The John Ericsson-class ironclads were designed to meet the need of the Swedish and Norwegian Navies for small, shallow-draft armored ships capable of defending their coastal waters.

The standoff between USS Monitor and the much larger CSS Virginia during the Battle of Hampton Roads in, early 1862, roused much interest in Sweden in this new type of warship, as it seemed ideal for coastal defense duties.

In response, they sent Lieutenant John Christian d'Ailly to the United States to study monitor design and construction under Ericsson.

D'Ailly arrived in July 1862 and toured rolling mills, gun foundries, and visited several different ironclads under construction.

[2] The John Ericsson-class ships had one twin-cylinder vibrating lever steam engines, designed by Ericsson himself, driving a single four-bladed, 3.74-meter (12 ft 3 in) propeller.

The engines produced a total of 380 indicated horsepower (280 kW) which gave the monitors a maximum speed of 6.5 knots (12.0 km/h; 7.5 mph) in calm waters.

[6] The John Ericsson-class ships had a complete waterline armor belt of wrought iron 1.8 meters (5 ft 11 in) high and 124 millimeters (4.9 in) thick.

[7] Mjølner ran aground at Kragerø on 21 June 1869, and could not be pulled off the rocks until the ship's ammunition, iron ballast and 120 long tons (120 t) of coal were removed.

The court of inquiry found the ship's commander and the pilot liable for the repairs, but the parliament released the two from their obligation two years later.