A ship's protection scheme must ensure that it can resist damage sufficiently to permit it to Float, Move and Fight.

"Float"

Colossus was designed as a central armoured citadel box with a watertight raft extending forward and aft protected by a 3 inch armoured deck lying a few feet below the waterline. At the ranges these ships were expected to fight (about 1,000 yards), gunfire trajectories were almost horizontal, so shells hitting the water would explode or be retarded before doing much damage below the waterline.

The spaces above the armoured deck were filled with stores, coal etc., and wing compartments were filled with cork. The intention here was that if the unarmoured ends were penetrated, not much water would be able to flow in, as the spaces would already be full and the ship will remain buoyant and stable.

Colossus's hull featured a double bottom for strength, rigidity and to minimise the risk of flooding. Bulkheads divided the ship into watertight compartments, although there were many watertight doors between compartments, and these were not always effective or quick to close (as was the problem when HM S Victoria was rammed and sunk by HMS Camperdown in 1893).

"Move"

The boilers and engines were well protected inside the armoured citadel. Additionally, coal bunkers were located outboard of the machinery spaces and the coal within was helpful in providing additional protection (2 feet of coal were found to be equivalent to an inch of armour).

"Fight"

This means that the ship must be able to continue to supply, aim and fire her guns. The loading mechanism and turret bases were protected inside the central citadel. The magazines and shell rooms as I shall show now, were located deep below the water line, with the lifts to the guns within the armoured citadel.

Move your mouse over the image to the right to see how the propulsive and fighting systems shared the protection of the citadel

Propulsion

Colossus was fitted with two Maudsley compound engines developing 7,488 Indicated Horse Power (IHP) driving two 4 bladed propellers of 17’8” diameter and giving her a maximum speed of 16.5 knots per hour (as speed was frequently expressed in the 1880s).[2]

The high pressure (HP) cylinder's diameter was 58”, and the two low pressure (LP) cylinders were 74” in diameter. The stroke was 39” long. Steam was exhausted to the surface condensers so it could be returned to the boilers for another cycle. Thrust blocks were situated just aft of the engine couplings.

Just forward of the engine spaces were the 4 boiler rooms. Their 8 main and 2 auxiliary elliptical boilers operated at a working pressure of 64 pounds on the square inch.

The main boilers each had three grates, the auxiliary boilers two. Coal bunkers were situated outboard of the boilers and engines, offering a little added protection while being near where it would be needed.