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.
Thornycroft & Co., Ltd.
Thornycroft & Co., Ltd.
It is not difficult to imagine a fight between a C.M.B. travelling at 40 knots, firing with its little Lewis gun at a big seaplane swooping down from the clouds at the rate of 70 miles an hour, and splashing the water around the frail little grey-hulled scooter with bullets from its machine gun. This actually occurred many times off the Belgian coast, and is a typical picture of guerrilla war at sea in the twentieth century.
Fig. 5.—Diagram illustrating method of attack by C.M.B. on surface ship (or submarine on surface). A. Object of attack travelling in direction indicated by arrow E. B. The position of the C.M.B. after delivering the attack. C. The torpedo, released by the C.M.B. at point D, travelling on course ending at F, which, allowing for movement of ship A, is the place where the torpedo should strike its object of attack. From this it will be seen that the torpedo, when released, actually follows the ship from which it is fired until the latter swerves from the straight course, when the torpedo continues until it strikes or misses the object of attack, the speed of the torpedo being about the same or a little less than that of the C.M.B. The total time occupied in such an attack over a course of two miles would be about 2½ minutes before the torpedo struck its object.
The C.M.B. was a purely British design, and the firm largely responsible for the success achieved was Messrs John J. Thornycroft & Company Limited. There were bases for these sea-gnats at Portsmouth, Dover, Dunkirk, and in the Thames Estuary at Osea Island. From all of these points mid-Channel could be reached in less than thirty minutes. Although useless in rough weather, a trip in a C.M.B., even on a calm day, was sufficiently exciting. The roar of the engines made speech impossible, and vision when sitting in the little glass-screened well, or conning-tower, was limited by the great waves of greenish-white water which curved upwards from either bow, and rolled astern in a welter of foam. There was an awe-inspiring fury in the thunder of the 700 h.p. engines revolving at 1350 per minute, and a feeling of ecstasy in the stiff breeze of passage and the atomised spray. When waves came the slap-slap-slap of the water as the sharp bows cleft through the crest and the little vessel was for a brief moment poised dizzily on the bosom of the swell caused tremors to pass through the thin grey hull, and, to complete the review of sensation, there may be added the human thrill of battle and the indescribable feeling of controlled power beneath one's feet.