HMS Inflexible was a Victorian ironclad battleship carrying her main armament in centrally placed turrets.

The ship was constructed in the 1870s for the Royal Navy to oppose the perceived growing threat from the Italian Regia Marina in the Mediterranean.

The Italian Navy had started constructing a pair of battleships, Duilio and Enrico Dandolo, equipped with four Armstrong 17.7-inch (450 mm) guns weighing 100 tons each.

Inflexible mounted larger guns than those of any previous British warship and had the thickest armour ever to be fitted to a Royal Navy ship.

Controversially, she was designed so that if her un-armoured ends should be seriously damaged in action and become water-logged, the buoyancy of the armoured centre section of the ship would keep her afloat and upright.

The ends were unarmoured, but with a 3-inch-thick (76 mm) armoured deck 6–8 ft below the waterline to limit damage to the underwater section to keep them buoyant.

There was also light superstructure to provide crew accommodation, and freeboard in rough weather, although anticipated to be seriously damaged in any major engagement.

Barnaby wanted a ship both broader than existing designs to minimise rolling and as short as possible to reduce its size as a target.

Making a ship broader compared to its length was known to reduce its speed, so the innovative technique of water tank tests on models, pioneered by William Froude, was used to finalise a design.

Later that year the MP Edward Reed, formerly Director of Naval Construction, visited the Italian ships and subsequently questioned their stability if the unarmoured ends were flooded.

An editorial in the same edition, 18 June, said "it is said that the unarmoured ends are, in fact, the corks on which she floats, that she cannot swim without them, and it would appear that if she lost one she would capsize".

In their report published in December 1877, they concluded that it would be hard for gunfire to completely flood the unarmoured but heavily compartmentalised and partially cork-filled ends.

Tests showed that the normal full charge of 450 pounds of brown prismatic gunpowder would produce a muzzle velocity of 1,590 feet per second (480 m/s), which could penetrate 23 inches (580 mm) of wrought iron armour at 1,000 yards (910 m).

For example Gerard Noel won the 1874 Royal United Services Institute essay contest with an article that asserted that "[i]n a general action I do not hold that the guns will be the principal weapon".

Whilst this showed the considerable potency of a ram, it also demonstrated the inadequate manoeuvring characteristics of many of the ships equipped with them.

The ram was designed to be removable to avoid damage during accidental collisions, but although other ships customarily carried theirs detached, Inflexible seems to have kept hers in place.

A piston in the brass cylinder forced out the torpedo when it was to be fired, and at the same time its own compressed air motor was started.

[6] Outside the citadel, above the 3-inch-thick (76 mm) armoured deck were a large number of small watertight compartments used to hold coal and stores.

Experiments were carried out with HMS Nettle firing 64-pounder shells into full scale replicas of the cork compartments and coffer dams.

Work by the hydrodynamicist William Froude had demonstrated that such a short length for the ship's width would not require excessive installed power at the design speed of 14.75 knots (27.32 km/h).

This was to help exercise and train the crew, especially as such an area of sail (less than 2 square feet (0.19 m2) per ton) would hardly move the ship.

The electrical installation provided 800 volts DC to power arc lamps in the engine and boiler rooms and Swan incandescent bulbs in other parts of the ship.