THE HYDROPHONE AND THE DEPTH CHARGE
OF all the weapons used in the anti-submarine war the two most important
were the _hydrophone_ and the _depth charge_. They were employed in
conjunction with each other and comprised the surface warship's
principal means of offence against submarines operating beneath the
surface.
The hydrophone resembles a delicate telephone. It is so constructed that
when the instrument is lowered over the side of a ship into the sea any
noise, such as the movement of a submarine's propellers, can be heard on
deck by an operator listening at an ordinary telephone receiver
connected to the submerged microphone by an electrified wire.
There were many different types of hydrophone in use during the Great
War. So important was this instrument for the work of submarine hunting
that money was spent in millions, and a corps of naval and civil experts
were engaged for several years, bringing it to a state of efficiency.
Each type introduced into the Service was an improvement on its
predecessor, and there were different patterns for the use of almost
each class of vessel. The fast destroyer required a different
instrument to the slow-moving trawler. The motor launch could only
employ successfully a totally different type to the submarine, and, to
add to the difficulties, the German submarines themselves were
generously supplied with similar instruments. The games of
"hide-and-seek" played on and under the seas with the aid of this
wonderful little instrument would have been distinctly amusing had men's
lives--and often those of women and children--not been dependent upon
the issue.
The portable hydrophone, used by some of the smaller and slower vessels
of the auxiliary fleet, consisted of a microphone, or delicate
mechanical ear, carefully guarded by metal discs from accidental damage,
and connected to ear-pieces or ordinary telephone receivers by an
electric wire which passed through a battery. Where the wire came in
contact with the sea water it was heavily insulated and lightly
armoured.
When it was required to use this instrument the vessel was stopped and
the microphone lowered overboard to a depth of about 20 feet. This was
the distance down from the surface at which submarine noises could be
heard most distinctly. The operator on deck or in the cabin then
adjusted the ear-pieces and sat listening for any noises coming through
the water. Although the sea is a far better conductor than air, the
range at which sounds could be heard varied considerably. On a calm day
or night the noise of a ship's propellers could frequently be
distinguished at from five to seven miles; whereas on a rough day, with
the sea splashing and the wind roaring, it was often difficult to hear
anything beyond half-a-mile.
[Illustration: FIG. 6.--Diagram showing essential parts of a portable
hydrophone. _A._ Head and ear pieces, by means of which a trained
listener hears submarine sounds. _B._ Flexible leads to enable an
officer to verify reports from listener. _C._ Battery box, containing
spare set of cells. _D._ Terminals. _E._ Terminals of spare cells. _F._
Flexible armoured electric cable which is lowered over side of ship.
_G._ Metal case protecting the microphone _H_. _H._ Microphone or
delicate receiver of submarine sounds, which is submerged (when
required, but not when ship is moving) to a depth of about 18 feet, as
in small diagram. The sound is detected by the microphone and
transmitted up the cable _F_ and wires _B_ to the ear-pieces _A_.]
In fine weather a submarine could usually be heard at a distance of
about two or three miles. There were, however, many microscopic noises
of the under-seas which were picked up and magnified by this type of
hydrophone. They were called "water noises," and often made it extremely
difficult to differentiate between them and the sound of a moving
submarine at a great distance. Later types were not so prone to these
disturbing influences.
To describe here the different natural and artificial noises heard on a
portable hydrophone is extremely difficult. One general statement can,
however, be made. It is the noise caused by the rapidly revolving
propellers of both surface ships and submarines that is the guiding
factor in the work of detection by submarine sound. A destroyer
travelling at full speed on a calm sea, when heard on a hydrophone
resembles the roar of a gigantic dynamo. The sound does not alter as the
distance between the _stationary_ listening ship and the _fast-moving_
warship increases or decreases; it continues to be a roar or low hum,
according to distance, until it fades out of hearing altogether. The
same statement applies also to a slow-moving cargo steamer, only in this
case the _single_ propeller is revolving very much slower, and, when
listening on a hydrophone about two or three miles distant, each
successive beat of the engines can be distinctly heard.
The simple movement of a vessel's hull through the water cannot be heard
on a hydrophone. Therefore for detecting the presence in the vicinity of
a _sailing_ ship at night or in a thick fog this instrument is quite
useless. The same drawback applies also to the location of a floating
derelict or iceberg, and restricts the use of the hydrophone to
faithfully reporting the presence of power-driven ships or special sound
signals at a range of a few miles.
