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Why did Japan wait until late 1943 to implement a central broad Antisubmarine Warfare (ASW) strategy for convoying merchant shipping with escort ships and where feasible, air cover? The Japanese Navy knew from 1939 the U-boat success with guerre de course especially against merchant ships sailing independently, yet did not act.

The Setting

Japan’s aggressive and successful early actions of December 1941 created within a few weeks greatly lengthened merchant ship trade routes covering distances up to 3000 miles from the homeland.

Within eight days of Pearl Harbor, the West Coast of Malaysia thousands of miles from Japan, was a destination for cargo ships supporting the Japanese invasion army. The next month Singapore fell, followed later by the Philippines. Other remote invasion points all required at-sea transport over long distances. In addition to significant activity south of the home islands, the long ongoing intrusion and exploitation in northern China and Manchuria also required continuous sea transport although the distances were shorter.

On December 7, 1941, the Japanese merchant fleet stood at more than six million tons. At war, the burden of this fleet would include both the Japanese Army and Navy. Further, the fleet addressed Japan’s extensive import requirements for her population as well as the huge demand for raw materials to meet extensive armament production and other industrial needs. A 20th century island, Japan survived on imports.

Size of the Japanese Merchant Fleet

1217/41 6,384,000 tons
8/14/45 1,465,900 tons

Accounting for the huge loss in Japanese shipping. foremost was the increasing effectiveness and skill of the United States submarine fleet growing and improving during each year of the war. The number of United States submarines in the Pacific Theater went from 47 in 1941 to 104 in February 1943 and 169 at the end of the war in 1945. United States ships. planes and submarines had the advantage of newly-developed sonar and radar systems. Japan’s military technology development and fleet implementation lagged that of the United States by four years.

Further consideration of the demise of the Japanese merchant fleet brings out other factors. The Japanese cult of the naval offensive made merchant ship convoying appear as a defensive role not in keeping with a Samurai’s view of fighting on the sea. Among some naval officers, ASW study and research fell into the category of only common sense.

It is not clear. why the 20th century Japanese Navy with its strong ties to British naval tradition, practices and strategy was not observant of Britain’s success with merchant ship convoying during the last years of WWI. There is no strong evidence that convoying was an important consideration in Japan’s inter-war years of naval planning.

Examination of the ASW state of readiness of Japan in late 1941 indicates ignorance of or disinterest in the heavy loss of merchant ships by Britain and others due to the improved U-boats during the first several years of WWII. Moreover. appreciation that air and sea convoy escorting of merchant ships at least moderated the losses seems to have gone unnoticed. Japan did not mount a significant focused merchant ship convoy effort until October 1943.

Before December 1941

The origins of modem Japanese naval heritage are from the successful Sino-Japanese War of 1894-5 and the Russo-Japanese War 1904-05. In both wars, success at sea came from the two Mahanian like clashes of fleet-versus-fleet with the Battle of the Yalu in the former and the Battle of Tsushima in the latter. At that time, the industrial needs of Japan were primarily agricultural and did not demand extensive seaborne support. Japan was not unique in its naval tradition of at sea-encounters with enemy battle fleets and the consequent large budgets for battleships and supporting craft.

After the Russo-Japanese War, the Army and the Navy began to diverge gradually in their perception of national objectives. The Army opted for a continental direction to the west of Japan on mainland Asia for expansion while the Navy inclined southward in the direction of oil and rubber resources. In the years ahead, this division took a toll in national preparedness, reduced inter-service cooperation, effective expenditure of resources, and, ultimately, in a rivalry for fiscal support.

Japan sided with England in WWI in accordance with an existing treaty and declared war against Germany 23 August 1914. Japan’s role involved occupation of the Marshall and Caroline archipelagos and capture of Germany’s Chinese port of Tsingtao in November 1914. By 1918, Japan’s destroyers were part of the extensive allied armada of support vessels in European waters in the successful convoy opposition to the U-boats.

As the fires of World War I abated in the late fall of 1918 with the armistice, attention turned to peace making and keeping. The new and hard won skills of ASW and the successful protection of merchant shipping by convoying with sea and air escorts were put aside and to some extent forgotten by the primary maritime nations. Awareness of the infrequent use of highly touted battleships by both sides during the almost five years of WWI dominated by the U-boat was forgotten. The concept of control of the sea with final decision based on the clash of great battle fleets again assumed its pre-World War I prominence among the primary powers of England, United States, France, Italy, and Japan. The battleship with its attendant high cost, long-term building requirements, manpower demands and support requirements was the weapon of choice.

