Topaze-class cruiser
The Topaze-class cruisers were a quartet of third-class protected cruisers built for the Royal Navy in the first decade of the 20th century.
As significant historical first, of this class was the first warship larger than a destroyer to be powered by turbine engines.
This class also embodied many historical lasts. Despite being units of the Edwardian-period Royal Navy, they represented the end of the Victorian-period lineage of protected cruisers in many ways. Amongst the many cruiser classes of the Royal Navy to have been rated as protected cruisers these were the last to be officially classified as such. They were the last British cruisers to feature the traditional arrangement of raised forecastle and poop connected by amidships Glossary of [nautical terms (A-L)#bulwark|bulwarks]. They were also the last to be designed for propulsion by reciprocating steam engines, except for the aforementioned HMS Amethyst.
Perhaps most significantly of all, the Gems were the very last third-class protected cruisers to be so rated in the Royal Navy. The classes which followed were of similar type, and like the protected cruisers featured internal protective decks instead of armour belts, but were faster though weaker, and intended for a very specialised role, thus according them the new official type designation scout cruiser. When the later small turbine-propelled cruisers of the Arethusa class appeared, the new ships were designated from the outset as light armoured cruisers, thus dispensing with the third-class rating entirely.
Design and description
Discussions had been ongoing for several years about a successor to the previous before the Admiralty decided on a design in 1901 that was very much larger and faster than the older ships. This represented a shift away from the traditional role of the third-class cruiser: previously best-suited to Imperial defence duties on foreign stations, but now shifting closer to the German concept of a light cruiser; a fast & potent small combatant, suitable for use with the Fleet for scouting duties in the presence of an enemy fleet. In this regard, the Topaze class demonstrates the effect of heightening tensions between the British and Germans, as the Royal Navy's procurement programmes began to emphasise potential combat in the North Sea in anticipation of a possible future war.The first small cruisers designed for the Royal Navy by the new DNC Sir Philip Watts, the Topaze-class ships had a distinctive three-funneled appearance, with a length between perpendiculars of, a beam of and a draught of. They displaced 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 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 which was intended to give a maximum speed of. The three ships easily exceeded their designed power and speeds during their sea trials. They carried a maximum of of coal which gave them a range of at and at.
Amethyst was fitted with a Parsons Marine [Steam Turbine Company|Parsons] steam turbine set that drove three shafts using the same number of boilers as her sisters. They delivered enough steam to allow the engines to reach and her designed speed of. She also easily exceeded her designed power and speeds during her sea trials, reaching from. The turbine proved to be more economical at high speeds than the reciprocating engines of her sisters, but less so at slow speeds, only having an endurance of at 10 knots, but at 20 knots.
The main armament of the Topaze class consisted of a dozen quick-firing (QF) QF 4 inch [naval gun Mk I – III| guns]. One gun each was mounted on the forecastle and the poop. The remaining ten guns were placed port and starboard amidships, with the forward and aftermost pairs of waist guns being sponsoned and the sides of the forecastle and poop embrasured to allow for axial fire from these four guns. This gave the ships a broadside of seven guns, with at least two guns able to fire through the extreme bow and quarter arcs, or three guns firing dead ahead and dead astern in a chase. Their shells were fired at a muzzle velocity of. The ships also carried eight QF 3-pounder Hotchkiss guns and two above water 18-inch (450 mm) torpedo tubes. The ships' protective deck armour ranged in thickness from, being at its thickest on the slopes amidships. The main guns were fitted with gun shields and the conning tower had armour thick.