[Illustration: FIG. 7.--An improved directional hydrophone fitted
through keel of motor launch. The tube _B_, at the lower extremity of
which is the microphone, can be raised or lowered from _C_, the cabin of
the M.L. This instrument is so arranged that the direction from which
the submarine sound is coming can be simply and quickly ascertained.]
A German submarine heard at a range of about a mile on a calm night
presents a curious sound which almost defies description. Its principal
constituent consists of a "clankety clank! clankety clank!" at first
barely distinguishable from the low swish of the water past the face of
the submerged microphone, then louder when the sound has been
distinguished and the human ear is on the alert. But when this sound was
heard in war there was little time for analysing or noting. It was the
call to action. The microphone was hauled to the surface and the chase
began, a halt being made every half-mile or so for a further period of
listening on the hydrophone. If the sound was louder the commander of
the pursuing vessel knew that he was on the right track, and if the
sound came up from the sea more indistinct the course was changed and a
run of a mile made in the opposite direction, when the vessel was again
stopped and the instrument dropped overboard.
Should this manoeuvre 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.
[Illustration: 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.
Nearly all U-boats were fitted with a number of hydrophones and
therefore were as well able to receive timely warning of an approaching
surface ship as the surface ship was of the presence of the submarine.
But the surface ship had the advantage of speed.
The result of all this was that when a German submarine heard a surface
vessel approaching she dived to the bottom, if the water was not too
deep or the sea-bed too rocky. Then shutting off her engines she
listened. The surface ship, mystified by the sudden cessation of the
noise she had been pursuing, also waited, and this stagnation sometimes
lasted for hours. Then if the surface ship moved, as she was often
compelled to do in order to avoid drifting with the tide away from the
locality, the submarine moved also, and the one that stopped her engines
first detected the other, but could not catch up to her again without
deafening her own listening appliance. In which case the next move would
probably be in favour of her opponent.
All of this is, perhaps, a little complicated, but a moment's pause for
reflection will make this curious situation clear to the reader. And so
the game went on, with decisive advantage to neither the surface ship
nor the submarine. Darkness usually intervened and put an end to further
manoeuvring, frequently allowing the submarine to escape.
A case of this kind occurred to a vessel, of a certain hydrophone
flotilla, commanded by the author. For over four hours the U-boat eluded
the pursuing surface ships by moving only when they moved and stopping
when they too had stopped, darkness and a rising sea eventually
favouring the escape of the submarine, which, a few hours later, was
able to attack (unsuccessfully) a big surface ship less than thirty
miles distant from the scene.
Nevertheless the hydrophone is a submarine instrument with a brilliant
future. It has already been improved out of all resemblance to its
original self, and more will undoubtedly follow. It is, however, purely
an appliance for the detection of submarines when cruising beneath the
surface, and not a weapon for their destruction. It should also be
remembered that any improvement made in the efficiency of the hydrophone
will benefit not only the surface ship, but also the submarine, for it
cannot be supposed that under-water craft will be left without these
wonderful submarine ears when their surface destroyers are equipped with
them.
The alliance between the hydrophone and the depth charge is a natural
one. The former instrument enables the surface ship to discover, first,
the presence of a submarine in the vicinity, and, secondly, its
approximate position. At this point its utility _temporarily_ ceases and
that of the depth charge begins. When a surface ship is hot on the track
of a moving submarine she endeavours to attain a position directly over
the top of her quarry, or even a little ahead, and then releases one or
more depth charges according to whether the chance of a hit is good or
only poor.
From this it will be apparent that whereas the hydrophone is the
instrument used for the initial detection of the submarine, and
afterwards for enabling the surface ship to get to close quarters with
her submerged adversary, it is the depth charge with which the attack is
actually made.
This weapon is really a powerful submarine bomb. It consists of several
hundred pounds of very high explosive encased in a steel shell, with a
special firing device which can quickly be set so that the charge
explodes at almost any depth below the surface after being released from
the above-water vessel.
The methods in use during the war for its release from the decks of
surface ships were very diverse, the most usual being for a number of
these weapons to be fitted on slides and held in place by wire slings
which could be released by simply pulling out a greased pin or bolt.
