This pictorial illustrates the shape of the detection
area for the 144 ASDIC, the ‘Q; attachment and the 147 Asdic.
Asdic dome, oscillator and housing equipment The
oscillator, a quartz crystal disc which converted electrical impulses into
sound and echoes back to electrical impulses , was lowered into the water under
the ship’s hull, protected from extraneous sounds by a streamlined dome which
allowed operation at speeds of up to 20 knots . At full speed or in rough seas
the dome and oscillator could be retracted into the hull.
The ultimate solution to sinking more U-boats depended not
on listening for the sounds that U-boats themselves emitted, but on generating
a pulse of sound that could be bounced off a U-boat’s hull to give an echo that
could be picked up by the transmitting ship. This was the principle of the
device that the British called Asdic (after the Allied Submarine Detection
Investigation Committee that sponsored the development). It involved a transducer
that could be made to send out a fan-shaped pulse of acoustic energy through
the water. If this struck a submerged object, enough of the energy would be
reflected towards the transmitting ship to be picked up as a sound echo. The
ship’s heading gave the bearing of the submarine (later, the transmitter head
of the detector could be turned by the operator to cover any direction from the
ship carrying the equipment) and the delay between the original pulse and the
receipt of the echo gave the range of the submarine.
Like the hydrophones, Asdic had its disadvantages, but they
were less restrictive. It could only be used when the transmitting ship was
moving at less than fifteen knots, and it tended to produce echoes from many
different kinds of submerged objects, only some of which were submarines. It
could give little indication of depth, and as the ship closed in on the target,
it lost contact when the range dropped to less than 100 yards. It was no use at
all against submarines on the surface.
Finally, it was too late for the war – only seven ships were
fitted with the equipment by the Armistice, and none used it against U-boats.
The result of this last-minute development of the one weapon that would have
made a genuine difference was that up to the end of March 1917, British
destroyers had made 142 attacks on U-boats, but had only succeeded in sinking
half a dozen of them. The chances were therefore 23 to 1 in favour of the
U-boat escaping its attackers, though simply forcing it to dive would usually make
it lose contact with potential targets.
Yet Asdic promised much for the future. In time, skilled
operators could learn to distinguish between echoes reliably enough to be sure
when a U-boat was in their sights. They could also estimate its depth well enough
for accurate attacks. It could also cause severe damage to the morale of the
U-boat crews: the shrill ping of the Asdic pulses travelling through the water
and striking the submarine hull told them escorts were searching for them and
very probably knew exactly where they were; crippling depth charge explosions
could be expected at any moment.
It would be 1920 before warships were equipped with Asdic in
quantity. In fact, it was developed at exactly the wrong time for British ASW
operations. Too late for the First World War, it was still early enough in
service to cause immense and crippling complacency over its effectiveness
during the inter-war years. The Royal Navy came to assume that if submarines
could not be abolished at the stroke of a pen in the clauses of the postwar
treaties, then any resurgent threat could quickly be seen off by Asdic and
depth charges. Had there been a chance to use these weapons to a larger extent
before the end of the First World War, it would have become clearer how difficult
it remained to sink U-boats, even with the aid of these powerful new weapons.
While this undoubtedly promised well for the future, the
Germans were already working on tactics of their own to reduce the advantages
conferred by Asdic. Since a U-boat’s speed and endurance while on the surface
were so much greater than when submerged, more and more skippers were choosing
to carry out attacks in darkness, when all the enemy would see was the small
silhouette of the conning tower against the blackness of the night. At the
time, this was advantage enough: when Asdic came into general use, it would be
even more powerful a tactic, since Asdic could not pick up the echo of a
surfaced submarine.
The fashion spread quickly, even among as individualistic a
group as submarine commanders. During the final year of war, more than a third
of U-boat attacks in the Atlantic and British home waters were night surface
attacks, and in the Mediterranean the proportion was almost doubled. Like
Asdic, this was a development that would prove even more effective when the
fighting resumed after the uneasy peace.
Because Asdic had been developed just too late to come to
the rescue in the First World War, its effectiveness had never been tested
under combat conditions, which would have revealed its very real limitations.
