Protected cruiser
Protected cruisers, a type of cruiser of the late 19th century, took their name from the armored deck, which protected vital machine-spaces from fragments released by explosive shells. Protected cruisers notably lacked a belt of armour along the sides, in contrast to armored cruisers which carried both deck and belt armour.
Outside of a handful of very large designs in the major navies, the majority of protected cruisers were of 'second-' or 'third-class' types, lighter in displacement and mounting fewer and/or lighter guns than armored cruisers.
By the early 20th-century, with the advent of increasingly lighter yet stronger armour, even smaller vessels could afford some level of both belt and deck armour. In the place of protected cruisers, these new 'light armored cruisers' would evolve into light cruisers and heavy cruisers, the former especially taking on many of the roles originally envisioned for protected cruisers.
Evolution
From the late 1850s, navies began to replace their fleets of wooden ships-of-the-line with armoured ironclad warships. The frigates and sloops which performed the missions of scouting, commerce raiding and trade protection remained unarmoured. For several decades, it proved difficult to design a ship which had a meaningful amount of effective armour but at the same time maintained the speed and range required of a "cruising warship". The first attempts to do so, large armored cruisers like, proved unsatisfactory, generally lacking enough speed for their cruiser role. They were, along with their foreign counterparts such as the French, more like second- or third-class battleships and were mainly intended to fulfil this role on foreign stations where full-scale battleships could not be spared or properly supported.First protective decks
During the 1870s the increasing power of armour-piercing shells made armouring the sides of a warship more and more difficult, as very thick, heavy armour plates were required. Even if armour dominated the design of the ship, it was likely that the next generation of shells would be able to pierce such armour. This problem was even more poignant where the design of cruising warships was concerned, with their requirement for long endurance needing much of their displacement to be devoted to consumable supplies – even where very powerful and space-consuming high-speed machinery was not required – leaving very little weight available for armour protection. This meant that effective side belt armour would be almost impossible to provide for smaller ships.The alternative was to leave the sides of the ship vulnerable, but to armour a deck just below the waterline. Since this deck would be struck only very obliquely by shells, it could be less thick and heavy than belt armour. The ship could be designed so that the engines, boilers and magazines were under the armoured deck, and with hopefully enough reserve buoyancy to keep the ship afloat even in the event of flooding resulting from damage above the protective deck. An armoured deck had actually been used for the first time in HMS Shannon, although she did rely principally on her vertical belt armour for defence: Her protective deck was only a partial one, extending from the forward armoured bulkhead of the citadel to the bow.
Early ships
The first of the smaller "unarmoured" British cruisers to incorporate an internal steel deck for protection was the of corvettes started in 1876; this was only a partial-length deck, with amidships over the machinery spaces. The Comus class were really designed for overseas service and were capable of only a speed, not fast enough for fleet duties. The following Satellite and Calypso classes were similar in performance.A more potent and versatile balance of attributes was struck with the four s. Ordered in 1880 as modified dispatch vessels and re-rated as second-class cruisers before completion, these ships combined an amidships protective armoured deck with the size, lean form and high performance of. They also featured a heavy and well-sited armament of modern breech-loading guns. Leander and her three sisters were successful and established a basis for future Royal Navy cruiser development, through the rest of the century and beyond. Their general configuration was scaled up to the big First Class cruisers and down to the torpedo cruisers, while traces of the protected deck scheme can even be recognised in some sloops.
