HNoMS Mjølner (1868)

HNoMS Mjølner was the fourth of five ships of the s built for the Royal Swedish Navy and the Royal Norwegian Navy in the mid-1860s. Impressed by the use of ironclads during the American Civil War, the design was based on that of the USS Monitor. They were designed under the supervision of the Swedish-born inventor, John Ericsson—coincidentally, designer of the Monitor—, and built in Sweden. Mjølner was delivered in 1868 and ran aground the following year, although she was not seriously damaged. The ship was reconstructed in 1897 and given modern breech-loading guns. Mjølner was sold for scrap in 1909.

Design and description
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 the 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. John Ericsson, designer and builder of the Monitor, born in Sweden—although becoming an American citizen in 1848—offered to share his design with the Swedes. 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. He returned to Sweden in 1863 having completed the drawings of a Monitor-type ship under Ericsson's supervision.

The ship measured 60.88 m long overall, with a beam of 13.54 m. She had a draft of 3.4 m and displaced 1522 t. Mjølner was divided into nine main compartments by eight watertight bulkheads. Over time a flying bridge and, later, a full superstructure, was added to each ship between the gun turret and the funnel. Initially her crew numbered 80 officers and men, but this increased to 104 as she was modified with additional weapons.

Propulsion
The John Ericsson-class ships had one twin-cylinder vibrating lever steam engines, designed by Ericsson himself, driving a single four-bladed, 3.74 m propeller. Their engines were powered by four fire-tube boilers at a working pressure of 40 psi. The engines produced a total of 380 ihp which gave the monitors a maximum speed of 6.5 kn in calm waters. The ships carried 110 t of coal, enough for six day's steaming.

Armament
Mjølner was armed with a pair of Armstrong 270 mm rifled muzzle-loading guns. Each gun was constructed of steel and weighed 18.5 LT. The ship also carried a 80 mm gun. During Mjølner's reconstruction in 1897 her gun turret was fixed in place and modified to serve as a barbette for her two new breech-loading Cockerill 120 mm guns. She also received two 124 mm and 65 mm Cockerill guns mounted in her superstructure. In addition two 37 mm Hotchkiss 5-barrel revolving guns were mounted in the superstructure. They fired a shell weighing about 1.1 lb at a muzzle velocity of about 2000 ft/s. This gave them a range of about 3500 yd. They had a rate of fire of about 30 rounds per minute

Armor
The John Ericsson-class ships had a complete waterline armor belt of wrought iron that was 1.8 m high and 124 mm thick. The armor consisted of five plates backed by 91 mm of wood. The lower edge of this belt was 74.2 mm thick as it was only three plates thick. The maximum thickness of the armored deck was 24.7 mm in two layers. The gun turret's armor consisted of twelve layers of iron, totalling 270 mm in thickness on the first four monitors. The inside of the turret was lined with mattresses to catch splinters. The base of the turret was protected with a 127 mm glacis, 520 mm high, and the turret's roof was 127 millimeters thick. The conning tower was positioned on top of the turret and its sides were ten layers (250 mm) thick. The funnel was protected by six layers of armor with a total thickness of 120 mm up to half its height.

Service
The Norwegians had built one monitor-type ship of their own, HNOMS Skorpionen, in 1865, and laid down several others, but the Norwegian Parliament authorized construction of Mjølner in 1867 in Sweden at the cost of 1,102,000 Norwegian krone. She was launched in 1868 and completed on 7 September of that year. The ship ran aground at Kragerø on 21 June 1869. Mjølner could not be pulled off the rocks until the ship's ammunition, iron ballast and 120 LT of coal were removed. Her repairs were completed on 7 July 1869 by the Royal Dockyard at Horten, at the cost of 5,000 krone. 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. She was visited by King Charles XV of Sweden on one occasion when visiting one of Sweden's west-coast ports in the early 1870s. Mjølner was laid up after her refit in 1897, but was mobilized during 1905 when the personal union between Sweden and Norway was dissolved. She returned to reserve afterward before being sold for scrap in 1909.