see-through, walk-through ironclad" for the public instruction. Many of the objects retrieved are on public display: everything from disarmed ammo and boiler room tools to footwear, razors, earrings -- even personal photographs owned by the crewmen. The ship has proven a valuable time capsule for historical study, and the Cairo exhibit at Vicksburg has made the Civil War accessible to thousands since its 1967 opening.
The Cairo's Steam Engines
Of particular interest is the Cairo's propulsion system. It is the only known original western rivers type steamboat engine, and was beautifully preserved when raised. Amazed experts from the Smithsonian reported rubber hoses in like-new condition after 102 years' immersion. Technically, the Cairo's power plant is an inclined 2-cylinder non-condensing steam engine, with a 22" bore and a 6-foot 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 secured to the piston rods, their aft ends to the eccentric cranks on the wheel's axle.
To power their machinery, the Cairo and her sisters each carried five sausage-shaped, coal-fired fire-tube boilers mounted longitudinally, filling the forward hold from side to side. A transverse steam drum mounted above the aft end of the boilers acted as a manifold, distributing the available steam to the two main engines and to the auxiliary engine (see diagram, below). Combustion gases were vented through a pair of lofty stacks aligned with the forward end of the boiler room. An auxiliary engine known as "The Doctor" fed steam to the capstains, pumps, a silt filter, and other machinery around the boat. This classic walking-beam engine was destroyed during salvage, though drawings of it remain. While being winched to the surface, cables bit into the waterlogged wood of Cairo's hull until it broke apart at the points of least rigidity: the boiler room and the paddle-wheel race. Both these areas contained large open spaces without many transverse timbers for stiffening. To the horror of salvors, the pieces tumbled back into the muddy waters of the Yazoo. The three sections then had to be salvaged individually, conserved, and gingerly fitted back together like the pieces of an historic jigsaw puzzle.
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.
The Pook Turtles deployed on muddy waters, as rendered by Daniel Dowdey; our thanks for permission to display his fine work.
View inside the gun deck of a City class ironclad, circa 1862. This was probably the Carondelet or St. Louis, both of them nearly 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 nearly identical to the models used on sailing warships of Nelson's day (circa 1800).
The fatal moment: underwater explosions rip open the Cairo's wooden hull as sailors from other vessels in the minesweeping operation watch in horror. Nearness of rescue ships and men averted a far worse disaster: an improvised but very effective evacuation made sure that no lives were lost, although Rebel bullets were whizzing through the air the entire time. Enlarge picture
An assortment of Cairo ordnance spread out along the wayside for conservation during the salvage.
The Cairo's octagonal conning tower was the fist piece lifted from the wreck, after being loosened by divers. It was displayed along with artifacts and a 32-pdr gun, at gatherings to raise funds for salvage. In the end a patchwork of private, county, and state funds was necessary to salvage the sunken warrior; the National Park Service eventually stepped in with the funds and exhibition space to complete the restoration.
Spiders of the Cairo's paddle wheel landed at Ingalls Shipyard, Pascagoula, Mississippi.
The Cairo's wreck being reassembled on the beach at Pascagoula. The stern section is furthest from the camera. Salvors compared the project to reassembling a giant jigsaw puzzle.
The Cairo today, reconstructed and displayed near Vicksburg. One of the mine holes can be seen in the hull at right here. Click to see photo in color.