Protected cruisers

The protected cruiser is a type of naval cruiser of the late 19th century, so known because its armoured deck offered protection for vital machine spaces from fragments caused by exploding shells above. Protected cruisers were an alternative to the armoured cruisers, which also had a belt of armour along the sides.

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 a 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 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 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 into 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.

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