and stern; it might also taper up from the central citadel towards the superstructure. The deck was typically lightly armoured with 2 to 4 inches of steel. This lighter armour was to prevent high-explosive shells from wrecking the superstructure of the ship. The battleships of the late 1880s, for instance the Royal Sovereign class, were armoured with iron and steel compound armour. This was soon replaced with more effective case-hardened steel armour made using the Harvey process developed in the United States. First tested in 1891, Harvey armour was commonplace in ships laid down from 1893 to 1895. However, its reign was brief; in 1895, the German pioneered the superior Krupp armour. Europe adopted Krupp plate within five years, and only the United States persisted in using Harvey steel into the 20th century. The improving quality of armour plate meant that new ships could have better protection from a thinner and lighter armour belt; 12 inches (305 mm) of compound armour provided the same protection as just 7.5 inches (190 mm) of Harvey or 5.75 inches (133 mm) of Krupp. Propulsion Almost all pre-dreadnoughts were powered by reciprocating steam engines. Most were capable of top speeds between 16 and . The ironclads of the 1880s used compound engines, and by the end of the 1880s the even-more efficient triple expansion compound engine was in use. Some fleets, though not the British, adopted the quadruple-expansion steam engine. The main improvement in engine performance during the pre-dreadnought period came from the adoption of increasingly higher pressure steam from the boiler. Scotch marine boilers were superseded by more compact water-tube boilers, allowing higher-pressure steam to be produced with less fuel consumption. Water-tube boilers were also safer, with less risk of explosion, and more flexible than fire-tube types. The Belleville-type water-tube boiler had been introduced in the French fleet as early as 1879, but it took until 1894 for the Royal Navy to adopt it for armoured cruisers and pre-dreadnoughts; other water-tube boilers followed in navies worldwide. The engines drove either two or three screw propellers. France and Germany preferred the three-screw approach, which allowed the engines to be shorter and hence more easily protected; they were also more maneuverable and had better resistance to accidental damage. Triple screws were, however, generally larger and heavier than the twin-screw arrangements preferred by most other navies. The French also built the only class of turbine powered