19th centuries, fleets had relied on two types of major warship, the ship of the line and the frigate. The first major change to these types was the introduction of steam power for propulsion. While paddle steamer warships had been used from the 1830s onward, steam propulsion only became suitable for major warships after the adoption of the screw propeller in the 1840s. Steam-powered screw frigates were built in the mid-1840s, and at the end of the decade the French Navy introduced steam power to its line of battle. Napoleon III's ambition to gain greater influence in Europe required a sustained challenge to the British at sea. The first purpose-built steam battleship was the 90-gun in 1850. Napoléon was armed as a conventional ship-of-the-line, but her steam engines could give her a speed of , regardless of the wind conditions: a potentially decisive advantage in a naval engagement. The introduction of the steam ship-of-the-line led to a building competition between France and Britain. Eight sister ships to Napoléon were built in France over a period of ten years, but the United Kingdom soon managed to take the lead in production. Altogether, France built ten new wooden steam battleships and converted 28 from older ships of the line, while the United Kingdom built 18 and converted 41. Explosive shells The era of the wooden steam ship-of-the-line was brief, because of new, more powerful naval guns. In the 1820s and 1830s, warships began to mount increasingly heavy guns, replacing 18- and 24-pounder guns with 32-pounders on sailing ships-of-the-line and introducing 68-pounders on steamers. Then, the first shell guns firing explosive shells were introduced following their development by the French Général Henri-Joseph Paixhans. By the 1840s they were part of the standard armament for naval powers including the French Navy, Royal Navy, Imperial Russian Navy and United States Navy. It is often held that the power of explosive shells to smash wooden hulls, as demonstrated by the Russian destruction of an Ottoman squadron at the Battle of Sinop, spelled the end of the wooden-hulled warship. The more practical threat to wooden ships was from conventional cannon firing red-hot shot, which could lodge in the hull and cause a fire or ammunition explosion. Some navies even experimented with hollow shot filled with molten metal for extra incendiary power. Iron armor The use of wrought iron instead of wood as the primary material of ships'