Japanese Submarines

Japan started the war with about 64 submarines. 41 were modern fleet boats, two were modern medium size boats and 21 were described as obsolete. They added 126 boats during the war for a total of 190. Since some submarines were lost during the war while others were being built, they never had a total of 190 at one time. Japan lost about 129 boats of the 190 total or almost 68%.

This does not include the mini submarines such as those used during the attack on Pearl Harbor or the Kaiten which were developed and used late in the war.

Most, perhaps about 80, appear to have been lost to our Anti-Submarine Warfare (ASW) forces. However, the exact numbers are difficult to determine. Our ASW forces included destroyers, destroyer escorts, aircraft and even a PT boat. The USS England (DE-635) sank five in just 7 days, and a sixth a few days later, all just before the Battle of the Philippine Sea. The Allied ASW forces of Britain, Australia, New Zealand and India sank about eight more.

U. S. submarines sank 18 Japanese boats, and British submarines sank two more. The Japanese submarines were on the surface when they were sunk. In WW2, U. S. submarines did not have the capability to safely attack hostile, submerged boats.

There were also about 22 losses to mines, collisions, accidents and unknown causes. (“Imperial Japanese Navy Submarines 1941-45” by Mark Stille, page 45.)

Some were smaller than American boats and many were the same size or larger. Japan built a variety of submarines of different sizes and capabilities. It appears that there were as many as 11 different classes in service at the outbreak of the war and that Japan built an additional 15 new classes during the war.

Some examples were:

1. The older RO boats displaced from 700 to 1,000 tons. These were one-half to two-thirds the displacement of the U. S. 1,500-ton Gato/Balao/Tench class submarines.
2. The early “fleet boats” (the I-53 and I-68 classes) were shorter range boats that displaced about 1,400 to 2,000 tons. They usually would carry one scouting aircraft.
3. The I-13 class that displaced about 3,600 tons and carried two aircraft.
4. The I-200 class boats that displaced only about 1,000 tons but were capable of higher submerged speeds.
5. The I-400 class boats that displaced 5,200 tons. These boats had a range of as much as 37,000 nautical miles and carried three aircraft. These were the largest submarines anyone built until we commissioned the USS Benjamin Franklin class of nuclear powered, Polaris submarines in 1965. Yes, these WW2 Japanese submarines were actually larger than our earliest Polaris submarines.

NOTE: The US built only three new classes of submarines during the war that were very similar to each other. This strategy of fewer classes made it easier and more economical to design and build the boats, purchase components, train the crews, complete repairs and stock spare parts. However, they were general usage boats and may not have had features that some special operations required.

Many Japanese submarines were unique. There were many different classes and variants within the classes.

1. As indicated above, many carried aircraft, mostly for scouting purposes. These were intended to locate the opposing fleet. After reporting the location of the enemy, the submarine was to attack and sink some of the enemy ships before the big gun battles.
2. Japan had a couple classes of transport submarines. They tried to import some raw materials, engineers and designs from Germany using submarines. They also used submarines to resupply garrisons that we had bypassed during our “island hopping” campaigns in the Pacific.
3. There were also a tanker submarine and a small class of minelaying boats.
4. Perhaps the most famous were the very large, aircraft carrying submarines that were intended to be used to attack New York and other American cities, as well as the locks in the Panama Canal. However, those attacks were never actually attempted.
5. The Japanese were given a couple German submarines. However, the Japanese did not appear to try to duplicate these boats. Instead, they used the U-boats as trainers.
6. There were midget submarines of about 50 to 60 tons and a crew of just two. Five of these small submarines were used during the attack on Pearl Harbor.
7. Late in the war, there were also the Kaiten submarines. These were very small submarines of less than 20 tons with usually only one man aboard. These are often thought of as the suicide submarines. The design of the Kaiten was essentially a variant of the Type 93 torpedo with a section added for the pilot.

They did complete many missions to resupply garrisons that had been cut off by Allied advances. Supplies were put into large barrel shaped containers. Then the submarines would get as close as they could to the isolated bases and release the containers from their decks. They would then let the tides and surf carry the supplies to shore where troops retrieved them.

Some submarines were used to refuel sea planes that were on reconnaissance missions or just doing harassment bombing. The refueling may have been a success, but the bombing usually wasn’t. On one mission to bomb Pearl Harbor in March of 1942, seaplanes managed to successfully drop four bombs during their attack. However, two landed in the ocean. The other two landed harmlessly in fields, which the local residents blamed on the U. S. Army.

