Submarine warfare is no exception to the historically proven rule that measure and countermeasure succeed each other in swift succession, with each new weapon more terrible than the last, and each defense more difficult to provide in time. VE-Day found the Germans launching a submarine capable of completely submerged operation for periods of weeks or months, equipped for incredibly deep diving, and with getaway speeds of 24 knots submerged. Allied opinion is that it could out-distance and probably shake oil any of our present escorts who attack it.
Such submarines will be with us for the rest of history. Their targets in the future will be cities as well as ships, for unlike the air bomber, they can approach a long coastline unseen, can surface, and project rocket and atomic missiles at will. Sub-surface ambush has now become an ever lurking danger to a nation which would prefer to mind other business while pioneering the high seas of international good will.
The menace of attack from beneath the sea, a grim actuality twice within our lifetime, has been met in the past and can be met again only by marshalling the resources of science for the perfection of SONAR. The word SONAR abbreviates SOund, Navigation, And Ranging, and includes all types of underwater sound devices used for listening, depth indication, echo ranging, and obstacle location. Radar, a relatively well publicized development, gives eyes to the fleet; sonar contributes cars and a voice underwater.
The Navy’s story of research and development in Subsurface Warfare, for two decades a closely guarded secret, was released to the American people by the Chief of Naval Operations on April 6, 1946. The scientific means by which the U-boats were overcome and by which our own submarines were prepared for assault on Japan’s life lines can now be described.
World War II, beginning on the first of September, 1939, found the United States with only 60 destroyers equipped with antisubmarine devices. When, caught short in December, 1941, we finally faced the packs of Axis submarines in full fledged war, we did so with utterly insufficient numbers of escort and patrol vessels. On this date we had 170 destroyers to protect the life lines of men, food, and war materials spreading to the far corners of the earth. This small group of escorts, often outnumbered two to one by the prowling U-boats, was, however, equipped with echo-ranging Sonar. Then as now, Sonar was the only effective means of detecting and locating completely submerged submarines.
The equipment developed between the wars by about twenty scientists working at the Naval Research Laboratory in Washington, in close cooperation with the sonar design section of the Bureau of Ships, constituted the primary techniques with which we entered and fought World War II. This Naval scientific development was an unspectacular, long, slow, steady conquest over the physical elements of the sea.
The basic principle of echo-ranging Sonar can be described in three steps: first, the production of pulses or bursts of highly intense sound energy in a cone shaped underwater beam somewhat similar to a slowly blinking searchlight beam in air; second, the rotation of the beam around and around the horizon in a search procedure, with the emission of “pings” at intervals of two or three seconds; and third, the reception, when an underwater target is present, of a returning echo as the pulse of outgoing sound strikes the target hull and returns, much enfeebled, to be detected by the practiced ears of the sonar operator. The device which changes electrical energy in the “driver” or transmitter into sound energy in the water is called a “Transducer” or underwater projector. The transducer, housed in a special protective blister beneath the ship’s hull, can be lowered through the hull when needed, and retracted again when the danger of submarines is past. The transducer, in sonar systems, plays exactly the same role under water that the radar antenna, turning around and around to search the horizon, does in air. Radar, radio, and other systems which use electromagnetic radiations are of no value under water, because water is a conductor of electricity, and serves as a shield against the penetration of such radiations. Sound energy alone can penetrate worthwhile distances below the surface of the sea.
The information obtained from echo-ranging is the direction or bearing of the target submarine from the attacking vessel, and the distance or range which separates attacker and attacked. The bearing is read by comparing the direction in which the transducer points when an echo is received, with the own ship’s heading given by the gyro-compass. The range of the target is estimated from the time it takes the sound pulse, or ping, to go from the transducer to the target and return as an echo. The Echo-ranging sonar with which we fought the war can detect submarines reliably to distances of about 2500 yards. This is, of course, much shorter than the range of radar in air, but it seems to be about the best that we can do to date with underwater sound.
