Coastal motor boat

This article is about WWI British Royal Navy boats. For coastal patrol, see patrol boat. For other coastal defence, see coastal defence ship.

HM Coastal Motor Boat 4 (1916) on display at the Imperial War Museum Duxford. View from the stern showing the torpedo launching ramp.

Coastal Motor Boat was a small high-speed British torpedo boat used by the Royal Navy in the First World War and up to end of the Second World War.

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 30 knots (56 km/h) 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,[a] was to not exceed the weight of the 30-foot (9.1 m) 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, though nearly half were built by eight other boat builders under subcontract.[2] The boat builders were Tom Bunn of Rotherhithe, Taylor & Bates of Chertsey, Camper & Nicholson of Gosport, Wills & Packham of Sittingbourne, Salter Brothers of Oxford, Rowhedge Iron Works of Rowhedge, Frank Maynard of Chiswick, J. W. Brooks of Lowestoft. 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.

  1. ^ "Torpedoes of the United Kingdom/Britain Pre-World War II". Navweaps.com. Tony DiGiulian. 7 October 2019. Retrieved 9 April 2022.
  2. ^ "Royal Navy Ships of World War 1". NAVAL-HISTORY.NET. Retrieved 27 August 2022.


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