Section III
Japanese Anti-Submarine Warfare and Weapons

3-1. Much interesting and hitherto unknown information on the Japanese anti-submarine warfare effort was gathered at the end of World War II by representatives of the U.S. Naval Technical Mission to Japan.¹ The most striking feature commented upon was the enormous gulf between the Japanese conceptions of such warfare and their actual accomplishments. The Japanese high command had given consideration to almost every phase of advanced anti-submarine warfare. Much thought and research effort was directed towards the development of echo-ranging equipment, influence and acoustic proximity-fuzed depth charges, deep-setting depth charge pistols, chemical recorders, ultra high frequency voice radio, surface search radar for escort ships and aircraft, magnetic detection equipment for aircraft, ahead-thrown rockets and standardized and improved doctrine for surface and joint air-surface offensive action. Satisfactory solutions to many of these problems were obtained and limited production of equipment and operational training were undertaken.

3-2. Nevertheless, at the end of the war Japanese anti-submarine warfare was still being waged without appreciable benefit from the results of their research effort. Lack of production facilities, inability to maintain adequate supply lines, and the necessarily large allocation of production to aircraft for the final defense of the Empire home islands greatly affected the anti-submarine forces, but there also existed a failure on the part of operating personnel to use what they did have to best advantage.

3-3. Although the importance of use of radar to detect submarines was recognized, many escorts were never fitted with surface search radar due to lack of equipment.² Even those escorts which had been equipped with search radar and improved echo-ranging devices were in most cases unable to obtain satisfactory results, due to defective equipment and poor training of maintenance and operating personnel. However, both combatant ships and escorts were generally provided quite early in the war with effective directional equipment to detect radar being used by U.S. submarines.²

3-4. The value of aircraft radar in anti-submarine work, both for search and blind attack, was also well recognized.Here again, their equipment was decidedly inferior and pilots generally were either poorly trained or lacked confidence in its use and are reported to have

had but little success in radar bombing attacks. Airborne radar was first used in medium bombers as early as September 1943, but there was no large-scale employment of radar-equipped planes for anti-submarine work until the fall of 1944. Their equipment was reported to have been capable of detecting a surfaced submarine at a range of twelve miles. However, Japanese doctrine called for use of airborne radar only during night or low visibility conditions because visual search was still considered more reliable. Towards the end of the war, Japanese radar-equipped planes were making numerous contacts with our submarines, but few attacks resulted. In 1945, a few anti-submarine aircraft were also provided with radar detection receivers, but the Japanese professed to have never reached the stage of homing on U.S. submarine radar. However, war patrol reports indicate that at least some enemy pilots achieved moderate success in using such equipment for initially detecting the presence of our submarines. Precise locating was probably then accomplished with aircraft search radar rather than radar detection equipment.

3-5. By late in 1943, the Japanese had successfully developed a magnetic airborne detector (MAD) and, commencing in March 1944, this equipment was put into operational use by both the Army and Navy for anti-submarine patrol.³ Range of detection was reported to have been about 120 meters under average conditions and about 250 meters was claimed under ideal conditions. Since expert pilots flew magnetic search planes only thirty to forty feet above the surface, the apparatus was therefore presumably capable of detecting a submarine at well over 300 feet submergence. Aircraft equipped with MAD were employed principally to search ahead of convoys or to exploit a submarine contact made by other means. Although it was planned to use such aircraft to sweep all heavily travelled convoy routes, lack of both aircraft and MAD equipment prevented this. The instrument was considered sufficiently reliable to warrant calling in surface craft whenever an initial contact had been established. The types of planes normally used for anti-submarine work, and equipped with MAD and/or radar when possible, were NELL, JAKE, KATE, JILL, DAVE, BETTY, ZEKE and EMILY.⁴ By the end of the war, only about one-third of the shore based anti-submarine planes had MAD, about one-third had radar, and only a very few were equipped with both. Anti-submarine planes were very seldom fitted with guns, which accounts for the low incidence of strafing attacks against U.S. submarines.

3-6. Only one small land-based "hunter-killer" air-surface group existed, and even this was not organized until early 1945.⁵ The group covered the East China Sea between Formosa and Shanghai and was comprised of five surface ships (DE types) known as the 102nd Surface Squadron and about 20 Navy fighters (ZEKES) of the 934th Squadron,

Shanghai Air Force. These planes were equipped with both radar and magnetic airborne detectors. For routine patrol, the planes usually carried two 60 Kg. depth bombs and, when sent out to attack a previously detected submarine, carried one 250 Kg. depth bomb, although the 250 Kg. size depth bombs were also sometimes carried on patrol. Both the planes and surface escorts were provided with voice radio but could communicate only for short distances and frequently were unable to communicate at all due to technical difficulties.

3-7. The Japanese repeatedly demonstrated facility in detecting and locating submerged submarines by using sonar. Their underwater listening gear was fairly good and could frequently pick up our submarines at ranges of 2,000 meters or more when the listening ship was stopped or proceeding at very slow speed. A damaged submarine with a high machinery noise level, such as a reduction gear click, could of course be heard at a far greater distance than an undamaged submarine running silent or creeping. Their echo-ranging equipment was of mediocre design, roughly corresponding to the early U.S. QC sonar of about 1937. The emphasis which the Japanese placed on sonar devices is clearly shown by the unnecessarily large amount of space allocated to such equipment in the already cramped quarters of their ships. At sea it was customary to man the hydrophone set continuously and to operate the echo-ranging gear at least fifteen minutes in every hour. Small escorts were generally provided only with listening gear, in some instances just a crude hydrophone lowered over the side. Japanese scientists were cognizant of density layers and temperature gradients in sea water and their effect on sound transmissions in echo-ranging detection work. However, anti-submarine vessels were not equipped with bathythermographs and no tactical use was apparently made of the small amount of information of operational value issued by the Japanese Hydrographic Office.

3-8. The depth charge was the primary Japanese surface ship anti-submarine weapon. Depth charges were placed aboard practically every type of ship capable of carrying them and even the smallest of patrol craft could generally be expected to have a few. Very slow craft used parachutes to reduce the sinking rate of their charges so as to reach a safe distance from the detonations. The Japanese used depth charge throwers extensively and even merchant ships were occasionally equipped with these. Fleet destroyers generally carried about 30 depth charges. Frigates (Kaikoban) could carry as many as 300 and were usually fitted with twelve single depth charge throwers, six to a side, and one stern rack. The Japanese PC-13 Class, by far the most numerous of their PC types, carried about 36 charges with two side throwers and one stern rack. Gun armament for the PC-13 Class consisted of one 8 cm. dual purpose and one twin 13mm machine gun.

3-9.There were only two types of depth charges in general use by the Japanese for surface ship launchings and both were almost exact copies of early British models of obsolete design. These were the Type 95, the Type 2, and the various modifications of each.⁶

The Type 95 was the regular issue depth charge until the development of the Type 2. Although the Type 2 charge was adopted in 1942, Type 95 charges are known to have still be manufactured up to 1943. Both charges were probably in use during 1943 until available supplies of the Type 95 were finally exhausted. The Type 95 depth charge was cylindrical in shape, 17.75 inches in diameter and 30.5 inches long. The Type 95, Mod. 0, was filled with 220 pounds of Type 88⁷ explosive and a Shimose⁸ booster; the Mod. 1 with 325 pounds of Type 97⁹ or Type 98¹⁰ explosive; and the Mod. 2 with 242 pounds on Type 1¹¹ explosive. The Type 2 depth charge was also cylindrical in shape, 17.56 inches in diameter and 30.5 inches long.The Type 2, Mod. 1, was filled with 357 pounds of Type 97 or Type 98 explosive and the Mod. 2 with 242 pounds of Type 1 explosive.