Breakthrough
By the start of the 1880s, ships were appearing with full-length armoured decks and no side armour, from the of very fast battleships to the torpedo ram. In the case of the latter, the armoured deck was of sufficient thickness to defend against small-calibre guns capable of tracking such a difficult, fast target. This was very much the philosophy adopted by George Wightwick Rendel in his design of the so-called 'Rendel cruisers' Arturo Prat, and. By enlarging the flatiron gunboat concept, increasing engine power and thus speed, Rendel was able to produce a fast small vessel and still have enough tonnage to incorporate a very thin partial protective deck over the machinery. Still small and relatively weakly built, these vessels were 'proto-protected cruisers' which served as the inspiration for a significantly larger ship; Esmeralda.The first true mastless protected cruiser and the first of the 'Elswick cruisers', the was designed by Rendel and built for the Chilean Navy by the British firm of Armstrong at their Elswick yard. Esmeralda was revolutionary; she had a high speed of , an armament of two and six guns and a full-length protective deck. This was up to thick on the slopes, with a cork-filled cofferdam along her sides. It would not defend against fire from heavy guns, but was designed to be adequate to defeat any gun of the day considered capable of hitting so fast a ship.
With her heavy emphasis on speed and firepower, Esmeralda set the tone for competitive cruiser designs into the early 20th century, with 'Elswick cruisers' of a similar design being constructed for Italy, China, Japan, Argentina, Austria and the United States. Cruisers with armoured decks and no side armour – like Esmeralda – became known as "protected cruisers", and rapidly eclipsed the large and slow armoured cruisers during the 1880s and into the 1890s.
The French Navy adopted the protected-cruiser concept wholeheartedly in the 1880s. The Jeune École school of thought, which proposed a navy composed of fast cruisers for commerce raiding and torpedo boats for coastal defence, became particularly influential in France. The first French protected cruiser was, laid down in 1882, and followed by six classes of protected cruiser – and no armoured cruisers.
Side armour abandonment
The Royal Navy remained equivocal about which protection scheme to use for cruisers until 1887. The large, begun in 1881 and finished in 1886, were built as armoured cruisers but were often referred to as protected cruisers due to the limited extent of their side armour – although what armour they had was admittedly very thick. Their primary role, as with the earlier Shannon and Nelsons, was still to function as small battleships on foreign stations, countering enemy stationnaire ironclads rather than chasing down swift commerce-raiding corsairs. While they carried a very thick and heavy armoured belt of great power of resistance that extended over the middle of the ship's length, the belt's upper edge was submerged at full load.Britain built one more class of armoured cruiser with the, begun in 1885 and completed in 1889. They were affected by a similar fault to the Imperieuse regarding their belt's submergence. In 1887 an assessment of the Orlando type judged them inferior to the protected cruisers and thereafter the Royal Navy built only protected cruisers, even for very large first-class cruiser designs, not returning to armoured cruisers until the introduction of new lighter and stronger armour technology.
The sole major naval power to retain a preference for armoured cruisers into the 1890s was Russia. The Imperial Russian Navy laid down four armoured cruisers and one protected cruiser during the late 1880s, all large ships with sails.
Elswick's influence
Following the Leander class, the next small cruisers designed for the Royal Navy were the of 1883. Derived from the previous class, these were also protected cruisers but with a full-length armoured deck for superior protection. The Merseys were born from a different tactical conception to their forebears and this was reflected in their armament arrangement. They were conceived as 'fleet torpedo cruisers' to carry out attacks on the enemy battle line and featured heavy guns fore and aft with excellent fields of fire. Despite public Admiralty criticism of Elswick designs, it is clear that the Mersey class was heavily influenced by the Italian 'torpedo ram cruiser' Giovanni Bausan, a design itself derived from Esmeralda. Thus, the British notion of the protected cruising warship was being shaped early on by the commercial export models coming out of Elswick.The protected cruiser remained a popular and economical type, rather stable in terms of its characteristics, right throughout the 1890s and into the early 1900s. During this period, protected cruiser designs of second- to third-class grew slowly in size, seeing few major changes to the common balance of design features. Perhaps the most significant paradigm shift came with the universal adoption of quick-firing guns by the world's navies in the middle of the 1890s; suddenly small and medium cruisers saw a swift increase in their fighting power for a slight reduction in gun calibre, yielding a very economical balance of attributes. This kept the protected cruiser competitive for a further decade.