Bringing supplies from Germany to Japan was also less than successful. Five I-boats carried raw materials to Germany but only one succeeded in making the round trip and returning to Japan with German technology. The rest were sunk on the return trip.

The aerial attacks planned for US cities and for the Panama Canal were never attempted.

On 24 February 1942, the Japanese submarine I-17 fired seventeen rounds from its cannon at an oil field near Santa Barbara, California. The first attack on the continental US since 1814 caused no real damage. The I-17 also sank two ships. (“Submarines of the Imperial Japanese Navy” by Carpenter and Polmar, Naval Institute Press, 1986, page 20.)

The IJN submarine I-25 sent its aircraft to bomb an Oregon forest in September of 1942, in retaliation for the Doolittle raid on Tokyo. Four incendiary bombs were dropped in two attacks in the hope of starting massive forest fires. Only one small fire was started. (“The Japanese Submarine Force and World War II” by Boyd and Yoshida, Naval Institute Press, 1995, page 110.)

None of their submarines successfully scouted and attacked American ships before the big sea battles.

Japanese submarines were not nearly as effective as the boats in other navies. That was more an issue of philosophy and deployment, although there were some design flaws.

There were some shortcomings with the boats themselves. Japanese submarines were not designed to go as deep as others. The deepest they were designed for was 100 meters or about 330 feet. They were also noisier than our boats and easier to track. Japanese boats were usually larger and would naturally be less maneuverable, particularly when submerged.

The biggest operational failures for Japanese submarines were not locating the American carriers before the attack on Pearl Harbor, again before the Battle of Midway and yet again before the Battle of Leyte Gulf. In each case, the submarines arrived too late to spot our forces.

Submarine warfare was not highly thought of by the Japanese, either offensively or defensively. Escort duty did not help an officer’s career. Japanese submarines did not get radar until June of 1944, about two years after most American boats were getting theirs. The Japanese did not take anti-submarine warfare seriously until 1944 when it was too late.

However, the biggest issue was how they were used. Japanese boats were envisioned to be “fleet boats”. They were expected to operate with the fleet. Their job was to use their scout planes to locate the enemy fleet and inflict damage before the big gun battles.

NOTE: At first, we also thought of our submarines as fleet boats. But that wasn’t practical. Our submarines couldn’t keep up with most of the fleet. Even if they did, submarines would give away their greatest advantage – stealth. Perhaps, in this regard at least, we were fortunate that, after the attack on Pearl Harbor, there wasn’t much of a fleet for our submarines to operate with. Out of necessity, our strategy and tactics had to change.

“And it was the Japanese Navy’s repeated use of submarines for purposes for which they were not designed that was a major reason for the failure of the submarine force to achieve a creditable combat record.” (“Submarines of the Imperial Japanese Navy” by Dorr Carpenter and Norman Polmar, Naval Institute Press, 1986, page 11.)

It certainly seems a bit strange that the Japanese Navy, which used aircraft carriers so effectively in the early days of the war, was still designing submarines to participate in an expected “Kantai Kessen,” the decisive big gun battle to determine the outcome of the war.

Japanese tactics also left much to be desired. Captains were only allowed so many torpedoes depending on the target. They stationed submarines at specific intervals when creating picket lines, trying to interdict our fleets. Their locations became too predictable – if you found one you could easily locate the rest. This last issue may help explain how the destroyer escort USS England (DE-35) was able to sink so many Japanese submarines in a few days.

Japan had done some exercises to determine whether their submarines could be used as commerce raiders.  “Curiously, when Japanese submarines were exercised against merchant targets, they were found to be effective. However, not only was this lesson not heeded with regard to the employment of Japanese submarines, neither was the converse possibility, that enemy submarines could cripple Japan’s seagoing commerce.”  (“Imperial Japanese Navy Submarines 1941-45” by Mark Stille, page 6.)

As the war went on, and the U. S. began to push the Japanese back to the west, they began to use the boats more often as supply transports. Beginning in November of 1942, the Japanese were using some of their submarines to supply troops in outposts that the U. S. had simply bypassed in the island-hopping strategy. This was not a good use of submarines even if they did the job well. The Japanese air base at Yap might be a good example of this. American planes regularly attacked the Japanese airfields at Yap by carrier aircraft, but the island was never invaded. It was just left isolated and the Japanese decided to deliver supplies.

