that the French iron industry could not supply enough, and the main reason why Britain built its handful of wooden-hulled ironclads was to make best use of hulls already started and wood already bought. Wooden hulls continued to be used for long-range and smaller ironclads, because iron nevertheless had a significant disadvantage. Iron hulls suffered quick fouling by marine life, slowing the ships down—manageable for a European battlefleet close to dry docks, but a difficulty for long-range ships. The only solution was to sheath the iron hull first in wood and then in copper, a laborious and expensive process which made wooden construction remain attractive. Iron and wood were to some extent interchangeable: the Japanese and ordered in 1875 were sister-ships, but one was built of iron and the other of composite construction. After 1872, steel started to be introduced as a material for construction. Compared to iron, steel allows for greater structural strength for a lower weight. The French Navy led the way with the use of steel in its fleet, starting with the , laid down in 1873 and launched in 1876. Redoutable nonetheless had wrought iron armor plate, and part of her exterior hull was iron rather than steel. Even though Britain led the world in steel production, the Royal Navy was slow to adopt steel warships. The Bessemer process for steel manufacture produced too many imperfections for large-scale use on ships. French manufacturers used the Siemens-Martin process to produce adequate steel, but British technology lagged behind. The first all-steel warships built by the Royal Navy were the dispatch vessels Iris and Mercury, laid down in 1875 and 1876. Armor and protection schemes Iron-built ships used wood as part of their protection scheme. HMS Warrior was protected by 4.5 in (114 mm) of wrought iron backed by 15 in (381 mm) of teak, the strongest shipbuilding wood. The wood played two roles, preventing spalling and also preventing the shock of a hit damaging the structure of the ship. Later, wood and iron were combined in 'sandwich' armor, for instance in HMS Inflexible. Steel was also an obvious material for armor. It was tested in the 1860s, but the steel of the time was too brittle and disintegrated when struck by shells. Steel became practical to use when a way was found to fuse steel onto wrought iron plates, giving a form of compound armor. This compound armor