An improvement on the preceding Passaics, modified in accordance with war experience, each vessel mounted two 15-inch (381 mm) Dahlgren guns.

When attacking the ironclad ram CSS Tennessee in 1865, the monitor Tecumseh was sunk by a naval mine, then termed a "torpedo".

Eight of the suspected conspirators for the assassination of Abraham Lincoln were incarcerated aboard Saugus and the monitor Montauk before they were transferred to the Arsenal Penitentiary.

The remaining four ships not commissioned during the war were built on the Ohio River, three at Cincinnati, and Ajax at South Pittsburgh.

A team, led by Alban C. Stimers, worked on a design based on a general plan developed by John Ericsson and in consultation with Gustavus Fox.

[1] An initial specification was issued to shipyards on 14 August 1862, that was very similar to the preceding Passaic-class monitor, to which 15 firms submitted bids.

The armor of the gun turret, of 21 ft (6 m) internal diameter, and the pilot house consisted of ten layers of one-inch plates.

A "rifle screen" of 1⁄2-inch (13 mm) armor 3 feet (0.9 m) high was installed on the top of the turret to protected the crew against Confederate snipers based on a suggestion by Commander Tunis A. M. Craven, captain of Tecumseh.

On 21 June, Commander Craven, of Tecumseh, spotted a line of breastworks that the Confederates were building at Howlett's Battery, and his ship opened fire at the workers.

[31] After commissioning, Manhattan steamed for the Gulf of Mexico and arrived at the Pensacola Navy Yard on 7 July, towed by the side-wheel gunboat Bienville.

Four hits were claimed, including one that broke the Tennessee's steering chains and another that jammed her stern gun port shutter in the closed position.

[44] Canonicus anchored at ranges from 900–1,200 yd (820–1,100 m) and fired 144 rounds and was hit four times, but suffered no casualties and no significant damage.

[48] After the assassination of President Abraham Lincoln on 15 April, eight of the suspected conspirators were incarcerated aboard Saugus and the monitor Montauk.

Briefly named Castor between 15 June and 10 August 1869, the ship was placed in reserve on 11 March 1872 at Hampton Roads, recommissioned on 21 November 1873 and then placed in ordinary at Richmond, Virginia, in 1889.

After spending time in Virginia at Norfolk, Brandon, City Point and Richmond, Manhattan was transferred to Philadelphia and laid up at League Island in 1888 before being struck from the Navy List on 14 December 1901 and sold on 24 March 1902 for breaking up.

Briefly recommissioned for local defense duties in response to the Spanish–American War, Wyandotte and Ajax were decommissioned in September 1898 and sold on 17 January and 10 October 1899 respectively.

[28][29] Decommissioned on 13 June 1865, Saugus was recommissioned on 30 April 1869 amidst reports of mistreatment of American citizens duringt the Ten Years' War in Cuba and patrolled along the Florida coast until end of 1870.

After being towed to Philadelphia, Pennsylvania, for repairs, Saugus was recommissioned there on 9 November 1872 and was based at Key West until transferred to Port Royal, South Carolina, in 1876.

[25][27] To prepare the ships for their lengthy voyage to Peru, around Cape Horn, Swift & Co. added a breakwater on the bow, stepped two masts with a fore-and-aft rig to supplement the engines, and provided closures to make vents and deck openings water tight.

[54] While this was going on, the United States was negotiating with Great Britain over the Alabama Claims, compensation for losses inflicted by British ships knowingly sold to the Confederacy during the Civil War.

Peru had been involved in an undeclared war with Spain, and the US was not willing to prejudice its claims against the United Kingdom by performing a similar action for a belligerent power.

Short on food, water and fuel, the monitor, was able to send a local schooner to Nassau to inform the authorities of their plight.

The ships finally reunited at St. Thomas in the Virgin Islands and had to wait for the Pachitea to arrive from Peru to tow Manco Cápac.

[55] Atahualpa was towed from Callao to Iquique, then part of Peru, from 11–22 May 1877, to defend that port from the rebel ironclad Huáscar during the Peruvian Civil War.

After striking the schooner Covadonga on blockade duty on 6 June, Manco Cápac was scuttled to prevent her capture when the city fell the following day.

Atahualpa, escorted by a tug, sortied to fight a long-range battle with the Chilean fleet, but failed to inflict any damage.