Submarine captains didn’t help themselves either. Reports are that they were very chatty, sending messages often. They also had a tendency to fire a few parting shots at American facilities as they were leaving or passing by. All of this made their boats easier to track.

Apparently not. “Japanese submarine doctrine clearly focused on the enemy’s battle fleet at the expense of developing the strategy and tactics required to attack the enemy’s commerce and sea lines of communication.” Mr. Stille adds: “All considered, the exploits of the [Japanese] Imperial Navy’s submarine force were among the more disappointing stories of the Pacific War.” (“Imperial Japanese Navy Submarines 1941-45” by Mark Stille, page 6.)

“Throughout the war, destruction of commerce was undertaken only when the fighting strength of the fleet allowed.” (“Sunk; The Story of the Japanese Submarine Fleet 1941-1945” by Mochitsura Hashimoto, page 62.)

“But the Japanese submarines never made a serious attempt to attack American merchant shipping in the Pacific, and none at all to disrupt the traffic through the Panama Canal …. The Japanese admirals stuck rigidly to the view that submarines were first and foremost an adjunct to the battle fleet, even when they were forced to use the boats to move soldiers and supplies unobserved during the Pacific Campaign.” (“The Pacific Campaign, the U.S.-Japanese Naval War 1941-45” by Dan van der Vat, pages 119-120.)

Kennosuke Toriso, Operations Officer on the Sixth Fleet (submarines) staff stated: “From the outbreak of the war, the Japanese Navy had stuck to its long-cherished ‘one-big-battle’ in which all naval arms - surface, air and underwater – were to be used once and for all. The employment of the underwater arm was not excepted, even after the loss of Guadalcanal turned the tide against us.”  (“The Japanese Navy in World War II: In the Words of Former Japanese Naval Officers”, by David C. Evans, ed. and trans., Naval Institute Press, 1986 page 440.)

Yet, when the larger battles of the Philippine Sea and, particularly Leyte Gulf, which were the closest things to the big battle the Japanese sought, their submarines were practically non-participants.

Only a few. Japanese submarines are credited with sinking the aircraft carrier USS Wasp (CV-7), the escort carrier USS Liscome Bay (CVE-56) and the cruiser USS Indianapolis (CA-35). They were also credited with finishing off the already damaged carrier USS Yorktown (CV-5) and the damaged cruiser USS Juneau (CL-52). Some smaller types of ships, such as destroyers, were also sunk by Japanese submarines.

The battleship USS North Carolina (BB-55) was seriously damaged by the same spread of torpedoes that sank the Wasp.  The carrier USS Saratoga (CV-3) was damaged by the IJN submarine I-6 in January of 1942. Both ships were repaired and put back into service.

We did have other large ships lost or damaged by Japanese torpedoes, particularly early in the war. However, those torpedoes were fired from destroyers and cruisers, not submarines.

It appears that Japanese submarines sank a total of “only 171 vessels, including warships, for a total of less than 1,000,000 tons.” (“The Japanese Navy in World War II: In the Words of Former Japanese Naval Officers” by David C. Evans, ed. and trans., Naval Institute Press, 1986.Page 440.) This is a quote from Kennosuke Toriso.

That was not a particularly effective record. Again, the fault is more the way Japanese fleet commanders assigned their boats, rather than the submarine crews themselves.

Japanese submarines sank only about one ship per submarine. German U-boats sank about 3,000 Allied ships of all types or about three per submarine built. American boats sank about 1,300 enemy ships of all types or about five per submarine.

Japanese submarines were directed to patrol off our West Coast shortly after the attack on Pearl Harbor. According to “Sunk; The Story of the Japanese Submarine Fleet 1941-1945” (by Mochitsura Hashimoto, page 63) the nine boats that patrolled between Hawaii and the mainland coast sank “about 10” ships. (In a related article, Wikipedia indicates that they sank just two ships and damaged six.It may be that neither number is reliable.) And that appears to be the end of the attacks against our shipping on the West Coast. Whether two ships or ten were sunk, the results were not significant.

NOTE: CDR Hashimoto was in command of the I-58 when it sank the USS Indianapolis (CA-35) in 1945.

After those patrols, most of the Japanese commerce raiding was to the south. The sinkings listed in Appendix B of “Sunk”; The Story of the Japanese Submarine Fleet 1941-1945” (by Mochitsura Hashimoto) were in the general areas of the Indian Ocean, the Coral Sea and around New Zealand. As the war progressed, Japanese captains considered the Central Pacific to be the “Hell War.” They were more likely to be sunk than to be successful.