In the 1920s and during the international depression period of the 1930s, economics began to play a more significant role in the restrained defense budgets of the primary maritime powers. In Japan, the actual ruling government power divided among the Army, Navy, and the premier’s cabinet with the Anny in the dominant position. Further, the potential enemies were Russia, China, and United States. The Army with a strong position and military needs directed toward China and Russia in Asia met its funding needs at the expense of Navy support. With limited fiscal means and the United States as its anticipated enemy, naval strategy focused on battle groups and the decisive at sea battles. This strategy obscured development of adequate wartime sea and air escort capabilities for shipping protection during armed conflict.

Smaller allocations insured continuing competition between navy and army priorities, and additional increasing attention to air power provided another factor in dividing the limited defense budgets.

Early in the 1930s, Japanese naval planning included ample recommendations for ships, boats, subchasers, air cover and wartime backup. Considerations were directed towards the need for better ASW and conversion of merchant escorts in time of war. There were other Navy voices that held opposing opinions which, when considering the U.S. as an enemy, held to the belief that enemy submarines like their own would not adopt the tactic of guerre de course. Budgetary restraints and lack of support prevented implementation of ASW-related developments.

In September 1940, Japan impressed by the Axis victories in Western Europe including the fall of France joined the Axis powers. Germany’s early 1941 success in the invasion of Soviet Russia triggered Japan’s excursions in southern Asia. On July 26, Japan occupied all of French Indochina with ensuing events leading to December 7′ s strike at Pearl Harbor.

Major Y. Horie, former member of the Imperial Japanese Army, provides some perspective regarding a Japanese view of convoying merchant ships. Horie spent most of the war years (World War II) with the Japanese Navy primarily concerned with the transportation of troops and materiel in his assignment with the Convoy Escort Fleet from its beginnings to its final days. Horie noted, “I found that Japanese high authority had done virtually nothing on convoy escort operation since the end of World War I.”

December 1941 – November 1943

The rule developed by the Allies in the battle with the U-boats based on analysis of the statistics of convoyed merchant ship losses revealed the following:

Number of escorts = (Number of merchant ships/10) + 3,
if with air escort
Number of escorts x 2, if no air escort.

The importance of escorts is seen in the numbers of escorts required per convoy. Before 1940, transatlantic convoys had 2 escorts; and in 1943, the number was 7. In peacetime, no Japanese ASW escort craft were built. “The war began without a single ship designed for commerce protection on the high seas.”

As the war opened, the Naval General Staff placed the responsibility for shipping protection in its Operations Division with a one officer billet. Regulations for masters of merchant ships in time of war varied, depending on the geographical locations of the ships. The navy commanders in the various locations issued separate regulations, which created confusion. In the fall of 1942, standardized regulations appeared.

In the early part of the war, Japanese convoys of 10 to 20 merchant ships included merely one warship as escon. Further, the merchant ships went to sea unarmed. It was not until April 10, 1942 that the Japanese Navy assigned units to duty escorting merchant vessels. A shortage of adequate officer personnel to assist in this effort created difficulties. Total Japanese escon support for the 2500-mile link from Japan to Singapore consisted of 10 overage destroyers, 2 torpedo boats, and 5 merchant ships converted to gunboats. The escort for the 2000-mile passage from Yokusuka to Truk was composed of four old destroyers, one torpedo boat, and two converted gunboats.

This disarray and escort shortage created additional problems. Inadequate escort capability and independent tanker and freighter sailings did not assure the arrival in Japan of the now available and much needed resources, particularly, oil from the recently conquered areas in Southeast Asia.

In 1940, the Japanese Navy approved construction of four frigates for coastal defense. Later this class of ship provided the basic design for the much-needed and belated merchant ship convoy escorts. Initially these frigates were equipped with 12 depth charges. The reluctance to embark on an extensive escort building program did not start until mid-November 1943 when the disastrous loss of merchant ships signaled the need to provide escorts was finally realized by Japan.

Negligent in building frigates until June 1942, the navy approved 40 frigates with a request for 360. Perspective regarding the risks of Japanese merchant shipping in July-August 1942 comes from an anecdote concerning the third war patrol of the USS Narwhal (SS167}. This older submarine commissioned in 1930 survived the bombing at Pearl Harbor and was then the first submarine to patrol the area between Honshu and Hokkaido. On patrol, the commanding officer Lieutenant Commander W. C. Wilkins observed the Japanese merchant ships and commented that the coastal traffic looked like “a street car line: fat targets chugging up and down the coast with no escorts. We could take our pick.” However, Japanese ASW was not to be overlooked. Three United States submarines were lost in 1941 and 15 the following year.

By late August 1943, the Japanese Navy became alarmed because of greatly increased merchant ship losses. The numbers of submarine attacks increased. Greater numbers of U.S. submarines equipped with communications, sonar, air and surface radar, and improved torpedoes resulted in further sinkings. Growing danger to merchant ships from American bombing planes caused additional dismay to the Japanese Navy.