As target range and bearing are furnished by the sonar operator, the course and speed of the submarine are plotted, and the attack is carried on by making runs across the track of the submarine, and dropping depth charges from the escort’s stern at the moment which is indicated as giving the best probability of a hit.
This, then, was the type of sonar equipment furnished our sub-chasers at the time of our entry into World War II. Our British allies, having pursued a parallel scientific course during the long armistice, possessed underwater sound equipment known as ASDIC, which was identical in principle and closely similar in detail to echo-ranging SONAR. The primary differences were: British asdic transducers consisted of sandwich arrangements of quartz and steel, whereas U. S. sonar transducers consisted of a battery of nickel tubes driving a steel plate by magnetostriction; the asdic protective domes were streamlined, whereas ours were spherical; and the asdic ranges were permanently recorded on a range recorder, whereas sonar ranges were indicated by a neon light flashing on a range-marked dial.
Of course the British bore the brunt of the U-boat war until December of 1941. During the first year of Blitzkrieg (through December 1940) some 1200 vessels, an average of about 80 per month, were sunk by the German submarines. During 1941 another 1118 ships had been sunk, averaging 93 per month. Many of these sinkings occurred within sight of our own coasts.
The primary answer to the destructive assaults of the U-boats was the construction of ever more escort vessels to protect our shipping, but there was no assurance that the escort program would succeed unless the effectiveness of the sonar equipment could be improved as rapidly as the Germans learned new tactics of evasion. The ships began coming in a steady stream from the shipyards, but the ideas for new equipment required time for development.
The answer to the scientific problem, if there could be an answer in time, was the formation of Division 6 of the National Defense Research Committee, operating under the Office of Scientific Research and Development. This Division rapidly and efficiently organized some of the best scientific talent in the country into new laboratories devoted to solving the technical problems posed by sub-surface warfare. Buildings were hastily thrown up and stocked with laboratory apparatus, shops were speedily equipped, hundreds of scientists were assembled from the universities and from industry, and their families were moved overnight to the chosen centers. The gymnasium of Harvard University, honeycombed with equipment, became the Harvard Underwater Sound Laboratory. Columbia University Division of War Research sponsored several groups, including the Underwater Sound Reference Laboratories at Mountain Lake?, New Jersey, and Orlando, Florida, the Sonar Analysis Group in New York, the Field Engineering Group, and the Columbia Laboratory at New London, Connecticut. On the west coast, the University of California Division of War Research operated a Laboratory at San Diego, devoted primarily to studies of sound transmission in water. In addition to these major laboratories, important groups of scientists at the Massachusetts Institute of Technology, the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography, and at a large number of university and industrial laboratories made important contributions, coordinated by the OSRD and by the Navy Department. The sonar stall of the Naval Research Laboratory was belatedly increased, and the staffs of other Naval laboratories in proportion. Thus the scientific problem of licking the U-boats was given to “the boys in the back room.”
From this point on, the story of sonar is one of mounting problems and ever more ingenious answers. In the grim days of spring, 1942, we were losing the Battle of the Atlantic with up to 143 ships going down in a month, and the U-boats were sinking ships faster than we could replace them. Our success in attacking the U-boats was less than five per cent. Ninety-five out of 100 echo-ranging contacts resulted in the submarine’s escape. Our equipment and our tactics were just not good enough. Close study of the contacts in which “the big one got away” was made by the Anti-Submarine Warfare Research Group (ASWORG) headed by Dr. P. M. Morse of M.I.T. This group, working through all fleet units and in close cooperation with the British, began to find operational attack answers. At critical moments sonar lost contact because the U-boats, hearing the pings growing louder, judged correctly the moment of attack and simply went deeper. Diving to 600 feet, if need be, they were below the sound beam of the attacking vessel. The U-boats also became adept at creating sound disturbances, backing down, turning sharply, ejecting bubble-making chemicals which sent back false echoes to the searching sonar. To counter these tactics a creeping attack was devised, whereby one destroyer using echo-ranging brought a second silent destroyer over the target by radio signals. The “bubble screens” no longer confused the sonar operators, who learned to distinguish fixed from moving targets by concentrating on the “Doppler” shift of pitch which occurs when sound bounces from a moving object.
New types of German torpedoes, some leaving no tell-tale white wake, some acoustically directed to “home” on the propeller sounds of the escort vessels, some weaving a looping path through the convoy under attack, challenged immediate scientific countermeasures. Towed “foxers,” or noisemaking devices trailed astern, decoyed the homing torpedoes away from the vulnerable propellers. Unescorted fast ships, equipped with hydrophone torpedo detecters, tried to outrun the U-boats, and one merchantman is credited with ramming a submarine after the torpedo had been dodged.
Faster sinking, stern-dropped depth charges, a result of hydrodynamics research and calculation, reduced the critical seconds in which the U-boat could get away. The escorts were equipped with “hedge-hogs” and “mousetraps,” new devices to project a pattern of depth charges several hundred feet ahead, which permitted much more accurate placing of the “shots,” and increased the percentage of sonar contacts resulting in kills.
The sonar equipment itself was greatly improved, by the introduction of the “console stack,” which increased the convenience of manipulation by the sound operator, rendered some of the operations more nearly automatic, and included devices to give the operator a graphic picture of the situation at all times.
The returns on scientific research, on whirlwind procurement, manufacture, and installation began to come in when history’s greatest invasion Armada of about 1065 escorted vessels attacked the North Africa Coast in November, 1942, with a loss of only 23 ships. Grand Admiral Doenitz affirmed our growing anti-submarine power in his communique to the German High Command: “Every U-boat that could reach these waters within ten days (about 40) was mustered. Defense in African waters was very effective, and U-boat losses were correspondingly high.”
Entering 1943 with 800 ocean going vessels fitted with ASDIC-SONAR equipment, the tide of battle, averaging 131 lost ships per month in 1942, slowly began to turn. Land- based and escort carrier planes, equipped with radar and directed by shore and ship- controlled high frequency radio direction finders, searched the general areas where surfaced U-boats were spotted. If the U-boat submerged in order to shake off radar contact, the chase was taken up again under water by sonar, through the use of expendable radio sonic buoys dropped by parachute from the searching planes. This device, on reaching the water, automatically lowered a listening hydrophone, picked up the U-boat screw noises, and broadcast its findings to the hovering planes. The planes then patrolled the area, holding the contact until sonar equipped surface vessels, in special killer groups, could reach the scene.
By summer of 1943 an average of 120 U-boats were operating in the Atlantic, with packs often outnumbering the convoy escorts two to one. Pulling up from behind during the months of May, June, and July, we sank 108 U-boats, a fatal blow to the wolf packs from which they never fully recovered. By late summer of 1943 we had survived, to be victorious, the Stalingrad of the sea. We did this, in Admiral Doenitz’ words, “by superiority in the field of science.”
From this point on until the end of the war, the U-boats were on the defensive, doing ever less damage, and taking more and more care to avoid detection and operating submerged a greater proportion of their patrols. Detection by asdic and sonar became increasingly effective, and early in 1944 we began sinking more U-boats than they sank ships. By D-Day, the Battle of the Atlantic had been decisively won, and the sonar screen maintained was so tight that the crevices and rocks of the ocean bottom no longer afforded safe hiding to the U-boats. None of our ships was lost to German submarines during the critical days of the great Normandy invasion.
The final score of the terrific 72 months’ battle for control of the seas shows a cost for the Allies of 4773 merchant ships (21,141,000 gross tons) sunk by enemy action, and a cost to the Axis of 996 submarines sunk, and 221 more plus scores of midgets captured after enemy capitulations.
Although final conquest of the submarines was obtained by cooperation of many arms and many weapons, the echo-ranging sonar and asdic played the final role in the majority of the attacks, and without this equipment victory would hardly have been possible.
As the war progressed, the center of interest, so far as sonar is concerned, gradually shifted from the Atlantic to the Pacific, and the urgency in research activity shifted from anti-submarine to pro-submarine developments.
