The havoc wrought by the submarine after the outbreak of the War of the Nations set naval architects to devising a boat that would destroy this menace to shipping, and a number of types were constructed. The general name applied to all craft of this type is submarine chaser. The essentials of the submarine chaser are speed, light draft and mobility. The boats varied in length from forty-five to 125 feet, and the draft does not exceed four feet six inches. Those of the larger size have engines of 600 horse power, and a speed of thirty-five to forty miles an hour is attainable. The armament consists of one or more 3-inch guns and from one to three machine guns. Because of its light draft such a boat cannot be struck by a torpedo, and its 3-inch gun throws a shot that will pierce the shell of any submarine. Its speed and ability to change direction quickly renders it comparatively safe from the guns of its enemy; consequently, it is an antagonist that the submarine does not care to encounter. In 1917 boats of this type did effective work in assisting Great Britain in fighting German submarines.
Each chaser is given a certain area of seaway to patrol. This she does, and if she is lucky she will soon see the periscope of an enemy submarine poking its hood above the water to take a peek around the horizon. This is the signal for the chaser to bear down on that periscope at full speed, the gunners doing their level best to hit the periscope or any other part of the submarine which shows itself above water. As it takes time for the submarine to dive or to get her own guns into action, the chaser stood a pretty good chance of either crippling or even sinking her. Further, the submarine cannot use her torpedoes on the chaser, for the latter craft is so short and has such a shallow draft that her hull does not offer much of a target for a torpedo, even though she were standing still, and much less when she is bearing down on the submarine at full speed.
The tactical employment of the submarine chaser was hunting submarines in limited areas, with hydrophones and listening devices, and by means of these ascertaining the submarine's course, speed, and position. When it was definitely located, it was attacked with depth charges according to certain doctrines. The mere method of hunting presented many difficulties. Listening for a submarine, a new development in Naval tactics, required a trained ear on the part of the listener; he had to be able to distinguish the peculiar sound of a "submarine beat" from that of surface craft, and learn how to ascertain its speed, course, etc. Moreover, one chaser alone could not accurately fix the position of a submarine; to do this it was necessary to have cross-bearings from other chasers. Accordingly, the training of the personnel in their particular duties, was one of the first tasks to be accomplished.
All American Chasers operating in foreign waters were equipped with several distinct types of listening apparatus for determining the position of a submarine by the sound of its propelling machinery transmitted through the water. The listening devices in common use were the "S. C." tube, the "M. B." tube, and the "K." hydrophone, of which the two former were used principally in the daytime, because of the ease with which they could be lowered and raised again. At night when hove to or when lying idly on station in the daytime, the "K." in addition to the other tubes was used.
The principle of all listening devices is based on the fact that to hear a sound most distinctly it is necessary to turn the head so that it faces the direction from which the sound comes, in other words, so that the ears are equi-distant from the source of sound. Both the "S. C." and the "M. B." tubes were built in the form of an inverted T, one L of which was connected to the listener's right ear and the other to his left ear. When the ship was stopped and the tubes lowered they were revolved with the same effect as though the listener were turning his head beneath the water. When a sound was heard it was "centered" by turning the tube until it came to the listener's ear with equal intensity. Its bearing relative to the ship's head was then read from a collar at the top of the tube marked in degrees. For example, if the sound came from the starboard side directly abeam, its bearing was 90 degrees. Knowing the ship's course, it was then a simple matter to convert this bearing into a magnetic compass bearing, to be transferred to the guide ship by telephone.
The "S. C." tube was installed on the port side of the keel well forward and when not in use was hauled up into a protective housing built along the garboardstrake. When lowered it projected about 3 feet below the keel. As shown in the sketch, the arms of this tube were fitted with hollow rubber ears connected by tubes of absolutely equal length to the stethoscopes. The sound coming through the water struck the outside of these sensitive rubber bulbs setting up a vibration of the air inside which traveled up the tubes to the ears of the listener. There were several objections to this type of tube. For example, when the chaser was rolling more than 20 degrees, and she usually was, the water washing against the ears made it difficult to get a good center. But a greater difficulty was that because of its construction all the sounds in the neighborhood were heard at once, producing a confusion which made it difficult to isolate any particular one.
The "M. B." tube was designed to overcome these difficulties. It was installed on the starboard side directly opposite the "S. C." and, while more accurate, was heavier and much more elaborate. It was constructed with 16 ears, eight on the right side and eight on the left, all on the bottom of the tube and all connected by passage of equal length to the stethoscopes. The principle of the ears equi-distant from the listener's ears was the same as in the "S. C." tube. The advantages of the "M. B." were that water noises had much less effect on it and that if there were more than one sound, all but one could be cut out provided they were more than 30 degrees apart. With the "S. C." any sound in the circle of the horizon could be heard while the "M. B." worked more as a telescope to concentrate on one spot.
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