There was, besides M.L.'s, another smaller but faster type of submarine chaser. These little vessels, of which there were about 80 actually in commission, possessed no cabin or other accommodation for long cruises. They were simply thin grey hulls with powerful high-speed engines. They were known as C.M.B.'s, or, to give them their full title, Coastal Motor Boats. The purpose for which they were constructed was to operate from coastal bases, and to be launched from ocean-going ships to chase a hostile submarine which had been located by seaplanes and reported by wireless in a given locality. This, however, was what they were _intended_ for, but bore little relation to the work they actually accomplished. Their nickname was "Scooters," and they certainly did "scoot" over the sea. [Illustration: FIG. 3.--Diagram showing principal characteristics of a coastal motor boat (C.M.B.). Speed 50 miles per hour. _A._ Hydroplane hull, so constructed as to rise on to surface when travelling at full speed. _B._ Covered wheel-house. _C._ Navigating well. _D._ Wireless aerials. _E._ Depth charges (2 small size). _F._ Manhole to engine-room.] There were three types of C.M.B.'s. One had a length of only 44 feet, and was intended for carriage on the decks of light cruisers or other moderate-sized surface ships. The armament was a Lewis machine gun and two depth charges for anti-submarine warfare. The next class were 55 feet in length and operated from coast bases. These were fitted with one or more Whitehead torpedoes, launched by an ingenious contrivance from the stern. Class III. were 70 feet in length, and were commissioned just before the signing of the Armistice. They were fitted for mine-laying close up to enemy harbours. [Illustration: FIG. 4.--Plan of coastal motor boat, showing torpedo in cleft stern. _A._ Whale-back or arched deck. _B._ Wheel-house. _C._ Navigating well. _D._ Engine-room. _E._ Foreward petrol tanks. _F._ Forepeak. _G._ Depth charges. _H._ Cleft stern with torpedo ready for launching. _I._ Whitehead torpedo, launched stern first.] The maximum speed of the 55-feet C.M.B.'s, which were the most numerous, was 40 knots, or nearly a mile a minute. They were driven by twin screws coupled to twin engines of 350 h.p. each--working at 1350 revolutions per minute. Being of very shallow draught, some 26 inches, these little vessels could skim, hydroplane fashion, over any ordinary mine-field, and a torpedo fired at them would merely pass under their keel. The risk of destruction from shell-fire was also reduced to a minimum by their small size and great speed. Their principal enemies were, however, seaplanes armed with machine guns. [Illustration: A 40-FT. COASTAL MOTOR BOAT TRAVELLING AT FULL SPEED _Thornycroft & Co., Ltd._] [Illustration: A 40-FT. COASTAL MOTOR BOAT TRAVELLING AT FULL SPEED _Thornycroft & Co., Ltd._]