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Imperial Japanese Navy Antisubmarine Escorts 1941-45

The Imperial Japanese Navy (IJN) placed little importance on dedicated anti-submarine warfare (ASW) capability. The IJN strategic emphasis was on decisive battles early in the war; thus dedicated ASW ships had no role. IJN first line destroyers carried almost no depth charges at all, and the few ASW escorts that were available only carried 12 to 18 depth charges. The lack of equipment was further complicated by poor tactics in responding to any attacks by US submarines. The IJN strategy saw no need to change early in the war as the US submarine fleet’s torpedoes were horribly ineffective. That changed in mid-1943 and the Japanese were completely unprepared. The IJN responded in November of 1943 by ordering newly dedicated ASW ships, but their arrival in 1944 was too little, too late. Ultimately, the IJN ASW ships proved to be most effective at rescuing the sailors of the ships they were supposed to protect.

This 48-page book contains two color paintings by Paul Wright along with a substantial description. The first painting is of Torpedo Boat IJN Hiyadori versus USS Amberjack which is also featured on the front cover. The second Paul Wright painting depicts Task Force 38 Avengers attacking a tanker while CD-35, an IJN Type C escort, tries in vain to fight back against the TBM Avengers. CD-35 took three bombs and ended up on the seafloor. I counted 21 tables, 40 black and white pictures. Paul Wright also contributes twelve color profiles along with a keyed cutaway color illustration of an IJN Number 1 Class (Type C) ASW Escort.

Mark Stille provides a revealing look at the evolution of the IJN ASW capabilities that occurred during World War II. The introduction covers the IJN ASW strategy, tactics, ship design, and weapons. A significant weakness for the ASW ships was the lack of capable depth charges. US Submarines could go down to 400 feet, but the IJN depth charge maximum setting was 295 feet. Complicating the IJN use of depth charges was the rather basic nature of their ASW sensors. The ‘meat’ of the book is a description of the individual ASW classes. Each class description includes a section on ‘Design and Construction’, a table detailing the individual ships and units they served with, ‘Armament and Modifications’, ‘War Service’, and a table of the class specifications. Most of the classes are also represented with color profiles.

Japanese Convoy Escort and ASW Capabilities

Between the world wars, however, ASW had progressed very little beyond its development at the end of World War I. One reason was the financially tightened circumstances in which most navies found themselves in the 1920s and 1930s. The slow progress was also related to the priorities most major naval establishments placed on the capital ship and to the misplaced confidence among these establishments that if need be, the tactics, technologies, and force structures of World War I could be reconstituted to defeat the submarine once again. The use of asdic, or sonar, as it was later called, had proved effective in detecting the direction if not the depth of submerged targets, but its various limitations had not been addressed in the years since its first appearance. Britain, which had pioneered the development of asdic, was so confident of its lead in this technology that it placed no great priority either on asdic’s radical improvement or on the perfection of those complementary elements of ASW—the convoy system, the escort vessel, and the patrol aircraft specifically dedicated to ASW—that had proven effective in World War I. During the interwar years, deficiencies in these tactics, technologies, and force structures were also manifested in the U.S. Navy, second only to Britain in its experience in ASW in World War I. The United States would learn the consequences of its neglect of these elements of ASW in the stunning losses it sustained during the German U-boat offensive along the American east coast in the spring of 1942.

If submarine warfare and ASW warfare had provided obvious evidence of major successes in World War I, amphibious warfare apparently had not. The most ambitious amphibious operation of the war, the landing at Gallipoli, had also been its most disastrous failure. It had, moreover, been one of the few amphibious operations undertaken in the face of determined opposition ashore (the assault on Zeebrugge being the other notable amphibious operation under such conditions). After the war, most military and naval establishments sifting the evidence of Gallipoli found it less a sorry record of mistakes that might be corrected by better planning, organization, and performance, than a confirmation of the difficulty, if not impossibility, of conducting a landing from the sea in the face of entrenched and presighted automatic weapons and artillery.

The Japanese navy was indifferent to the problem of protecting the nation’s shipping lanes. As early as the Russo-Japanese War, this neglect had led to several serious yet avoidable injuries to Japanese sea transport. In the period between the conflict with Russia and World War I, commerce protection was still neglected by the Japanese navy, since the two greatest future threats to merchant shipping, the submarine and the airplane, were still in a primitive stage of development. From the outset of World War I, of course, the submarine caused enormous havoc. Ultimately, however, it was the great gun duel at Jutland, not the ravages of the U-boat, that had the greater impact upon the thinking of the Japanese naval high command. Much the same would be said of the British and American navies. Yet, as major participants in convoy escort and ASW in World War I, Anglo-American efforts in these tasks during World War II were aided at least partly by institutional memory at the high command level of the requirements for convoy protection.

In contrast, Japanese understanding of the rigors of commerce protection against submarines was limited in time, distance, and practical experience. The singular exploits of a handful of Japanese destroyer men who served with the Special Service Squadron on convoy escort and antisubmarine patrol in the distant Mediterranean, 1917-18, had made little impression on the outlook of the Japanese Navy General Staff.

To be sure, the navy took several measures both during and immediately after World War I to keep itself informed on the techniques and technologies used by the Allies in defeating the German U-boats. To begin with, naval observers like Comdr. Suetsugu Nobumasa sent back reports on German U-boat warfare. Moreover, as part of a major study undertaken by a special research commission and by Japanese naval attachés in Europe, the general staff’s Intelligence Division amassed voluminous materials on Britain’s protection of its sea trade during the war. From these sources, the navy collected detailed information on the convoy system and the techniques of ASW, as well as on the British naval organization to deal with these problems, the last being the focus of a special report in 1922 drafted by Lt. Comdr. Niimi Masaichi of the Intelligence Department. After the war, based on the information gathered, the Naval Staff College held lectures and map exercises on commerce protection. But none of these studies and presentations did much to arouse interest within the navy as a whole. Even a memorial to the throne in 1929 by chief of staff Katō Kanji on the importance of protecting Japan’s sea lanes failed to find any resonance in Japanese naval policy. Moreover, Admiral Katō failed to back up his general recommendation with specific proposals that would alter the navy’s force structure to provide for greater commerce protection.

Thus, on a theoretical level, the Japanese navy acknowledged the problems of protecting Japan’s merchant shipping, but it failed to undertake any concrete measures that would make such protection effective. The reasons for this were several. To begin with, the restricted budgets in the 1920s and early 1930s (not unlike those of the U.S. and British navies) did not allow any deviation from the priority the navy had placed on the construction and modernizing of fleet units. Moreover, the failure of contemporary Japanese naval thinking to link commerce protection with the requirements of ASW derived considerably from the navy’s view of its own submarine force. Seeing that force as directed primarily against American fleet units, the Japanese naval high command apparently assumed that their opposites in the U.S. Navy harbored similar plans, as indeed they did, up until the beginning of the war.

Furthermore, while the navy considered protection of its vital sea lanes to be important, the geographic definition of which sea lanes were considered “vital” was quite limited. This limitation can best be understood by reference to the navy’s operational plans. For example, in case of war with the United States, pre- 1941 plans called only for commerce protection in the waters north of the Taiwan Straits and along the littoral of the northeast Asian continent. The China Sea and the waters of Micronesia were only provisionally included. With hindsight, the official history of Japanese escort operations points out that by so limiting the area of Japanese interest, the navy had virtually abandoned the idea of commerce protection in much of the ocean where the Pacific War would be fought.

Essentially, the navy saw commerce protection as an extension of the problem of providing for coastal defense of the Japanese home islands. Immediately prior to the London Treaty both the Navy General Staff and the Naval Affairs Department of the Navy Ministry had done some careful study of the navy’s needs for coastal defense. They had recommended building substantial light forces for this purpose, including escorts, submarine chasers, and land-based air groups. However, the restricted naval budgets of the treaty era aborted such plans.

Undoubtedly, however, the overriding consideration in limiting the construction, training, and equipment devoted to commerce protection was the navy’s absolute priority on the heavy ships with which the navy hoped to win the decisive surface battle. In combination with the interwar budgetary limitations, this fixation repeatedly set aside “lesser” considerations, such as the security of the nation’s maritime transport. Indeed, until the Pacific War, the navy viewed the safety of Japanese merchant shipping as a matter of coastal defense, which the navy assumed would be indirectly guaranteed by Japanese battle forces in any event. The 1928 Nomura Report gave a clear priority to heavy warship construction and, with the exception of a few special units, left the matter of coastal defense to a few overage warships still in service. In the 1936 Imperial Defense Policy, references to the creation of an escort force for Japanese shipping were vague, feeble, and unrealistic: Preparations were to be limited in peacetime to the creation of a few core units; further study was to be made of the kind of training and ships that would be required for protection of Japanese shipping; and, in the event of hostilities, the navy would somehow undertake the rapid and wholesale construction of escort vessels as they were needed. The 1941 operational plan, for its part, did not make specific reference to the preparations necessary for commerce protection, these being subsumed under the problem of the general defense of the home islands. In the plan, sea communications in this rear area would be protected by first-line sea and air forces. Direct protection of coastal areas would be left to elements allotted from the navy’s slender defense units, to wartime construction, and if necessary, to fleet units detached from first-line forces. The view of most Japanese naval officers reflected this official disdain for commerce protection. They had little understanding or experience in either convoy escort or antisubmarine operations and, trained as they were in the principles of decisive fleet action, had no interest in learning what seemed a marginal and unrewarding trade.

At the highest levels of the navy there was Olympian complacency concerning the safety of Japanese shipping in the event of war. Civilian leaders were far less confident. In the summer of 1941, the Cabinet Planning Board had warned of dire shipping losses that might well exceed Japan’s capacity to replace them. In important meetings that autumn, civilian cabinet members had tried to raise the issues of convoy protection, shipping safety, and the American submarine threat with chief of the general staff Adm. Nagano Osami. On such occasions, Nagano either replied with utter assurance about the navy’s ability “to bring American submarines under control” or, under the pretext of national security, refused to discuss the navy’s preparations to deal with these problems.

Thus, on the eve of the Pacific War, the Japanese navy had given almost no thought to the possibility of a submarine campaign in its rear areas, and commerce protection, specifically including convoy escort, was a mission to be improvised as the need arose. These deficiencies in outlook would have been serious enough had Japan been planning to fight a defensive war within its home waters. But with the plans emerging in 1941 for a campaign of territorial aggrandizement, the potential difficulties for commerce protection increased enormously. The strategic rationale for the conquest of Southeast Asia was based on Japan’s acquisition of the region’s vast resources. This being the case, the occupation of the region would require not only the transport of sufficient military forces to bring it under Japanese control, but also forces for the defense of the area and the ships carrying strategic materials back to Japan. Both these operations, moreover, would require attention to the problem of escorts, as well as to the problem of shipping.

No aspect of this twin logistical-naval problem was more important than tanker tonnage and the means to protect it, but only a few Japanese naval officers seem to have grappled with these issues. If his memoirs are correct, Comdr. Ōi Atsushi was one of those who did. In the autumn of 1939, Ōi, then a lieutenant commander and a junior member of the Intelligence Division, studied the convoy system resurrected by the British navy at the outset of the war. He lacked the influence, however, to have his enthusiasm for it converted into concrete measures. Two years later, in an informal meeting, a handful of middle-echelon officers, including Ōi and several civilian petroleum specialists, met to discuss the problem of Japanese access to the oil of the Netherlands East Indies. Ōi argued that the issue was not getting the oil out of the Indies, but getting it back to Japan. Pointing to the threat of British and American oceangoing submarines, Ōi noted the unwarranted complacency of the Navy General Staff and Navy Ministry concerning convoy escort in the face of this potential danger. Ōi’s views led to an extended discussion of the number of tankers available for regular transport of East Indies oil back to Japan, the amount of naval strength sufficient for convoy escort for such shipping, and the expected losses of tankers to enemy submarines. Lacking an official forum, however, neither the views nor the concerns expressed by the participants in the meeting ever reached higher levels to affect policy. Thus, a month before the war, when the emperor queried Admiral Nagano about the ability of Japan “to obtain and transport oil without hindrance when faced with attacks by planes and submarines based in Australia” and what measures the navy would take to deal with the problem, Nagano was still unshaken in his complacency.

Given the general neglect of the problem of commerce protection by the Japanese navy, it is not surprising that on the eve of the Pacific War, there existed very little in the way of organization, naval units, weapons, tactics, or training to deal with the tasks of convoy escort and ASW. To begin with, no organization within the naval high command had exclusive responsibility of planning for these missions. Such responsibility that existed was the concern of a single individual, and not his only concern at that. In the Second Section (Defense Planning) of the Operations Division of the Navy General Staff (much less glorified than the First Section, whose ten members dealt with the heady operational plans for the Combined Fleet), three or four officers were charged with an assortment of planning duties, including those for “rear area” defense. This last was the responsibility of a single officer and was considered to include, inter alia, the task of commerce protection. In 1941, with the imminent possibility of war, rear area defense was assigned to two staff officers. Yet only one of them had responsibility for commerce protection, and until October of that year, he held the concurrent position of aide-de-camp to the emperor.

The actual direction of convoy escort and ASW was assigned to the naval districts, which were responsible for Japan’s coastal defense. Not only did the district staff officers command few resources for this considerable task or even have much understanding of it, but also, by precedent and inclination, they were far more concerned with accommodating the tactical and logistical demands of the Combined Fleet. Before the Pacific War, therefore, there was no major line command for commerce protection and none was established until the war was four months old. Even then, the quality and quantity of these forces were completely inadequate for the task at hand, and Combined Fleet headquarters became exasperated with having to detach destroyers and other fleet units for convoy escort. This would change only with the establishment of a full-fledged escort force in 1943.

The marginal attention given by the navy to protection of sea transport was reflected in the quantity of warship construction devoted to it. Just before the London Treaty, the navy briefly considered building a sizable force of specialized craft for commerce protection, but budgetary considerations had aborted the plan. The meager result of this brief interest in escort ships was the building of four kaibokan (coastal defense vessels) that had originally been designed for fishery protection and security tasks in Japan’s Kurile Islands. Proposed under the Circle One program and then again under Circle Two, the four ships of this Shimushu class were finally authorized under the Circle Three program when the navy concluded that they could fill the role of a general-purpose escort able to perform several coastal defense duties, including minelaying, minesweeping, and antisubmarine patrols. But the low priority that the navy assigned to commerce protection was reflected not only in the navy’s originally laying down no more than four ships of this class, but also in the ships’ reduction from a planned displacement of 1,200 tons to an actual 860 tons. The ships were reduced because a portion of the funds set aside to build them was used instead to help defray the costs of the Yamato and Musashi. These Shimushu vessels were the closest thing the Japanese navy had to the British and American destroyer escorts. Sufficiently satisfied with the Shimushu class, the navy ordered fourteen more ships of an improved Shimushu design (the Etorofu class), but they took so long to construct, because of stringent naval construction standards, that none was completed when the Pacific War broke out. The kaibokan were well designed and sturdily built to withstand the enormous stresses of the North Pacific. But their modest armament, few depth charges (none of which could be thrown forward), and their speed of less than 20 knots (slightly lower than the surface speed of the average U.S. fleet submarine) made these ships less than ideal as antisubmarine vessels. Ultimately, the most serious failure of the kaibokan effort was that not enough were built.

In addition to building the kaibokan, the Japanese navy ordered the construction of vessels that, although constructed for other purposes, could serve as escorts (such as the minelayer-cruiser Okinoshima). Further, the navy converted several torpedo boats and older destroyers for escort work. Beginning in 1931, the navy also ordered a few submarine chasers averaging 250 tons and carrying two 40-millimeter antiaircraft guns and thirty-six depth charges. Finally, it commissioned about seventy-five auxiliary submarine chasers averaging 130 tons. But the auxiliary submarine chasers were only good for harbor and coastal patrol, because their 11-knot speed, feeble armament, and low freeboard made them ineffective for their intended purpose. Destroyers, of course, were the most formidable warships for both convoy escort and ASW, but the priorities of the Japanese navy made it reluctant to release fleet units for either of these missions. In any event, the number of ships assigned escort duty was wholly inadequate for the defense of Japanese shipping.

Not only was the quantity of these vessels wholly inadequate for their assigned missions, but their weapons and equipment were largely ineffective for such tasks. They were equipped with no forward-throwing ASW weapons, and their main armament was usually inferior to the larger deck guns of enemy submarines. Their depth charges were frequently set too shallow because faulty intelligence had underestimated the maximum diving depth of American submarines.

In underwater detection, the Japanese navy had not greatly progressed beyond the technology developed during and immediately after World War I, much of which had been acquired from Britain and Germany. The navy had undertaken research on hydrophones in the 1920s and, in 1930, had imported the American MV-type hydrophone, from which it developed its own hydrophones, the type 93 and type 0. The former was the navy’s standard underwater detection device throughout the Pacific War, despite its limited range of 1,000 yards. In the field of sound ranging—sonar—the Japanese had made some progress when the war broke out; about twenty destroyers had been outfitted with type 93 sonar. Development of sonar continued throughout the war, but the pace of the program was slow, and Japanese sets remained rudimentary.

Detection of submarines by interception of their radio communications offered promise. The Japanese navy had some experience in tracking American naval maneuvers through radio intercepts since the early 1930s. The slow collation of bearings, however, made the navy’s otherwise efficient high-frequency-radio direction-finding (HF/DF) system of little use for the kind of quick response demanded by convoy escort and ASW operations in the Pacific War.

In sum, the Japanese navy was ill prepared, in outlook, doctrine, training, ships, and equipment to deal with either commerce protection or ASW. The official history of the navy’s maritime protection operations during the war recapitulates the reasons for this failure. First, the navy failed to foresee the protracted nature of the war and that it would inevitably involve the protection of sea transport over a vast area. Second, the navy’s fixation on the decisive surface battle monopolized its armaments, its preparations, and its training, and left it blind to the possibility that destruction of the nation’s commercial shipping might also be decisive. Third, the British and American advocacy of limitations on submarines at the London Naval Conference left the Japanese navy with the impression that submarines were a weak point in the U.S. Navy’s force structure. This led to the fatal conclusion that the possibility of an American submarine campaign against Japan was slight. Last, the defensive character of both convoy escort and ASW, and the tiresome, repetitive duty that they entailed, gave them short shrift in a navy whose traditions had held that offense was the best defense.

Forschungsmitarbeiter Mitch Williamson is a technical writer with an interest in military and naval affairs. He has published articles in Cross & Cockade International and Wartime magazines. He was research associate for the Bio-history Cross in the Sky, a book about Charles ‘Moth’ Eaton’s career, in collaboration with the flier’s son, Dr Charles S. Eaton. He also assisted in picture research for John Burton’s Fortnight of Infamy. Mitch is now publishing on the WWW various specialist websites combined with custom website design work. He enjoys working and supporting his local C3 Church. “Curate and Compile“
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