Twenty ships of the class were eventually built at great expense, but proved so unseaworthy when trialed that they were quickly sidelined, causing a public scandal.

The specifications for the Casco class originally called for a vessel with a light draft, not exceeding six feet, and a low freeboard to present the smallest possible target to Confederate guns.

Ericsson came up with a design for a 225-foot (69 m)-long vessel with a single revolving turret containing two 11-inch (280 mm) guns, an armored upper deck, and twin screw propellers giving a top speed of around eight knots.

The new office, located across the hall from Ericsson's design bureau, was nominally headed by Rear Admiral Francis H. Gregory, but was effectively run by Chief Engineer Alban C. Stimers, to whom was entrusted the power of setting general plans and ship specifications.

[7] The final design called for a total of thirteen auxiliary engines and pumps per ship, fancy brasswork in place of cast iron, and a complex system of pipes for draining and filling the water tanks.

[9] A second trial, of USS Tunxis, confirmed the disaster, with waves washing over the deck and the ship only able to make a speed of 3½ knots as opposed to the original specification of eight.

The vessels were redesigned and refitted in order to improve buoyancy, but few of them saw active service before the end of the war and those that did were decommissioned and laid up within months, while the majority were never commissioned at all.