Breastwork monitors were the most powerful ships of their day but had very short range because contemporary steam engines were not very efficient. A solution was the masted turret ship, which was a monitor-like vessel but with masts and full rigging. To prevent the rigging from being damaged when the guns fired through it, it was attached to a platform mounted above the gun turrets known as the hurricane deck instead of brought down to the main deck. Tripod masts were also used to minimise standing rigging.
The major drawback of this type of ship was that the weight of the heavily armoured turrets added to that of the masts reduced the metacentric height and care had to be taken to make the ship stable. One way of limiting this was to reduce the freeboard to bring these weights closer to the centre of gravity, but making them wet and vulnerable to flooding in rough weather.
Captain was designed by Captain Cowper Phipps Coles, who had previously designed the then-revolutionary gun turrets fitted to contemporary turret ships. Coles had been highly critical of Admiralty-produced designs for masted turret ships, a subject of considerable political antagonism. Coles had won wide backing from parliament and the public and the Royal Navy was compelled to allow him to build a ship to his design. The vessel was launched on March 27 1869 after two years construction at Cammell Laird, Birkenhead, England.
Insufficent supervision of weights during the building - partly due to Coles' protracted illness - meant that she was 735 tons heavier than planned. The designed freeboard was just 8 feet and the additional weight brought it down to just 6 feet 6 inches. She was commissioned on April 30, 1870 under Captain Hugh Talbot Burgoyne, V.C.
Shortly after midnight on September 7 1870 whilst cruising off of Cape Finisterre as part of a squadron of 11 ships, she heeled over under the force of the wind on her 50,000 square feet of sail. Before the captain's order to cut away the topsail could be carried out the roll increased and she capsized and sank with the loss of around 480 lives, including Coles. Only 18 of the crew made it to a boat which had broken free and survived.
A subsequent investigation showed that the ship was insufficiently stable - at 14 degrees heel (when the edge of the deck touched the sea) the torque due to the bouyancy pushing the ship upright again was just 410 foot-tons (4,080 kNm). HMS Monarch, another masted turret ship which was present at the sinking, had a righting torque of 6,500 foot-tons (64,700 kNm) at the same angle.