Some of the shortcomings of the Passaic class were recognized early enough to be corrected in the design of the 9 ships of the Canonicus class. The side armor of the raft, made up of five 1-inch thicknesses of iron, like the Passaics, was strengthened by the addition of two "armor stringers," iron belts 6 1/2 inches wide, running around the ship under the armor. For a distance of about 70 feet from the bow, these belts were 6 inches thick, and for the rest of the distance they were 4 inches thick. This made the armor near the water line at least 9 inches thick, at what experience had proved to be a vulnerable area. The deck armor was also increased to 1 1/2 inches thickness.
An additional band of armor 5 inches thick and 15 inches high was added to the base of the turret, while the turret itself was only 10 inches thick as opposed to the 11 inches of the Passaics. A more sophisticated recoil system also was introduced for the two 15-inch guns—a gear train from the gun carriage to a friction wheel was used. The guns themselves were 16 inches longer than those of the Passaics, eliminating the cumbersome smokebox.
The pilothouse on top of the turret was strengthened with two additional layers of 1-inch plate, giving its walls a thickness of 10 inches.
In order to handle the additional load of armor, the main engines were supplied by four boilers instead of two, and the ships were driven by a 14-foot-diameter propeller. In an effort to overcome the perennial ventilation problem, three, more powerful air pumps and blowers were provided.
The first of this class, the Canonicus, was launched less than 18 months after the Monitor, and just less than a year after the Passaic.
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