The Japanese did have a Torpedo Data Computer (TDC) with inputs similar to the American version. From the scant information available, it appears that the Japanese TDC did not maintain a running solution. It likely was only a point-in-time solution similar to the British “Fruit Machine” or the German TRW. Those machines worked, but if the firing was delayed, the solution would have to be recalculated. The American TDC kept a running solution, constantly updating the target’s position relative to the submarine.

“Japanese TDCs were generally quite reliable, but like similar equipment in U. S. Navy submarines in the 1930s, torpedo firing solutions were only as good as the input data provided to the TDC; estimates of a target’s speed and range, in particular, depended largely on the ‘seaman’s good eye.’ Thus, throughout the war Japanese TDCs were incapable of generating the range or bearing, but they effectively solved the firing problem in accordance with the input data.” (“The Japanese Submarine Force and World War II” by Carl Boyd and Akihiko Yoshida, Naval Institute Press, 1995. Page 38.)

The relative lack of success in sinking Allied shipping, in spite of high-quality torpedoes, might indicate some weakness in the Japanese TDC.

At the beginning of the war, the Japanese torpedoes were probably the best of any navy. The Type 93 torpedoes, fired from destroyers and cruisers, were particularly effective.“During the course of the war, 22 Allied warships were sunk after Type 93 hits: 10 cruisers, 11 destroyers, and one fleet aircraft carrier [the USS Hornet]. Thirteen of these had been fatally hit solely by the Type 93, with the rest succumbing to a combination of bombs, gunfire, and torpedoes.”

https://en.wikipedia.org/wiki/Type93torpedo

Submarines did not use the Type 93. However, the Type 95 submarine torpedo had many of the same strengths as the Type 93.

The lengthy range at high speeds and the large warheads. They did not seem to experience the issues with their torpedoes that the U. S. and Germany did. But there appear to have been some other issues specific to Japanese torpedoes.

At least one senior Japanese officer complained at the end of the war about the submarines and weapons they produced. However, it is not clear whether he was just shifting the blame or there were more problems with their boats. It could well be the former – blaming someone not in the operational chain of command. (His comments are found in “Sunk; The Story of the Japanese Submarine Fleet 1941-1945” by Mochitsura Hashimoto, page 244.) A conclusion by S. Fukutome, Former Chief of Staff of the Combined Fleet:

“What were the reasons for these deplorable results? They were largely due to our inferiority in the fields of ordnance and ship construction. Our submarines, built and equipped with inferior weapons, were counterattacked by the enemy before they ever got to their targets, or were evaded. Against the superior equipment of the enemy submarines our ships under attack had no opportunity to counterattack or evade. Many of them were alive [sic] to the enemy’s presence only after they were attacked.”

Whether his comments are legitimate or not, Japanese torpedoes at the beginning of the war were still the best by far even with a few issues. Although most of these problems were related to the torpedoes fired by surface ships, the submarine Type 89 and then the Type 95 submarine torpedoes were very similar.

The first issue is related to the fact that the Japanese used pure oxygen in their torpedoes rather than air for combustion. This was a great feature for these torpedoes because it gave them a much greater range and left virtually no bubble trail. Oxygen is about 21% of normal air. Therefore, pure oxygen would last about five times as long as the same amount of air. Most of the products of combustion with pure oxygen were water and a small amount of CO2 that would largely be absorbed by the sea.

The Japanese worked on oxygen torpedoes from 1920 until 1935 before the weapon was satisfactory. During the development of these torpedoes, the Japanese experienced explosions when the engines first started. This was solved by using another, smaller flask of normal air when the torpedo was first fired. The air flask was used first. As that flask was used, the air was replaced by the pure oxygen in a second larger tank. By the time these torpedoes reached the fleet, this problem had been solved. However, handling pure oxygen is always a hazard.

Because of the dangers in handling pure oxygen, torpedomen were described as the most highly skilled of the crew. The rest of the crew were told that the second (pure oxygen) tank was just another air flask. The Japanese installed bulky oxygen-producing equipment in both cruisers and destroyers but not in submarines. If a submarine’s Type 95 torpedoes needed to have the oxygen topped off, the boat would have to go alongside a cruiser or destroyer.

NOTE: It is hard to imagine that such information could be kept from the rest of the crew in the close confines of a submarine. However, Japanese culture was more structured and more formal than others and that may well have been the policy. Still, there would be so many clues that the rest of the crew could not help but see.

