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 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 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.
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 where it is on loan from the Imperial War Museum, Duxford with details of these boats and the action. Agar’s VC is at the War Museum in London. CMB 9 has been restored and is based at Avonmouth near Bristol. She took part in the 2014 Remembrance Day events in Bristol. CMB 9 was converted to a Distance Control Boat in 1918, the first CMB so converted and in so doing became DCB1. It is in her DCB outfit that the vessel currently exists. The hull of CMB9 is identical to that of CMB4 the hull of which was for many years thought to be the sole survivor her crew were Archibald Dayrell Reed and Lieutenant Harold Drew, The first ever loss on such a vessel was made on 9 The vessel was given Battle Honors and the crew a bounty. The DCB role was and still is in part Top Secret, completely autonomous, unmanned and controlled via aircraft therefore considered to be our first autonomous drone vessel in 1918
55 foot Coastal Motor Boats
Larger versions of the 40-footer were ordered in 1916 In 1917, John I. Thornycroft & Company 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 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 or torpedo work. 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.
