HMS Captain was a major warship built for the Royal Navy as a semi-private venture, following a dispute between the designer and the Admiralty.

In early 1863 the Admiralty gave Coles permission to work with Nathaniel Barnaby, head of staff of the Department of Naval Construction, on the design of a rigged vessel with two turrets and three tripod masts.

Stunned by the committee's decision to cancel his single-turret ship and his proposal for a two-turret vessel, and objecting to the Monarch's design, Coles launched a strong campaign against the project, attacking Vice Admiral Robert Spencer Robinson, Controller of the Navy, and various other members of the committee and the Admiralty.

[10] On 17 April 1866, Coles submitted to the Admiralty his critique of the proposed Monarch (designed by the Controller's department and the Chief Constructor), stating that he could not publicly endorse a vessel which did not represent "my views of a sea going Turret-ship, nor can she give my principle a satisfactory and conclusive trial."

Sensing that such an increasingly acrimonious and high-profile debate would only continue, the First Naval Lord, Admiral Sir Frederick Grey, minuted four days later (21 April) that Coles should at last be allowed to build what he felt would be a 'perfect' seagoing turret-ship.

Robinson noted that the low freeboard could cause flooding issues on the gun deck, and Reed criticised the design in 1866 both for being too heavy and for having too high a centre of gravity.

[15] Insufficient supervision during the building, owing partly to Coles' protracted illness,[16] meant that she was 735 long tons (747 t) heavier than planned.

In trials versus the Monarch, she performed well and returned to sea in July and August, travelling to Vigo, Spain, and Gibraltar in separate runs.

[20] On the afternoon of 6 September 1870 Captain was cruising with the combined Mediterranean and Channel Squadrons comprising 11 ships off Cape Finisterre.

The commander in chief, Admiral Sir Alexander Milne,[22] was on board to see her performance, and speed had risen to 11–13 knots before he departed.

Not being accustomed to ships with such low freeboard, he was disturbed to note that at this speed with the strengthening sea, waves washed over the weather deck.

The First Lord of the Admiralty, Hugh Childers, and Under-Secretary of State for War, Thomas Baring, both lost sons in the disaster.

[27] The subsequent investigation on the loss of Captain, in the form of a court-martial,[28] under Admiral Sir James Hope, took place on board HMS Duke of Wellington, in Portsmouth Harbour.

For years they had demanded that Coles be allowed to produce a super-ironclad—armed with turrets—which could restore confidence in the primacy of the Royal Navy in a way which neither broadside ironclads like the partially armoured HMS Warrior nor Reed's central-battery versions seemed able to.

[citation needed] Coles fatally added the requirement that a fully-rigged, seagoing turret-ship like HMS Monarch also be as low in the water as possible, like the low-freeboard (though mastless) American monitor USS Miantonomoh; which had crossed the Atlantic under escort in June 1866, and which both Coles and the Board of Admiralty toured when she was anchored at Spithead.

HMS Monarch, the masted turret ship proposed by the 1865 committee and designed by Reed, and which was in the area at the time of the sinking, had a righting moment of 6,500-foot-tons (20 MN·m) at the same angle.

This aims to raise funds in an effort to discover the wreck of the Captain, whose sinking was the worst disaster suffered by the Royal Navy in the 'Pax Britannica' era.

Fuller and Sir Sherard Cowper-Coles, Captain Cowper Coles great-grandson, suggested that the chances of finding the wreck were good and that fund raising had reached the half-way mark by June 2023.