The ships were basically improved Passaics, with wood and iron upper and iron lower hulls, modified in accordance with war experience. Catawba and Oneota were named Atahuallpa and Manco Capac by the Peruvians, who acquired them in 1868. The six other ships were renamed Scylla, Castor, Ajax, Neptune, Centaur and Vesuvius on 15 June 1869. They reverted to their original names on 10 August 1869, except Ajax who kept its name and Vesuvius who became Wyandotte. The four ships not commissioned during the war were built on the Ohio river, three at Cincinnati and Manayunk as far up as South Pittsburgh.
The hull lines were improved and designed speed is given as 13 knots but there was no hope of getting near this. The 5 × 1 in side armour was backed by two iron stringers 6½ in deep and 6 in thick for 70 ft from the bows, but 4 in elsewhere, and the armour lower edge was 3 × 1 in. The turret, of 21 ft internal diameter, had 10 × 1 in plates as did the pilot house above, and the funnel base was also armoured. The turret skirt was protected by a 5 in thick and 15 in high ring fixed to the deck, and as in other later monitors the 15 in guns were longer than in the Passaic class and fired with their muzzles outside the turret.
Ships of the Canonicus class: