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.