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.