Noun
coastal motor boat (plural coastal motor boats)
(nautical) a small warship used in the Royal Navy during the First World War that carried torpedoes, mines or depth charges.
In the summer of 1915, Lieutenants Hampden, Bremner, and Anson approached John I. Thornycroft & Company about developing a small high speed torpedo boat, and this effort eventually led to the Coastal Motor Boat which first went into service in April 1916. Source: Internet