Topaze-class cruiser

HMS Amethyst
Class overview
Operators:	Royal Navy
Preceded by:	Pelorus class
Succeeded by:	None
Subclasses:	Amethyst
Built:	1903–05
In commission:	1905–21
Planned:	8
Completed:	4
Cancelled:	4
Scrapped:	4
General characteristics
Class and type:	Topaze-class protected cruiser
Displacement:	3,000 long tons (3,048 t)
Length:	360 ft (109.7 m) (p/p)
Beam:	40 ft (12.2 m)
Draught:	16 ft (4.9 m)
Installed power:	9,800–12,000 ihp (7,300–8,900 kW) 10 water-tube boilers
Propulsion:	All except Amethyst: 2 Shafts, 2 triple-expansion steam engines Amethyst: 3 Shafts, steam turbine set
Speed:	21–22 knots (39–41 km/h; 24–25 mph)
Complement:	318
Armament:	12 × QF 4-inch (102 mm) guns 8 × QF 3-pounder (47-mm) guns 2 × 18-inch (450 mm) torpedo tubes
Armour:	Deck: 0.75–2 in (19.1–50.8 mm) Gun shields: 1 in (25 mm) Conning tower: 3 in (76 mm)

The Topaze-class (often referred to as the Gem-class) cruisers were a quartet of third-class protected cruisers built for the Royal Navy in the first decade of the 20th century. Four additional ships of the class were cancelled before their keels were laid. They were the last class of protected cruisers built for the Royal Navy.

Discussions had been ongoing for several years about a successor to the previous Pelorus class before the Admiralty decided on a design in 1901 that was very much larger and faster than the older ships. The Topaze-class ships had a length between perpendiculars of 360 feet (109.7 m), a beam of 40 feet (12.2 m) and a draught of 16 feet (4.9 m). They displaced 3,000 long tons (3,048 t) and their crew consisted of 313 officers and other ranks.

One objective for the Admiralty with these ships was to evaluate steam turbines against traditional triple-expansion steam engines in a ship larger than a destroyer and Amethyst became the first cruiser to be equipped with them. Her three sisters used a pair of 4-cylinder triple-expansion steam engines, each driving one shaft, using steam provided by 10 water-tube boilers. The engines were designed to produce a total of 9,800 indicated horsepower (7,300 kW) which was intended to give a maximum speed of 21.75 knots (40.28 km/h; 25.03 mph). The three ships easily exceeded their designed power and speeds during their sea trials. They carried a maximum of 700 long tons (710 t) of coal which gave them a range of 7,000 nautical miles (13,000 km; 8,100 mi) at 10 knots (19 km/h; 12 mph) and 2,000 nautical miles (3,700 km; 2,300 mi) at 20 knots (37 km/h; 23 mph).

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