The Highflyer-class cruisers were essentially repeats of the previous Eclipse class, albeit with a more powerful armament and propulsion machinery. They were designed to displace 5,650 long tons (5,740 t). The ships had an overall length of 372 feet (113.4 m), a beam of 54 feet (16.5 m) and a draught of 21 feet 6 inches (6.6 m). Their crew consisted of 470 officers and other ranks.
The ships were powered by two 4-cylinder triple-expansion steam engines, each driving one shaft, using steam provided by 18 Belleville boilers, which were lighter and more powerful than the cylindrical boilers used by the Eclipses. The engines were designed to produce a total of 10,000 indicated horsepower (7,500 kW) which was intended to give a maximum speed of 20 knots (37 km/h; 23 mph). The ships easily exceeded their designed power and speeds during their sea trials. They carried a maximum of 1,125 long tons (1,143 t) of coal.
The ships' protective deck armour ranged in thickness from 1.5 to 3 inches (38 to 76 mm). The engine hatches were protected by 5-inch (127 mm) of armour. The main guns were fitted with 3-inch gun shields and the conning tower had armour 6 inches thick.
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