NOTE: Some sources refer to these torpedoes as “oxygen-fueled.” That is not technically accurate. The fuel in the Japanese torpedoes was kerosene. Pure oxygen was used rather than normal air as the oxidizer to burn the kerosene fuel.

The second issue was that the explosive in their warheads appeared to be very sensitive. (Warheads were filled with Shimose, a mixture of 60% TNT and 40% hexanitrodiphenylamine or HND, a booster class explosive.) 

One possible factor in the sensitivity of the Type 93 warheads might be the setting by the crews. “Many accounts of the 1942 battles describe these torpedoes prematurely detonating after running the arming distance or detonating once they crossed the target’s wake. For example, it was estimated that about a third of the torpedoes launched at the Battle of the Java Sea (Sea Engagement off Surabaya) either prematured or detonated on wakes. An investigation by CDR Takedai Takashi of the Navy Technical Department, who was in charge of torpedoes at that time, found that approximately half of the torpedo fuzes returned for examination from warships involved in this battle would activate at pressures much lower than those specified. However, torpedo fuzes examined at the naval arsenals and at military supply departments worked properly. Further investigation found that the main cause for the self-destruction was that the torpedo crews on the ships were resetting the fuzes to a lower level in a misguided effort to ensure detonation. Post-war, Rear Admiral of Engineering Oyagi Shizuo, an authority on the Type 93 torpedoes … remarked in his recollections: ‘It was a matter of eternal regret that we had provided each vessel with a sensitivity adjuster for the fuzes.’”

(http://www.navweaps.com/Weapons/WTJAP_WWII.php" rel="noopener noreferrer" target="_blank">http://www.navweaps.com/Weapons/WTJAP_WWII.php; http://www.navweaps.com/Weapons/WTJAP_WWII.php" rel="noopener noreferrer" target="_blank">, section for Type 93 torpedoes.)

There were a couple incidents when Japanese cruisers experienced the sensitivity of their Type 93 torpedo warheads. Perhaps the best example of the issue is the outcomes for the cruisers IJN Mogami and IJN Mikuma in the later stages of the Battle of Midway. The two cruisers, along with others, had been sent to shell Midway after the carrier battle had already been decided. They were then recalled before they reached Midway. However, during the maneuvering to change course and return to the fleet, the two cruisers collided. Mogami then jettisoned her Type 93s, whereas Mikuma did not. As they were limping home, our aircraft found them and attacked. Mogami appeared to absorb more blows than the Mikuma but survived. Mikuma sank during the fighting when her own torpedoes detonated while still onboard. Mogami made it home for major repairs but was lost a couple years later when she was sunk during the Battle of Leyte Gulf.

Another example of the danger was thought to be the heavy cruiser IJN Chokai during the battle off Samar in October of 1944. In that case, a desperately lucky 5-inch shot from the fleeing escort carrier USS White Plains (CVE-66) reportedly hit the Chokai in the torpedo tubes. The resulting explosion then severely damaged the cruiser, and she had to be scuttled. However, another source disagrees with the identity of a cruiser sunk by her own torpedoes during that battle. “[T]he 2019 discovery by the RV Petrel of the wreck of the Chokai with her torpedoes intact disproved this theory. The same Samar engagement saw the heavy cruiser IJN Suzaya sunk by the detonation of her Type 93 torpedoes: a bomb near miss starboard amidships set off the torpedoes in the starboard tube mounts; the resultant fires propagated to other torpedoes nearby and beyond; the subsequent explosions damaged one of the boilers and the starboard engine rooms and eventually reached the main magazines.” It appears that at least one Japanese cruiser in this battle was sunk by her own exploding torpedoes although it is not clear which.

https://en.wikipedia.org/wiki/Type93torpedo" rel="noopener noreferrer" target="_blank" style="background-color: white; color: black;">https://en.wikipedia.org/wiki/Type93torpedo

Most of the issues would seem to have applied since the torpedoes were of a very similar design. Of course, Japanese submarines likely did not experience the number of air attacks that affected the cruisers and destroyers. There does not seem to be any record of incidents on submarines, although they could still have been an issue or a contributing factor in some losses by depth charge or hedgehog attacks.

No. The Type 93 Long Lance was longer than the submarine torpedo and was used by surface ships such as cruisers and destroyers. Submarines used the Type 89 prior to the war. That was replaced by the Type 95 kerosene/oxygen torpedo prior to the war. They also used the electric Type 92 torpedo.

