Protected Cruiser - Evolution

Evolution

From the late 1850s, navies began to replace their fleets of wooden ships-of-the-line with armoured ironclad warships. However, 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 any meaningful amount of protective armour but at the same time was capable of the speed and range required of a 'cruising warship'. The first attempts to do so, armoured cruisers like HMS Shannon, proved to be unsatisfactory, generally being too slow for their cruiser role.

During the 1870s, the increasing power of armour-piercing guns made armouring the sides of a ship 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 guns would be able to pierce it. The alternative was to leave the sides of the ship vulnerable, but to armour a deck just below the waterline. Since this deck would only be struck very obliquely by shells, it could be rather less thick and heavy than belt armour. The ship could be designed so that under the armoured deck were the engines, boilers and magazines, and enough displacement to keep the ship afloat and stable even in the event of damage. Cruisers with armoured decks and no side armour became known as protected cruisers, and eclipsed the armoured cruisers in popularity in the 1880s and in to the 1890s.

Shannon was the first warship to incorporate an armoured deck; hers stretched forward from the armoured citadel to the bows. However, Shannon principally relied on her vertical citadel armour for protection. By the end of the 1870s ships could be found with full-length armoured decks and little or no side armour. The Italian Italia class of very fast battleships had armoured decks and guns but no side armour. The British used a full-length armoured deck in their Comus class of corvettes started in 1878; however the Comus class were designed for colonial service and were only capable of 13 knots speed, not fast enough for commerce protection or fleet duties.

The breakthrough for the protected cruiser design came with the Chilean cruiser Esmeralda, designed and built by the British firm Armstrong, at their Elswick yard. Esmeralda had a high speed of 18 knots (dispensing entirely with sails), and an armament of two 10in and six 6in guns. Her protection scheme, inspired by the Italia class, included a full-length protected deck up to 2in thick, and a cork-filled cofferdam along her sides. Esmeralda set the tone for cruiser construction for the years to come, with "Elswick cruisers" on a similar design being constructed for Italy, China, Japan, Argentina, Austria and the United States.

The French navy adopted the protected cruiser 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 coast defence, was particularly influential in France. The first French protected cruiser was the Sfax, laid down in 1882, and followed by six classes of protected cruiser - and no armoured cruisers.

The British Royal Navy was equivocal about which protection scheme to use until 1887. The large Imperieuse class, begun in 1881 and finished in 1886, were built as armoured cruisers but were often referred to as protected cruisers. While they carried an armoured belt some 10 in thick, the belt only covered a 140 ft of the 315 ft length of the ship, and the belt was also submerged below the waterline at full load. The real protection of the class came from the armoured deck 4 in thick, and the arrangement of coal bunkers to prevent flooding. These ships were also the last armoured cruisers to be designed with sails. However, on trials it became clear that the masts and sails did more harm than good. The masts, sails and rigging were removed and replaced with a single military mast with machine guns.

The next class of small cruisers in the Royal Navy, the Mersey class, were protected cruisers, but the Royal Navy then returned to the armoured cruiser with the Orlando class, begun in 1885 and completed in 1889. However in 1887 a comparison of the Orlando type judged them inferior to the protected cruisers and accordingly the Royal Navy built exclusively protected cruisers, even for very large first-class cruiser designs returning only to armoured cruisers in the late 1890s with the Cressy class, laid down in 1898.

The only major naval power to retain a preference for armoured cruisers during the 1880s was Russia. The Russian Navy laid down four armoured cruisers and one protected cruiser during the decade, all being large ships with sails.

Around 1910, armour plate began to increase in quality and steam turbine engines, lighter and more powerful than previous reciprocating engines, came into use. Existing protected cruisers became obsolescent as they were slower and less well protected than new ships. Oil fired boilers were introduced, making side bunkers of coal unnecessary but losing the protection they afforded. Protected cruisers were replaced by "light armoured cruisers" with a side armoured belt and armoured decks instead of the single deck, later developed into heavy cruisers.