Postwar accounts by Army Major Y. Horie and Navy Captain Atsushi Oi in the US Naval Institute Proceedings addressed the basis of the inability of the Japanese Navy to cope. Oi suggests failure in ASW largely because the Navy disregarded the importance of the problem. Horie found the Navy indifferent to the problem of escort protection for merchant ships.

It became essential to confront these extreme shipping losses. On November 15, 1943, the Japanese navy established the Grand Escort Command Headquarters with centralized responsibility over all matters of shipping protection. Frequently throughout the war years, the Navy took various steps to improve the protection of merchant ships but always without a cohesive centralized plan, adequate manpower, and material support.

En route from Fremantle, Australia (one of seven trips), to deliver cargo and commandos to the Philippines in November, 1943, the above-mentioned USS NARWHAL encountered what appeared to be a lone Japanese oil tanker. However, three destroyers escorted the tanker. Packed with tons of supplies and armed only with the torpedoes in its tubes, the submarine attacked the tanker but missed. Evading the destroyer escorts, NARWHAL went on to fulfill its mission, delivering the supplies and personnel and rescuing thirty-two.

Frigates previously mentioned and called “Kaibo-kan” (coast defense), initially not intended for escort duty began to be used as merchant ship escorts. The characteristics of these 220-foot frigates of 800-1000 tons included Diesel or steam engines with deck guns and 60 depth charges. Later versions carried 120 depth charges. Ranges of the order of 6000 miles were typical. Speed of 16-20 knots and adequate sonar made them almost exclusively an oceangoing convoy escort. Construction of these frigates was initiated in October 1943. By May 1944, 145 were completed. Now two years into the war, Kaibo-kans began to operate effectively in the southwest Pacific. In contrast, Britain built and had 100 convoy escorts available before the start of WWII.

Regular convoying started in mid-November 1943 but only on the Singapore run. By this time, damage to the merchant fleet was beyond repair and new construction limited. Wooden ~ton cargo carrying sampans became numerous along the coasts as the number of merchant ships sharply decreased. Somewhat improved convoy methods were still forthcoming the following year, 1944. Late in that year after the battle of Leyte Gulf, the Japanese Navy became a minor factor. However, it was during that fall when the U.S. lost eight submarines in six weeks, the highest rate of the war, possibly due in part to the almost after-the-fact convoy escorting of merchant ships.

According to Pacific submarine war naval historian Theodore Roscoe, “Throughout the Pacific War the behavior of the Japanese escort was completely unpredictable. The escort’s lack of adequate communications equipment, only at this late date being equipped with primitive radar detection devices, could be one of the reasons for Roscoe’s comment. In addition to the deficiency of adequate strategy and tactics for convoying, escort ships, planes, and trained personnel were in short supply.

The Tecbnology Gap

At the start of the war, no Japanese ship was equipped with radar. It was many months before a limited number were supplied. Another year would be required to install radar on the combatant ships. The United States Navy entered the war with radar available and improvements forthcoming.

The delay in the introduction of advanced technology reveals some of Japan’s lag. In other systems as well, the United States continued to excel and increase Japan’s technological lag even further.

Japanese Technology Introduction

1942 Shipboard radar detector
Aircraft warning radar
1943 Battleship, medium bomber
lOcm radar
1944 Air convoy escort radar
Escort ship radar detector (in Dec.)


Data from Parillo’ display the final tonnage of the sinkings of Japanese merchant ships during the nearly four years of engagement primarily with U.S. Naval forces over a wide area of the Pacific. During the years 1942-44, U.S. submarines accounted for more than 2/3 of the sinkings of Japanese merchant shipping for each of the years. At the end of 1944, remaining Japanese merchant tonnage was close to or below the 2,000,000 tons required to meet the food supply needs of the country.

Failure to consider and plan for protection of merchant shipping, particularly in view of the industrial power of the United States and the neglect of historical evidence in support of convoying, contributed greatly to the collapse of Japan. This negligence and the presence of more than 150 U.S. submarines in the Pacific by 1945 hastened Japan’s defeat.


1. Major Y. Horie, “The Failure of the Japanese Convoy Escort”, Naval Institute Proceedings, October 1956, p. 1073.
2. Mrk P. Parillo, “The Japanese Merchant Marine in WWII”, U.
S. Naval Institute, 1993, p. 95, 97.
3. Atsushi Oi, “Why Japan’s Anti-submarine Warfare Failed”, Naval Institute Proceedings, June 1952, p. 592.
4. Clay Blair, Jr., “Silent Victory: The U.S. Submarine War Against Japan”, J.B. Lippincott Co., 19075, p. 497.
5. Theodore Roscoe, “United States Submarine Operations in World War”, US Naval Institute, 1949, p. 216.
6. Parillo, op, cit. p. 242.


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