A mortar boat is simply a timber raft 60 feet by 25 feet, and 45 inches deep. The timbers are firmly held together – and the whole is covered with heavy plank for a deck. Each boat has eight hatches 5 feet square and 8 feet deep, designed for ammunition. The only implements on them for locomotion are two or three pair of oars. They have also, each, two anchors and cable chains, two coils manila cable, two pairs tackle blocks and lines, an awning, and some have a canvas tent, the spoil of Forts Henry and Donelson.
Above the coarse plank flooring there is laid a water tight deck, and around this sort of low bulwark square timber two feet high and eight inches thick. It extends along the sides of the rafts, and falls short of covering the corners, but extends out at each end forming an obtuse angle. Attached to this, on the outside, is a bullet proof protection for the men. It is 7-8 inches boiler iron, 6 1-2 feet high – pierced at the side with several openings to give egress to the men and holes to look out.
Attached to each mortar boat is a sailing crew, consisting of a captain familiar with flat-boating experiences and the river channel, and two sailors.
The mortar is a most formidable looking thing. Its length is 4 1-2 feet, diameter 43 inches; diameter of bore 13 inches, leaving walls 15 inches thick; depth of bore 3 feet; weight of each a little over 17,000 pounds; weight of each carriage about 5,000 pounds. The fighting crew for a mortar is two lieutenants, twelve men, and one captain to two mortars.
– Published in The Davenport Daily Gazette, Davenport, Iowa, Saturday Morning, March 29, 1862, p. 2