Coastal Motor Boat 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 torpedo es , depth charge s or for laying mine s . 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 , 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 . In January 1919 a force of 12 CMBs was dispatched to the Caspian Sea ( travelling by rail from Batumi on the Black Sea coast to Baku ) to join a British naval unit supporting the anti - Bolshevik governments of Armenia , Azerbiajan and Georgia . Survivors . The hull of CMB 4 in which Augustus Agar won his VC for the attack on Kronstadt naval base in 1919 and sank the cruiser " Oleg " was , for many years , at the Vosper Thornycroft works on Platt ’ s Eyot on the Thames near Kingston . When these works closed it was restored and can now be seen at the Imperial War Museum , Duxford with details of these boats and the action . Agar ’ s VC is at the War Museum in London . 55 foot Coastal Motor Boats . Larger versions of the 40 - footer were ordered in 1916 In 1917 Thornycroft produced an enlarged overall version . This allowed a heavier payload , and now two torpedoes could be carried . A mixed warload of a single torpedo and four depth charges could also be carried , the depth charges released from individual cradles over the sides , rather than a stern ramp . Speeds from were possible , depending on the various petrol engines fitted . At least two unexplained losses due to fires in port are thought to have been caused by a build - up of petrol vapour igniting . It was these larger boats that entered the harbour during the Kronstadt raid and torpedoed the Soviet ships . The design was so successful that more were built during World War II . The last survivor , MTB 331 , is of this group , built in 1941 . Survivors . MTB 331 , owned by Hampshire County Council and on - loan to the British Military Powerboat Trust ( BMPT ) at Marchwood , is the sole surviving 55 ' CMB . Built in 1941 , the penultimate 55 ' built , her design was based on that of the CMBs of 1917 with two V12 engines . Her post - war history is incomplete , but she was registered as the " Jonrey " at Teignmouth , then later at Bristol . She was acquired by the Council , around 1990 . Some restoration after this was carried out at Priddy ' s Hard , then she was transported by road to BMPT Marchwood in March 2000 . 70 - foot Coastal Motor Boat . Twelve 72 ft long CMBs were ordered in early 1918 for minelaying ( 7 magnetic mines ) or torpedo work ( 6 torpedoes ) . Five were cancelled ; of the remainder , 3 survived the Second World War , with CMB 103 MT preserved as a museum ship . CMB 103 was restored in August 2011 and is on display at The Historic Dockyard at Chatham .