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Steam power

The first major change to the ship-of-the-line concept was the introduction of steam power as an auxiliary propulsion system. The first military uses of steamships came in the 1810s, and in the 1820s a number of navies experimented with paddle steamer warships. Their use spread in the 1830s, with paddle-steamer warships participating in conflicts like the First Opium War alongside ships of the line and frigates.

Paddle steamers, however, had major disadvantages. The paddle wheel above the waterline was exposed to enemy fire, while itself preventing the ship from firing broadsides effectively. During the 1840s, the screw propeller emerged as the most likely method of steam propulsion, with both Britain and the US launching screw-propelled warships in 1843. Through the 1840s, the British and French navies launched ever larger and more powerful screw ships, alongside sail-powered ships of the line. In 1845, Viscount Palmerston gave an indication of the role of the new steamships in tense Anglo-French relations, describing the English Channel as a "steam bridge", rather than a barrier to French invasion. It was partly because of the fear of war with France that the Royal Navy converted several old 74-gun ships of the line into 60-gun steam-powered blockships (following the model of Fulton 's Demologos), starting in 1845. The blockships were "originally conceived as steam batteries solely for harbour defence, but in September 1845 they were given a reduced rig rather than none at all, to make them sea-going ships.… The blockships were to be a cost-effective experiment of great value." They subsequently gave good service in the Crimean War.

The French Navy, however, developed the first purpose-built steam battleship with the 90-gun Napoléon in 1850. She is also considered the first true steam battleship, and the first screw battleship ever. Napoléon was armed as a conventional ship of the line, but her steam engines could give her a speed of 12 knots (22 km/h; 14 mph), regardless of the wind conditions – a potentially decisive advantage in a naval engagement.

Eight sister ships to Napoléon were built in France over a period of ten years, but the United Kingdom soon took the lead in production, in number of both purpose-built and converted units. Altogether, France built 10 new wooden steam battleships and converted 28 from older battleship units, while the United Kingdom built 18 and converted 41.

In the end, France and Britain were the only two countries to develop fleets of wooden steam screw battleships, although several other navies made some use of a mixture of screw battleships and paddle-steamer frigates. These included Russia, Turkey, Sweden, Naples, Prussia, Denmark, and Austria.