In introducing the screw, two difficulties were encountered. The engines of the day gave only about twenty-five revolutions to the minute, and it was therefore necessary to introduce some sort of multiple gearing between the engine-shaft and the screw shaft; for a screw should turn at seventy-five times a minute, or more. The other defect of screw propulsion was found in the strain of the shaft upon the stern of the ship. No combination of timbers in a wooden ship could resist that strain for any great length of time. To the English, however, this was a matter of no moment, for iron had already been used for building hulls. The first of these iron ships was the _Aaron Manby_, launched in London, in 1820, but the iron ship that first really influenced the British merchant marine was the _Great Britain_, built for the Great Western Steamship Company at Bristol, in 1843. She was a big ship for her day (322 feet long), and she was not only a profitable cargo carrier, but, having been stranded on the coast of Ireland, she endured the poundings of the storms of an entire winter, and was then hauled off, repaired at small expense (considering the storms she had endured), and when put at work again was found to be as serviceable as ever. As the _Great Britain_ was driven by a screw the use of iron screw ships soon became fashionable in the British merchant marine, and the more rapidly because they were much more economical in the use of coal. Lindsay (_History of Merchant Shipping_) says that the repeal of the ancient British navigation laws helped to turn British merchants to the screw steamer. They were unable to compete with the Americans in the use of sails, and had to take up the new ship or abandon the sea. The Cunard Company would have adopted iron ships promptly but for the contract with the Admiralty. The naval officers supposed that wooden walls were better for keeping out shot than iron, and it was not until 1855, as noted, that this company was allowed to use the best material. The fact that a subsidy thus restrained enterprise seems important here.