Coastal Motor Boat - 40-foot (12 M) Coastal Motor Boats

40-foot (12 M) Coastal Motor Boats

In 1910, Thornycroft had designed and built a 25' speedboat called Miranda IV. She was a single-step hydroplane powered by a 120 hp (89 kW) Thornycroft petrol engine and could reach 35 knots (65 km/h).

A 40' boat based on the 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" 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 that 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.