After several years of inactivity, she was selected for conversion into an experimental turret ship instigated by Captain Cowper Coles, who believed that a mastless ship armed with turret-mounted guns was the best possible design for a coast-defence ship. The order to proceed with the conversion was issued on 4 April 1862.
She was razed down to the lower deck, leaving her with between 7and of freeboard. The decks and hull sides were strengthened to carry the planned armament, and to absorb the force when the guns were fired. There was some delay when it was found that she had been cut down too far, necessitating some re-building of the sides. On the completion of her conversion on 20 August 1864, she was the first British turret-armed ship, and the only one with a wooden hull. Her length-to-beam ratio was slightly under 4:1, which was the smallest ever ratio used in British armoured ships.
The original design included five turrets, each containing either two 68-pounder smoothbore or one 100pdr smoothbore cannon. This was modified to a four-turret configuration, with one twin turret, and three single turrets. The initial guns carried were NaNinches smoothbores that fired a 150-pound spherical steel shot. In 1867 they were all replaced by 9inches muzzle-loading rifles.
On 15 January 1866 three shots were fired at close range against the after turret of Royal Sovereign by one of the 9inches guns carried by HMS Bellerophon to evaluate how well Coles' turrets held up to gunfire. While the armour plates of the turret were displaced, and one shot pierced the back of the turret, the ability of the turret to turn and the guns to fire was not impaired.
She was commissioned at Portsmouth for service in the English Channel, where she undertook limited operational service and was used for gun and turret testing and evaluation. She paid off in October 1866, being then re-commissioned in July 1867 for the Naval Review. She was thereafter attached to the naval gunnery school as gunnery ship until 1873, when she was replaced by and demoted to fourth class reserve. She saw no further service until her sale in May 1885.
. George Alexander Ballard . The Black Battlefleet . 1980 . Naval Institute Press . Annapolis, Maryland. 0-87021-924-3.