The hull was to be formed of two portions, a kind of barge-like structure or lower hull, built of iron, and mostly under water when the ship was afloat, and fixed over this the upper hull, a raft-like structure, wider and longer, and with overhanging armoured sides and lighter deck-armour. The dimensions were—
- Upper part of hull, length 172 feet, beam 41 feet.
- Lower hull, length 122 feet, beam 34 feet.
- Depth, underside of deck to keel-plate, 11 feet 2 inches.
- Draught of water, 10 feet.
Engines and boilers were aft, and the long overhang of the armoured deck astern protected the under-water rudder and screw propeller. In the overhang at the bow there was a well, in which the anchor hung under water. Forward, near the bow, there was a small armoured pilot-house, or, as we now call it, "conning-tower." Amidships, in an armoured turret, were mounted two heavy smooth-bore guns, of large calibre, and throwing a round, solid shot.
The conning-tower was built of solid iron blocks, nine inches thick. The sight-holes were narrow, elongated slits. This was the helmsman's station.
The committee to which Ericsson's plans were referred was at first hostile; some of the members declared that the ship would not float, that her deck would be under water, and she would be swamped at once. Further objections were that no crew could live in the under-water part of the hull. But at length all objections were met, and the Swedish engineer was told that his plans were accepted, and that a regular contract would be drawn up for his signature. Ericsson knew the value of time, and before the contract was ready the keel plates of his turret-ship had been rolled and a dozen firms had started work on her various parts. While the ship was being built, he proposed she should be named the "Monitor," and the name became a general term for low-freeboard turret-ships.
the "MERRIMAC" & "MONITOR" drawn to the same scale
The keel of the ship was laid at Greenpoint Yard, Brooklyn, in October, 1861. She was launched on 30 January, 1862. The work of completing and fitting was carried on day and night, and she was commissioned for service on 25 February, 1862. But even when her crew were on board there were a number of details to be completed. Workmen were busy on her almost up to the moment of her departure from New York harbour nine days later, so there was no chance of drilling the men and testing the guns and turret.