As a result, it had come to be regarded as the panacea for future ASW; sound
location and the depth charge were assumed to have virtually rendered the
submarine obsolete as a threat.
Nevertheless, some work had been done to develop tactics to
use this new combination to sink submarines, and these had been tested at sea,
using Royal Navy submarines as targets. The anti-submarine warfare specialists
at HMS Osprey at Portland, under the direction of ‘that devoted father of the
Asdic, Professor Jack Anderson’, had devised what came to be known as the
‘pounce’ and ‘MRCS’ tactics, which set out to reduce the freedom of a submarine
to take evasive action during the last stage of a depth-charge attack, when
Asdic effectively became deaf. The ‘pounce’ attack involved the attacking
warship moving at slow speed to avoid being picked up on the submarine’s
hydrophones. In the meantime another escort monitored the submarine’s
movements. When the time was right, the first escort would accelerate to full
speed for the attack, being homed in on the target by its sister ship.
At first this seemed to work quite well, until the skippers
of the target submarines realised how the tactic worked and became adept at
outwitting it once they realised the high-speed dash had begun. The next step
was the Medium Range Constant Speed, or MRCS attack, which involved shadowing a
submarine at low speed from half a mile away, and then accelerating to the
limiting speed at which Asdic could still hold the echo of the submarine,
adjusting the escort’s course to match the submarine’s movements. This
succeeded in reducing the area of uncertainty between the point at which the
echo was lost and the dropping of the depth charge pattern to some 250 yards,
but this was still ample for a skilled submarine skipper to take successful
evasive action.
One of the Royal Navy’s particularly strong suits was in the
field of training aids, and before the war they introduced an Asdic mobile
target and a depth-charge attack analyser, which could be used to assess the
success or failure of anti-submarine exercises. The Admiralty Research
Laboratories also developed a course plotter as a navigational aid, but it also
proved valuable when plotting the course of an antisubmarine attack.
The first attack teaching aid for training officers and
ratings in anti-submarine tactics and drills was set up at the Portland
Anti-Submarine School by 1925, and consisted of the control equipment of an
Asdic set together with a glass-topped attack table covered with a sheet of
thin plotting paper. Two spots of light were projected on to this representing
the positions of the escort and the submarine, and these were moved
independently under the orders of the pupil and the instructor. Each Asdic
pulse was represented by beams of light corresponding with the settings of the
Asdic controls, and if one of these struck the submarine the sound of the echo
was triggered through the pupil’s headphones. Other aids trained operators in
the techniques of sweeping for a possible target and what to do if a target was
lost.
However, the greatest defect of this sound practical
training is that so few of the people who would use the equipment in wartime
were ever persuaded to specialise in ASW before the war. In spite of the
lessons of 1917–18, ASW remained more of a career backwater in the Royal Navy
than U-boats in the Kriegsmarine. Captain Donald Macintyre, who became one of
the Royal Navy’s foremost sub-killers and who sank the U99 and captured the ace
Otto Kretschmer, spent the pre-war years flying with the Fleet Air Arm and
commanding fleet destroyers (apart from a stint running HMS Kingfisher, the
experimental ship of the Anti-Submarine School). The greatest submarine hunter
of all, ‘Johnny’ Walker, suffered being passed over for both promotion and
command for selecting to specialise in ASW in a navy still dominated by the
battleship and the big gun.
Ironically, the Germans themselves were to prove they had
their blind spots. Since the end of the First World War, they had concentrated
much more on passive developments like hydrophones because for several years
active sound location methods were seen as being linked to attack rather than
defence and were therefore proscribed by the Versailles Treaty. As a result,
they had little knowledge of what Asdic and the other ASW weapons could do.
Doenitz was firm in his conviction that the British were too complacent
regarding Asdic’s value and capabilities. Werner Fürbringer disagreed, on the
grounds that the Royal Navy’s defences would be too formidable to risk wasting
U-boats and their crews on a blockade campaign. The problem, from Doenitz’s
point of view, was that Fürbringer was a rear-admiral, was responsible for
submarine planning at the Naval High Command, and was effectively his boss.
