They were basically improved Passaics, modified in accordance with war experience. 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.
Tecumseh was sunk in Mobile Bay by a mine that exploded under her turret, and went down in 25–30 seconds. Canonicus, the last survivor, was finally decommissioned 31 years before being sold.