HMS Hydra was the second ship completed of the four Cyclops-class breastwork monitors built for the Royal Navy during the 1870s.

Their accommodations were rated the worst in the fleet, referred to by ordinary seamen as "ratholes with tinned air".

[3] Hydra had two 4-cylinder inverted compound steam engines made by John Elder, each driving a single 12-foot (3.7 m) propeller.

The engines produced a total of 1,472 indicated horsepower (1,098 kW) on 4 July 1872 during the ship's sea trials which gave her a maximum speed of 11.2 knots (20.7 km/h; 12.9 mph).

Hydra carried 250 long tons (250 t) of coal,[4] enough to steam 3,000 nmi (5,600 km; 3,500 mi) at 10 kn (19 km/h; 12 mph).

The gun had a muzzle velocity of 1,365 ft/s (416 m/s) and was credited with the ability to penetrate a nominal 12.9 inches (330 mm) of wrought iron armour at 100 yards (91 m).

The superstructure and conning tower was fully armoured, the reason it was called a breastwork, with 8–9 inches (203–229 mm) of wrought iron.

This refit also strengthened the breastwork and upper decks, added another watertight bulkhead as well as a false keel.