With a slenderness ratio of 4.6:1 Inflexible was a stable gun platform.
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).
However, the same proportions were adopted in the similar but smaller HMS Ajax and HMS Agamemnon, but resulted in a serious lack of directional stability in those ships.

The ship had two compound steam engines manufactured by John Elder and company.
Each had one high-pressure and two low-pressure cylinders connected to a crankshaft.
The connecting rods were 9 inches (230 mm) in diameter attached to 17.5-inch-diameter (440 mm) bearings on the crankshaft.
A hollow steel shaft drove each of the two screws at a maximum 75 rpm.
There were two boiler rooms, one each end of the engine room.
Each contained two 17-foot (5.2 m) and two 9-foot (2.7 m) boilers operating at 61 psi.
Similar high pressure systems had been used on HMS Alexandra launched in 1875 and Temeraire in 1876, but they were a recent innovation and more economical than the previous low pressure engines.
Gangs of stokers were continuously bringing coal from the bunkers to feed the fires.
The ship had a further 39 smaller engines for various purposes including bilge pumps capable of shifting 300 tons of water per hour, pumps for cooling water through the steam condensers, fans to draw air through the ship through a system of ventilation ducts, steering gear, hydraulic pumps for the guns, air compressors, winches and for generating electricity.
The engine room was noisy, wet, greasy, oily and steamy.
It would be a normal occurrence for engines to leak steam and for bearings to run hot so that they had to be hosed down to keep them operating.
All the essential equipment was contained within the armoured citadel.

Although she was propelled principally by steam, she was equipped with a pair of masts and yards, so that 18,500 square feet (1,720 m²) of sail could be deployed.
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.
As Jackie Fisher wrote: "The sails had so much effect upon her in a gale of wind as a fly would have on a Hippopotamus in producing any movement."

The masts and sails were removed after four years in service, and replaced by simple pole masts for carrying signal flags and circular fighting tops, platforms carrying quick firing guns.
With a slenderness ratio of 4.6:1 Inflexible was a stable gun platform. 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). However, the same proportions were adopted in the similar but smaller HMS Ajax and HMS Agamemnon, but resulted in a serious lack of directional stability in those ships.
The ship had two compound steam engines manufactured by John Elder and company. Each had one high-pressure and two low-pressure cylinders connected to a crankshaft. The connecting rods were 9 inches (230 mm) in diameter attached to 17.5-inch-diameter (440 mm) bearings on the crankshaft. A hollow steel shaft drove each of the two screws at a maximum 75 rpm. There were two boiler rooms, one each end of the engine room. Each contained two 17-foot (5.2 m) and two 9-foot (2.7 m) boilers operating at 61 psi. Similar high pressure systems had been used on HMS Alexandra launched in 1875 and Temeraire in 1876, but they were a recent innovation and more economical than the previous low pressure engines. Gangs of stokers were continuously bringing coal from the bunkers to feed the fires. The ship had a further 39 smaller engines for various purposes including bilge pumps capable of shifting 300 tons of water per hour, pumps for cooling water through the steam condensers, fans to draw air through the ship through a system of ventilation ducts, steering gear, hydraulic pumps for the guns, air compressors, winches and for generating electricity. The engine room was noisy, wet, greasy, oily and steamy. It would be a normal occurrence for engines to leak steam and for bearings to run hot so that they had to be hosed down to keep them operating. All the essential equipment was contained within the armoured citadel.
Although she was propelled principally by steam, she was equipped with a pair of masts and yards, so that 18,500 square feet (1,720 m²) of sail could be deployed. 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. As Jackie Fisher wrote: "The sails had so much effect upon her in a gale of wind as a fly would have on a Hippopotamus in producing any movement."
The masts and sails were removed after four years in service, and replaced by simple pole masts for carrying signal flags and circular fighting tops, platforms carrying quick firing guns.
HMS Inflexible (1876)