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During the First World War, following a suggestion from three junior officers of the Harwich destroyer force that small motor boats carrying a torpedo might be capable of travelling over the protective minefields and attacking ships of the Imperial German Navy at anchor in their bases, the Admiralty gave tentative approval to the idea and, in the summer of 1915, produced a Staff Requirement requesting designs for a Coastal Motor Boat for service in the North Sea. These boats were expected to have a high speed, making use of the lightweight and powerful petrol engines then available. The speed of the boat when fully loaded was to be at least and sufficient fuel was to be carried to give a considerable radius of action. They were to be armed in a variety of ways, with torpedoes, depth charges or for laying mines. Secondary armament would have been provided by light machine guns, such as the Lewis gun. The weight of a fully loaded boat, complete with 18-inch (450 mm) torpedo,〔The British "18 inch" torpedoes were 17.72 inches (45.0 cm) in diameter (British torpedoes pre WWII )〕 was to not exceed the weight of the long motor boat then carried in the davits of a light cruiser, i.e. 4.5 tons. The CMBs were designed by Thornycroft, who had experience in small fast boats. Engines were not proper maritime internal combustion engines (as these were in short supply) but adapted aircraft engines from firms such as Sunbeam and Napier. ==40-foot Coastal Motor Boats== In 1910, Thornycroft had designed and built a speedboat called ''Miranda IV''. She was a single-step hydroplane powered by a Thornycroft petrol engine and could reach . A boat based on ''Miranda IV'' was accepted by the Admiralty for trials. A number of these boats were built and had a distinguished service history, but in hindsight they were considered to be too small to be ideal, particularly in how their payload was limited to a single 18-inch torpedo. Several companies were approached, but only Thornycroft considered it possible to meet such a requirement. In January 1916, twelve boats were ordered, all of which were completed by August 1916. Further boats were built, to a total of 39. The restriction on weight meant the torpedo could not be fired from a torpedo tube, but instead was carried in a rear-facing trough. On firing it was pushed backwards by a cordite firing pistol and a long steel ram, entering the water tail-first. A trip-wire between the torpedo and the ram head would start the torpedo motors once pulled taut during release. The CMB would then turn hard over and get out of its path. There is no record of a CMB ever being hit by its own torpedo, but in one instance the firing pistol was triggered prematurely and the crew had a tense 20 minutes close to the enemy whilst reloading it.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Coastal Motor Boat」の詳細全文を読む スポンサード リンク
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