All Royal Navy destroyers were fitted with ASDIC during the
early 1930s. This underwater detection device to locate U-boats using sound
echoes was refined before and during World War II by British and other
anti-Nazi scientists. Improved hydrophones had long been able to detect a
U-boat’s bearing. When grouped to receive echoes of sound pulses, they also
determined range. ASDIC worked by sending out acoustical pulses that echoed off
hulls of U-boats, but also sometimes off the sides of whales or schools of
fish. The echoes were heard by grouped hydrophones on the sending ship, so that
an ASDIC screen and operator provided the escort’s captain with estimated range
and position of the enemy submarine. It was limited by the sounds of other
ships’ screws, rough seas, and onboard machinery of its host ship. Such
interference enabled U-boats to hide from escorts inside the “noise barrier”
created by a convoy. More importantly, even in optimum conditions early ASDIC
could not determine a U-boat’s depth.
British and Commonwealth ASDIC operators could locate
U-boats to a distance of 2,000 meters by 1940. However, from 200 meters range
to source, pulse and echo merged. That meant U-boats were lost to detection
before the moment of attack, just as a destroyer closed on its position.
Because forward-throwing technology for depth charges had not been developed,
the explosives were dropped astern of the charging destroyer across the last
known position of the U-boat. Loss of contact, stern attack, and the time it
took charges to sink to explosive depth combined to permit many U-boats to
escape destruction simply by turning hard away from the closing destroyer or
corvette. Admiral Karl Dönitz, head of the Kriegsmarine U-boat arm, countered
the threat from ASDIC by instructing U-boat captains to attack only on the
surface and at night. That countermeasure was lost to U-boats once the Western
Allies deployed aircraft equipped with Leigh Lights. Dönitz next ordered
research into absorbent coating and rubber hull paints to reduce the ASDIC
signature of his U-boats, but with little success. Similarly, release of a
Pillenwerfer noise-maker only tricked inexperienced ASDIC operators. An
advanced Type 147 ASDIC set was developed later in the war that tracked U-boats
in three dimensions, giving readouts of bearing as well as range and depth.
Note: All Western Allied navies adopted the U. S. Navy term for ASDIC in 1943:
sonar.
Major wartime sonar developments attempted to address these
deficiencies. Power rotation and improved displays enhanced operating rates,
and streamlined steel domes raised useful search speeds. Dual-frequency sets
(operating at either 14 or 30 kilocycles) enhanced ranges, and tilting
transducers eliminated the dead zone. Britain also developed a specialized
sonar (Type 147B) for accurate depth determination. A simultaneous line of
development, the scanning sonar using an omnidirectional transmitter coupled to
an array of fixed receiving transducers, offered a possible solution to the
search problem. Such equipment required greater power to maintain its range but
could be larger (since rotation was eliminated) and hence could operate at
lower frequencies, enhancing performance.
Wartime submarines also carried sonar. Most navies relied on
active sets for target detection, but Germany pursued a different course with
its Gruppen-Horch-Gerät (GHG) equipment, a standard installation from 1935 on.
An array of sound-receiving diaphragms on each side of the bow connected to a
pulse-timing compensator provided bearings of received noise. This apparatus
could detect single ships out to 16 miles and large groups to 80 miles, but the
bearings it provided were insufficiently precise for accurate attacks. At short
ranges, however, a supplemental swiveling hydrophone (Kristall-Basisgerät)
generated bearings accurate to within 1 degree. Finally, to obtain ranges
U-boats carried an active sonar (SU-Apparatus) developed from surface warship
sets, although this device was rarely used because its emissions would reveal
the submarine’s presence. Late-war trials, however, using GHG together with
SU-Apparatus demonstrated that as few as three active impulses sufficed to
determine target distance, course, and approximate speed.
Fixed-array scanning formed the basis for active sonar development after World War II, while passive systems evolved from the original German GHG. In the process the two types converged; most modern ship-mounted sonars operate in both active and passive modes, often simultaneously.
