JR Potts, AUS 173d AB (Updated: 8/26/2016):
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.