Coles died in a maritime accident in 1870 when HMS Captain, an experimental warship built to his designs, capsized and sank with him on board.

On 24 October 1853, he was posted to Agamemnon as flag lieutenant for his uncle, Rear Admiral Sir Edmund Lyons.

It was during the Siege of Taganrog that he and other British naval officers and sailors constructed a 45-foot (14 m) raft named Lady Nancy from twenty-nine casks lashed together with spars.

The raft supported a long 32-pounder gun and because of its small draft could be moved into shallow water from where it was used to attack Russian government stores in Taganrog.

Coles expanded the idea by drawing up plans for a better raft, mounting a gun enclosed within a hemispherical shield.

Coles' design aim was to create a ship with the greatest possible all round arc of fire, as low in the water as possible to minimise the target.

This proved to be a weakness in designs he created, because he was unwilling to compromise these aims for the practical necessities of sailing ships' rigging, decks sufficiently high to be clear of heavy seas and other necessary superstructures which restricted the guns' rotation.

In January 1862, the Admiralty agreed to construct a ship, HMS Prince Albert, which had four turrets and a low freeboard, intended only for coastal defence.

The low freeboard was countered by hinged sections increasing the height of the sides above the deck; these were dropped down to allow the guns to fire.

However, once Royal Sovereign was completed and had received favourable reports, he requested Admiralty assistance in creating a new design.

[5] Coles once again resorted to appealing to public opinion to obtain support for a ship more closely in accord with his design ideas.

Plans for HMS Captain were submitted to the Admiralty as would be normal, but Reed declined to 'approve' them, instead marking all drawings 'not objected to'.

[7] Captain was designed to have a freeboard of only 8 feet (2.4 m), but owing to mistakes in construction leading to increased weight, the ship eventually floated 14 inches (360 mm) lower in the water.

By contrast, Monarch had a maximum restoring force at an angle of 40 degrees, so that any heel up to this limit would always meet increasing resistance.

[citation needed] A son was Sherard Osborn Cowper-Coles, metallurgist and inventor of the sherardising process of galvanisation.