3-10. The pistols for both the Type 95 and type 2 depth charges were also almost exact copies of early British models and both operated on the same depth setting and firing principle, although they were not interchangeable between the two types of charges.¹² The firing mechanism of both pistols was actuated by the hydrostatic pressure of sea water flowing through a small orifice and slowly filling up an inner cylinder. Depth setting was accomplished by varying the size of the water inlet, thus determining the amount of time required to fill the inner cylinder. The smaller the inlet, the deeper the setting. The Type 95 depth charge pistol used early in the war could be set for operation only at depths of 98 feet, 98 feet with parachute, 197 feet, and "Safe". Later modification of this pistol provided for an additional setting of 292 feet. The maximum range of depth settings for the Type 2 depth charge pistol was much greater, a choice being available of 98 feet, 197 feet, 292 feet, 390 feet, 480 feet, and "Safe". It was apparently not possible with either type pistol to choose depth settings other than those enumerated above, even though detonation at some intermediate depth might be considered desirable.

3-11. No data are available as to how much variation could normally be expected between the prescribed depth settings on the pistols and the depths at which charge detonation would actually occur. The amount of error would naturally be affected by variations in sinking rates due to the following factors: (a) the method of launching, i.e., whether the charges were projected from throwers or dropped from stern racks; and (b) by the condition of the sea, i.e., whether still or disturbed. The accuracy

with which the orifices in the pistols were initially machined would also affect the detonation depth. Tests conducted by the British on their early pistols indicated that discrepancies of as much as 60 feet should be expected for charges set for detonation at 250 foot depth. It should be noted that even though a Japanese depth charge were to come to rest in water more shallow than that for which its pistol had been set, it would still fire when sufficient water had seeped through the entry orifice to provide the minimum pressure required. This is believed to account for many of the delayed depth charge detonations which have been reported by U.S. submarines.

3-12. The maximum depth setting possible with the Type 95 depth charge is believed to have been 292 feet and the maximum setting normally used for the Type 2 charge was 390 feet. A proportionate number of charges with these maximum settings were usually included in each attack pattern. Our submarines reported only a few cases where depth charges were set for depth in excess of 400 feet.¹³ As the Japanese were unable to estimate the depth of a submerged target, no specific technique was developed for attacking submarines that took refuge in deep submergence. The only deep setting depth charge pistol developed was the Type 3, Models 1 and 2, designed for operation at 131, 262,393, 524 and 656 feet.¹⁴ The design of this pistol was similar to that used in both the Type 95 and Type 2 depth charges, but the extra time required to reach the greater depth was provided by a delay train initiated by the firing pin. However, issue to ships had not become general by the end of the war and no specific doctrine for its use had been developed.

3-13. Although the need for proximity-fuzed depth charges seems to have been appreciated, none were developed in time for operational use during the war. The Japanese Type 4 depth charge, designed for magnetic actuation, had been developed, but trials on it were still in progress when the war ended.¹⁴ An acoustic type depth charge was also scheduled for development but work on this did not proceed beyond the preliminary design stage and no prototypes were made.

3-14. At least two types of anti-submarine explosive weapons designed for underwater tow by small surface vessels are known to have been developed and used by the Japanese, particularly early in the war. The first of these was called the "Yokosuka depth charge," and was set to explode upon contact with a submerged object. The charge was cylindrical in shape, about 1 foot in diameter by 5 feet in length, and contained 55 pounds of Type 88 explosive filler.¹⁵ The

second weapon was designated by the Japanese as the Mark 2 Explosive Hook and, although developed primarily for minesweeping, was occasionally used for anti-submarine work. This device was a cast iron cylinder, 8 inches in diameter and ten inches long with four grapnel-like arms projecting from the main body, each 7¹/2 inches long. The body contained a charge varying from 8 to 19 pounds of Type 88 explosive.¹⁶ Firing was accomplished after the hook secured on a submerged target; the Mod. 0 either electrically by an observer on the towing ship or automatically when an additional tension of 550 pounds was put on the towing line, and the Mod. 1 by electrical control from the towing ship. Although several reports were received from U.S. submarines of small Japanese vessels apparently using these two weapons, no large-scale employment was made and there is no information to indicated that damage was ever inflicted.¹⁷

3-15. Three sizes of aerial depth bombs were developed by the Japanese Navy specifically for anti-submarine work and were used for this purpose by both Army and Navy aircraft.¹⁸ These were designated by the Navy as Mark 2 bombs and by the Army as Type 4 bombs. They were fitted with anti-ricochet nose rings and had considerably higher explosive-loading factors than ordinary general purpose bombs. The smallest of the three was a 60 Kg. bomb, designated by the Navy as the Type 99, No. 6, Mk. 2, Mods. 0 and 1, and was loaded with 85 pounds of Type 98 explosive. These small bombs were carried primarily by planes on routine anti-submarine patrol and were considered to have a lethal range of 12 to 15 feet. The second was a 180 Kg. bomb designated as the Experimental 19, No. 25, Mk. 2, loaded with 308 pounds of Type 98 explosive. This bomb was developed primarily for use in conjunction with planes equipped with magnetic airborne detectors and was considered to have a lethal range of about 25 feet. The third and largest was a 250

Kg. bomb designated as the Type 1, No. 25, Mk. 2, Model 1, Mods. 0 and 1, and was filled with 317 pounds of Type 98 explosive. This 250 Kg. bomb was considered to have a lethal range of about 33 feet and was generally only loaded on aircraft when a definite contact had been established by other planes or surface craft.

3-16. All three of the above Mark 2 depth bombs carried time rather than hydrostatic fuzes, and depth settings could only be varied by changing the fuze gaines prior to loading the bombs on aircraft. Four different fuze gaines with different time delays were used for anti-submarine work, each of which could be used with all standard Navy bomb fuzes, which in turn would fit any of the three Mark 2 bombs. The first gaine, the only variable setting design adopted by the Japanese, was designated as the Type 15 and could be set for detonation at any desired delay between 0 and 1.5 second after striking the surface of the water, equivalent to between 0 and about 50 foot depth.¹⁹ The desired setting had to be selected prior to insertion of the gaine in the bomb fuze. Although the Type 15 gaine was not developed specifically for anti-submarine bombs, it is known to have been used extensively for the purpose of attacking surfaced or submerging submarines. Each of the other three fuze gaines had a pre-fixed time delay and all three were designated as the Type 1, Mark 2 design. The Model 5 was set for detonation after 3.5 second delay, about 80 foot depth; the Model 1A after 6 seconds delay, about 150 foot depth; and the Model 1B after 17 seconds delay, about 300 foot depth.¹⁹ The Japanese referred to bombs fitted with the Models 5 and 1A gaines as "25 meter" and "45 meter" bombs and these were the types generally carried, although the gaines were naturally varied to suit the expected conditions.

3-17. When anti-submarine Mark 2 Navy bombs were not available, the Japanese substituted common types of 60 Kg. and 250 Kg. Navy GP bombs or Army 50 Kg., 100 Kg., and 250 Kg., Type 3 bombs. Army aircraft frequently carried a mixed load of their own Type 3 bombs along with Navy Mark 2 bombs, the Type 3 bombs being fuzed instantaneously, with short delays, or with a nose plug and a special 3.5 second delay tail fuze.

3-18. The Japanese directed considerable effort toward the development of gun projectiles for anti-submarine work which would continue an undisturbed trajectory after striking the water. After extensive tests in 1943, a flat-nosed projectile was adopted in which the area of the flat front was equal to half the area of the base. Projectiles of this non-ricochet type were produced for guns up to and including 8-inch.

3-19. Many U.S. submarines reported hearing small "sono-bomb" or "light" explosions while submerged and in contact with Japanese surface units. Salmon (SS-182) likened the noise of such successive explosions to a "string of fire-crackers". The "sono-bombs" are believed to have been the Japanese Mark 3 and Mark 4 "Emit Sound Missiles" (HATSUONTO). This device consisted of a can 8 inches long and 5 inches in diameter, filled with Type 88 explosive. When dropped

into water, a small quantity of sodium under the top cap explodes, blowing the cap off and operating a pull igniter. Then after a short delay, the explosive charge detonates. These missiles were employed by the Japanese for the training of their own submarine crews, to simulate depth charges in an attempt to frighten Allied submarines away,²⁰ and later were employed very successfully in sweeping U.S.-laid acoustic mine fields. Many of these "light" explosions heard just prior to depth charge detonations may also have been caused by the sound of depth charge throwers or possibly even gunfire in some cases.

3-20. Although mines were used by the Japanese as an anti-submarine measure and certainly constituted an appreciable hazard, such ming never became really extensive during any phase of the war, probably due to lack of mine-laying vessels.²¹ Both contact and controlled mine fields were laid in Empire coastal waters and harbors, the accesses to the Sea of Japan, the southern approaches to the Empire, Formosa and the Philippines. The most commonly used contact mine was the Type 93, containing 220 pounds of Type 88 or Type 1 explosive.²² This was a moored mine and was designed to arm only with tension on the mooring spindle, a drifting Type 93 mine therefore being disarmed except for the cases where the arming mechanism was locked closed by corrosion. The most commonly used controlled mine was the Type 92, containing 1,100 pounds of Type 88 explosive.²² This was also a moored mine and could be either acoustically or magnetically monitored from shore or small vessels. The Japanese copied the design of two German magnetic and acoustic proximity-fuzed ground mines which were brought to Japan by a German vessel in September 1942 and produced their own adaptations by March 1944, both designed for planting by submarine. There is evidence that such mines were laid off Truk²³ and in one area off the east coast of Australia,²¹ but aside from these two locations there is no indication that proximity-fuzed mines were used anywhere else. The development late in the war of the QLA sonar mine detector made possible the penetration of enemy minefields by our submarines and permitted spectacular operations in areas long considered invulnerable by the Japanese, such as the Sea of Japan.

3-21. The greatest disparity between the weapons employed in the Japanese anti-submarine program and ours was their failure to develop an ahead-thrown contact weapon similar to our Mark 10 Projector ("Hedgehog"). Some development work was done on ahead-thrown rockets fired in salvos of five from "rocket guns" but results were not satisfactory and the project was abandoned. This was the only type of ahead-thrown anti-submarine weapon under consideration by the Japanese. The U.S. Mark 10 "Hedgehog" projectile is fuzed to detonate on contact and the charge size (33 pounds Torpex) is

such that rupture of the pressure hull is to be expected even when detonation occurs at the superstructure. The tactical problem is simplified, for usually contact is not lost prior to launching of a "Hedgehog" barrage and no interference with sound apparatus will occur until a hit is obtained. Probability studies based upon the various factors entering into an attack on a submerged submarine and statistical analysis of action reports indicate that a submarine's chance of surviving a depth charge barrage are at least four times as great as the probability of surviving a "Hedgehog" attack.²⁴

3-22. The Japanese failed to achieve optimum use of their principal anti-submarine weapon, the depth charge, simply by repeatedly setting their pistols too shallow. There are many instances reported by our submarines, especially during the first two years of the war, in which depth charge patterns detonated directly overhead without causing appreciable damage but where an increase in depth setting would probably have resulted in fatal or serious damage. Above all, the Japanese generally lacked persistence in both their search efforts and exploitation of a positive contact, once made. They were prone to accept the slightest evidence that a submarine had been destroyed and then depart. Many a U.S. submarine owes its escape to the Japanese predisposition to take the most optimistic view.

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Footnotes

1. NavTechJap Target Report, Index. No. S-24, of 8 February 1946 (Japanese Anti-Submarine Warfare).

2. Japanese anti-submarine escorts did not begin receiving radar surface search equipment in quantity until about September 1944. Previous to that date, very few escorts were equipped with search radar since most of the available production was allocated to combatant ships. Radar detection receivers were installed on most escorts by April 1944. U.S. Strategic Bombing Survey, Naval Analysis Division, Interrogation No. 61.

3. NavTechJap Target Report, Index No. E-14 (Japanese Magnetic Airborne Detector).

4. U.S. Strategic Bombing Survey, Naval Analysis Division, Interrogations Nos. 200 and 371.

5. NavTechJap Target Report, Index No. S-24 (Japanese Anti-Submarine Warfare).

6. Handbook of Japanese Explosive Ordnance, OpNav 30-3M of 15 August 1945 and Bureau of Ordnance Pamphlet 1507 of 20 April 1945.

7. Type 88 explosive -- 75% ammonium perchlorate, 16% ferro-silicon. More powerful that TNT. Power compares favorably with explosives containing aluminum.

8. Shimose -- Almost pure picric acid. Slightly more powerful than TNT.

9. Type 97 explosive -- 60% trinitrotoluene, 40% hexanitrodiphenylamine. Slightly less powerful than TNT.

10. Type 98 explosive -- 60% trinitroanisole, 40% hexanitrodiphenylamine. Power is approximately that of TNT.

11. Type 1 explosive -- 81% ammonium picrate, 16% aluminum. Power compares with that of Torpex or about one-third to one-half greater than TNT.

12. Handbook of Japanese Explosive Ordnance, OpNav 30-3M of 15 August 1945 and Bureau of Ordnance Pamphlet 1507 of 20 April 1945.

13. While being depth charged on 4 July 1944, during her fifth patrol, Seahorse (SS-304) reported that her DCDI indicated a few charges were definitely detonating below the ship even though her depth at that time was 470 feet. Threadfin (SS-410) reported that while being depth charged on 28 March 1945, during her second patrol, over half of the detonations occurred at depths greater than 450 feet as indicated by DCDI.

14. NavTechJap Target Report, Index No. O-08 of 16 December 1945 (Japanese Depth Charges).

15. Handbook of Japanese Explosive Ordnance, OpNav 30-3M of 15 August 1945.

16. Handbook of Japanese Explosive Ordnance, OpNav 30-3M of 15 August 1945.

17. On 8 September 1943, during her first patrol, Billfish (SS0286) sighted a Japanese convoy escorted by a small converted merchantman and a sampan, the latter dragging two cables over the stern. During the subsequent depth charge attack, a distinct scarping sound was heard down the port side of the hull while at a depth of about 300 feet. This probably was an explosive sweep. In at least two instances, U.S. submarines were caught by grapnels but managed to escape without damage. (a) While submerged off the entrance to Kwajalein on 9 July 1942, during her fourth war patrol, Thresher (SS-200) was hooked in her after superstructure by an unknown type of Japanese grapnel. It was reported that the pull of the grapnel line made the boat noticeably light aft. The grapnel was finally cleared, after ten minutes, by running at high speed with full right rudder and by increasing depth to 350 feet. (b) On her sixth war patrol, Crevalle (SS-291) was hooked in her periscope shears by a crude four-pronged grapnel. The grapnel line broke and the grapnel itself fell to the deck and was recovered upon surfacing.

18. Handbook of Japanese Explosive Ordnance, OpNav 30-3M of 15 August 1945 and NavTechJap Target Report, Index N. O-23 of December 1945 (Japanese Bombs).

19. The time delays given here are based on U.S. tests and do not agree with the figures contained in Japanese documents. The depths corresponding to these time delays have been computed for bombs dropped from an aircraft traveling at 100 knots at an elevation of 100 feet.

20. JICPOA captured Document No. 78745 records their use in this manner by a Japanese merchant ship and CinCPac-CinCPOA captured Item No. B-10202B also records such use by Japanese destroyers during the central action of the Battle for Leyte Gulf, 24-26 October 1944.

21. NavTechJap Target Report, Index No. O-05 (Japanese Naval Mining Organization and Operational Techniques).

22. Handbook of Japanese Explosive Ordnance, OpNav 30-3M of 15 August 1945.

23. Field Survey of Japanese Defenses at Truk, Part I, CinCPac-CinCPOA Bulletin 3-46 of 15 March 1946.

24. From Operations Evaluation Group Report No. 51 [Antisubmarine Warfare in World War II]. This applies only to depth charges having pistols with preset depth settings, such as those used by the Japanese, and not to proximity-fuzed depth charges. An attack made with the latter is theoretically only slightly less effective than a "Hedgehog" attack.