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[Page 38]
"fish" always being full of water, which percolates into the body of the fish through hundreds of very small holes bored at intervals round its entire circumference and length. The cable was then paid out some 200 feet, the engines started at a maximum of ten knots, and listening commenced.
It may be asked "Why was the "fish" filled with water?" The answer is this:- a microphone, being a very sensitive instrument if surrounded only by the air contained in the vessel in which it is fixed, would first of all register the sounds of the water rushing past that vessel to the elimination of all other sounds. But by filling the "fish" with water, you were thus towing, as it were, a small pond of absolutely still water through the water rushing past the outside of the "fish". The sound of the outside water was not sufficiently intense to be registered by the microphones once they were cushioned by the still water surrounding them.
Secret of Microphones
This in fact was the whole secret of the Nash "Fish" Hydrophone. Before this, a hydrophone - and there are many kinds - could only be used by a vessel which had lost all way, and even then the microphones they contained were really handicapped, not only by the rise and fall of the ship and the eddies caused by the shape of her hull as the seas licked round her, but even by people walking about or talking. But the inventor of the "fish" decided that a hydrophone to be of any offensive use in tracking down the elusive Fritz must be, not only a mobile instrument enabling it to be towed a good deal faster than Fritz could travel while submerged, but must be immune from all noises caused by the ship and the water rushing past her.
Fitted in small vessels such as drifters or trawlers and in some cases T.B.D's with reciprocating engines, the "fish" was responsible for the tracking down and ultimate destruction of a great many enemy submarines. Indeed, it did all the inventor claimed for it. In one instance a submaring was heard by two drifters fitted with the "fish" off the Isle of Wight and was tracked and destroyed twenty hours later in the neighbourhood of Lands End, this in spite of the fact that she had come to the surface several times during the night in order to use her Diesel engines for greater speed.
An Utter Failure
Unfortunately for our hopes in the Adriatic Barrage, the "Torrens" - who by the way was the first turbine vessel fitted with "fish" - as regards her "fish" proved an utter failure. The reason was