Should this man[oe]uvre have placed the surface ship in close proximity to the submarine, one or more depth charges were released, and if the explosion of these damaged the comparatively delicate hull or machinery of the under-water craft, she had either to rise to the surface and fight for her life with her two powerful deck guns, or, if badly damaged, sink helplessly to the bottom, emitting oil in large quantities from her crushed tanks.
Before entering upon a description of the depth charge, however, there is more to say of the hydrophone, which has played such an important part in the defeat of the U-boats.
When the advantages of this instrument had been fully demonstrated in the stern trial of war, successful efforts were made to improve upon the original crude appliances. The "water noises" were reduced and, greatest improvement of all, the hydrophone was made "directional." By this is meant that when a sound was heard its approximate direction north, south, east or west of the listening ship could be more or less accurately determined. What this improvement meant to a vessel hunting a submarine in a vast stretch of sea will be easily realised. When the sound came up the wires from the submerged microphone the operator had simply to turn a small handle in order to determine from which direction the noise was coming.
If, for example, the sound was first heard away to the east, the instrument was turned to another quarter of the compass. Then, if the noise was plainer, the instrument was turned again until the sound decreased in intensity. In this way the line of maximum sound was obtained, and this showed the direction from the listening ship in which the U-boat was operating.
Fig. 8.—Plan showing how microphones or ears B are fitted in a submarine A to enable it to detect the approach of surface craft.
With the perfection of this invention the hydrophone section of the naval service came into being. Special courses in the detection of submarine sounds were instituted for officers and also for seamen listeners. The actual movements of a submarine under water at varying distances from a hydrophone were recorded by a phonograph, and records made so that the sounds might be reproduced at will for the education of the ear. Surgeons with aural experience estimated the physical efficiency in this respect of would-be volunteers for the hydrophone-listening service, and vessels were formed into special hydrophone flotillas, whose duties consisted of listening in long lines for submarines and when a discovery was made attacking them in the most approved tactical formation, with the aid of depth charges and guns.
A considerable measure of success attended these arrangements, and the author spent many cold hours listening at night for the sound of the wily submarine. On more than one occasion an exciting chase resulted.
It must, however, be pointed out that there is one great drawback to the successful use of the hydrophone. It exists in the necessity for the listening ship to stop before the hydrophone is hoisted outboard, it being quite impossible to hear anything beyond the roar of the engines of the carrying ship so long as they are in motion. Furthermore, all progress through the water must have ceased and the listening ship have become stationary before artificial sounds, such as the propellers of a submarine, can be distinguished from the natural noises of the sea water.
Now it will at once be apparent that not only does a stationary ship offer a splendid target for under-water attack, but also it allows a somewhat humorous game of hide-and-seek to be played between a hunting vessel and a hunted submarine.