In the Pacific, the roles of submarine warfare were reversed. The Japanese, having but 70 submarines, used them for reconnaissance, for daily check-ups off Guadalcanal, for supplying outlying garrisons, and for the protection they might afford to their surface vessels. We, in turn, became the attackers, using the submarine as an aggressive instrument, with the pride of the fleet lying disabled at Pearl Harbor. The problem, therefore, was not that of spotting subsurface raiders, but of becoming silent and effective raiders ourselves. To this end, submarine science worked to reduce ship noise, and to increase the sensitivity and accuracy of listening equipment; for, cut off from radio contact, submarines became blind moles, probing the depths for Japanese fleet units, and later the tangled and mined approaches to Honshu itself in preparation for invasion.
Just as the submarine, when wholly submerged, can be detected only by echo-ranging sonar, so also the undersea craft can stalk its prey, communicate with its friends, identify its targets, and evade its attackers only by making full use of listening sonar. As a matter of fact, our submarines are equipped with both echo-ranging and listening sonar. The echo ranging gear is seldom used because of the danger of giving the enemy an audible warning of impending attack. Usually a single ping suffices to fix the target’s range just before the torpedoes start on their way. The listening sonar, on the other hand, is in constant use as the sole means, when fully submerged, of keeping in touch with what is going on. Each submarine has two types of listening sonar—the JK system for listening to super-audible, or in Navy lingo, supersonic frequencies; and the JP system for listening to the audible, or sonic frequencies.
More important than any specific sonar equipment is the trained team of men who operate the instruments, interpret the incoming data, and provide the information which the submarine commander must have in order to fight his ship, destroy the enemy, and return safely to port. The sonar team not only uses the equipment, but is itself part of the equipment, each man a special instrument for the duration of the attack. Each man, known to be temperamentally suited for the submarine service by having passed psychological tests devised by the San Diego Laboratory, is chosen for sonar aptitude through musical tests for hearing, pitch, and sound differentiation. His native aptitude is increased by special training at one of the Navy’s Fleet Sonar Schools (at San Diego and Key West), in addition to basic radio materiel training. He has had intensive practice, simulating actual conditions, with listening teachers, sound classification trainers, bearing and range recorder interpreters, and attack teachers, all special devices to give him in record time the multitude of skills he needs. He has had a graduate course at sea, using towed underwater targets and actual submarines.
The sonar team in their steel shell has at hand hydrographic charts showing the types of bottom over which they travel, for they may need to seek a hiding place over soft absorbing mud, dulling all sounds, rather than take chances on a hard sand bottom from which sounds are clearly reflected. They have maps showing areas where snapping shrimps are plentiful, making noise which mask the sounds of ships and pings. The team is prepared with the best which science, industry, and training, working hand in glove, can provide.
With such teams our submarines, patrolling thousands of miles from friendly shores, annihilated more than 2000 Japanese merchant ships, totalling 7,600,000 tons. They also sank a fair fraction of the 684 warships which Japan lost. So good was our equipment, and so skillful our crews, that this result was achieved with the loss, under operational conditions, of only 46 American submarines.
That the modern submarine is a formidable offensive weapon is amply proved by the magnificent record made by our submarines against the Japanese. The German U-boats, obsolescent though they were, chalked up a frightful toll (over 21 million tons) of Allied shipping before they were vanquished. The final outcome might well have been different had the Germans been able to introduce their new high-speed submarines at the height of the battle instead of at the very end. All previous submarines were essentially surface vessels with ability to submerge. The newest U-boats are true submersibles, equipped with schnorchels (breathing tubes) for diesel intake and exhaust while running beneath the surface. For detection of these vessels, radar is virtually useless, and for them sonar must search farther, faster, and deeper than ever before.
Measure and countermeasure have completed another cycle. With the ever-present threat of multiple Pearl Harbors, Secretary Forrestal’s words become a forceful warning: “A military research program literally may be the price of survival.”