When the depth charge rolled off the stern of the surface ship it sank
to the "set depth" and then exploded like a submarine mine. The result
was a shattering effect exerted through the water for several hundred
feet around. If the submarine was close to the explosion her
comparatively thin plates were nearly always stove-in. When she was over
a hundred feet away, however, the rivets holding her plates together
were often loosened, and the resulting leak frequently compelled her to
come to the surface, where she could be destroyed by gun-fire.
[Illustration: DROPPING DEPTH CHARGES
_British Official Photograph_]
It often happened, however, that neither one nor the other of these
things occurred, but that the submarine's delicate electrical machinery
was thrown out of order by the violence of a depth-charge explosion,
even when a considerable distance away. With the electric engines
used for submerged propulsion no longer available, and possibly the
interior of the vessel in darkness, there were only two courses open.
She could either rise to the surface and endeavour to fight it out with
the aid of her powerful deck guns, or else sink to the bottom and trust
to luck that other depth charges would not be dropped close enough to
seriously damage her hull. In the open sea, however, the latter chance
was denied because of the depth of water. Three hundred feet may be
taken as the greatest depth to which an ordinarily constructed fighting
submarine can safely descend without running a grave risk of having her
plates crushed in by the great water pressure. Even at this depth the
weight on every square foot of hull surface exceeds 8 3/4 tons.
If the damaged submarine rose to the surface the guns of her pursuers
were ready and could generally be relied upon to place her at least
_hors de combat_ before the hatches of the under-water vessel could be
opened and her own guns brought into action.
[Illustration: FIG. 9.--Diagram showing how depth charges are carried on
the stern of a motor launch. _AA._ Depth charges, each containing 300
lb. of high explosive. _B._ Hydrostatic device by means of which the
charge can be made to explode when it has sunk from the surface to a
depth of 40 or 80 feet, and by which it is rendered comparatively safe
while on deck. _C._ Slings holding charges in place on inclined
platform. _D._ Greased bolts which, on being pulled out, allow wire
slings to fly free and depth charge to roll into the sea. Depth charges
can only be released from vessels under way, otherwise the explosion
which occurs a few seconds after release damages surface vessel.]
In shallow water where there was a fairly smooth bottom it generally
happened that a submarine damaged by depth charges elected to sink to
the sea-bed and trust to luck. This was also frequently resorted to as a
means of eluding pursuit even when the U-boat was not damaged by the
first few charges dropped. It was then that the hydrophones carried by
the surface ships were again brought into use to ascertain if the
submarine was still under way. When no sound was heard those on the
surface knew that "Fritz" was lying doggo, or else that he had escaped.
If a number of ships were available a few waited over the spot where it
was considered the U-boat was lying, while the others scoured the
surrounding seas in circles trying to pick up the sound of the runaway's
engines if she had escaped in the melee. When nothing further was heard
they returned to the scene and set about the work of systematically
bombing the surrounding sea-bed.
[Illustration: FIG. 10.--Diagram illustrating a depth charge attack on a
submerged submarine. _A._ Motor launch, which has dropped a depth charge
to destroy a submarine _B_ travelling at a depth of 90 feet below the
surface. _C_ is the depth charge sinking as the M.L. steams away from
the danger area. _D_ is the point (80 feet below the surface) at which
it will explode, and _E_ indicates the danger area for the submarine
_B_.]
As many as one hundred depth charges were dropped in quite a small area
of sea and yet a submarine known to have been lying "doggo" in the
locality was not damaged. In cases such as this other means, which will
be described in a succeeding chapter, were then resorted to.
All the foregoing sounds very thorough and hopeful, but in fairness it
must be said that submarine hunting is a heart-breaking task. The
reader may have noticed that the method of depth-charge attack
presupposes the surface vessel to have attained a position almost
directly over the top of her enemy, a manoeuvre extremely difficult of
achievement even with the most efficient hydrophone. Heavy seas, snow
and fog have also to be taken into consideration, to say nothing of
darkness, the presence of a second submarine, a surf-beaten rock or
sandbank and the confusing sounds of passing merchant ships, making a
difficult task more difficult, as will be seen when we come to the
actual fighting.