The “Long Lance” was not a Japanese name for the torpedo. After the war, the Type 93 was nicknamed the “Long Lance” by the U. S. Naval historian Samuel Eliot Morison. The Japanese knew the submarine torpedoes as the Type 89, the Type 92 and the Type 95. The Type 89 was an older kerosene/compressed-air torpedo. The Type 92 was their electric torpedo, and the Type 95 was the kerosene/oxygen version.

The Japanese designated their torpedo (and TDC) types based on the last two digits of the year of their ancient empire. The Type 93 torpedo was first perfected in 1933, or 2593 years after the founding of the Japanese empire.

Emperor Jimmu (Jinmu-tenno) was the legendary first emperor of Japan according to the chronicles Nihon Shoki and Kojiki. His ascension is traditionally dated as 660 BC [BCE]. In Japanese mythology, he was a descendant of the sun goddess Amaterasu, through her grandson Ninigi, as well as a descendant of the storm god Susanoo. …. In modern Japan, Emperor Jimmu’s accession is marked as National Foundation Day on February 11. There is no evidence to suggest that Jimmu existed, and he is regarded by most modern scholars as a legendary figure. However, there is a high probability of a powerful dynasty in the vicinity of Miyazaki Prefecture during the Kofun period. (Wikipedia)

The following table is a comparison of the standard, high-speed torpedoes used by Japan and the US during WW2. The Japanese Type 95 is a little larger and a little faster. However, using pure oxygen rather than compressed air gives the Type 95 much greater range at 46 knots. The range of the Type 95 was nearly three times that of the Mark 14 torpedo, carried by U. S. submarines, at that same 46 knot speed. It also carried a warhead that was one-third larger than that of the American Mark 14.

The Japanese Type 95 Mod 2 had an even larger warhead but sacrificed some range. That warhead weighed about 550 kg or 1,210 pounds.

Because there was almost no bubble trail, it would be much harder to detect the kerosene/oxygen torpedoes but not impossible. Torpedoes made a lot of noise. Still, it seems unlikely that it would have been detected in time to avoid being hit.

Bioluminescence along the torpedo track may have also been detected at night. That too may not have been noticed soon enough to avoid being hit.

Yes, they did. Japan sent five mini submarines to take part in the attacks. They were carried to the Hawaiian Islands on the larger submarines that were also part of the plan. However, they were generally a failure. Our first warning of the attack – which we obviously didn’t take seriously enough – came when we discovered one of the mini submarines trying to enter the harbor and sank it.

It was originally thought that none of the mini submarines were able to accomplish their missions. However, recent video analysis indicates that one of the small subs may have penetrated the harbor and successfully fired one torpedo against Battleship Row, hitting its target.

These very small submarines displaced only about 60 tons. They had a crew of two men and carried two torpedoes.

No. The Kaiten were much smaller and were not developed and put in service until late 1944. They were developed as a modification of the Type 93 torpedo and had a crew of just one pilot.

The Kaiten were not very effective, although the Japanese certainly thought they were.  The first Kaiten operations were delayed until November of 1944. The attack at Ulithi on 20 November sank the USS Mississinewa (AO-59). Our ASW forces then spotted the other Kaiten and sank them.

The Japanese claimed great successes by the Kaiten in the last three months of the war. However, the claims of success were dubious: “[Japanese] Sixth Fleet Headquarters exaggerated the effectiveness of the Kaiten in the second Gen operation, especially before the loss of the I-48. Indeed, no ships were sunk, but as a U. S. naval intelligence officer noted, ‘The Japanese assessed the results as eighteen vessels sunk, including one converted aircraft carrier, nine large transports, one cruiser, and six other large ships including aircraft carriers, battleships and transports. Such optimistic estimates can only be the result of inexcusably poor staff work.” (“The Japanese Submarine Force and World War II” by Carl Boyd and Akihito Yoshida, page 170.)

This is confirmed in “Suicide Submarine” by Yutaka Yokota and Joseph D. Harrington, (also known as “The Kaiten Weapon”, page 256.) That text indicates in the Epilogue: “When World War II closed, the Japanese thought they had sunk some 40 Allied ships, including a British cruiser of the Leander class through their Kaiten effort. A check of all available sources reveals they sank only two U. S. Navy ships, the tanker Mississinewa in the Kikusui sortie and the destroyer escort Underhill. This second ship was actually sunk by friendly forces on July 24, 1945, after being hit by a Kaiten. Only one U. S. merchant ship, SS Canada Victory, was the victim of a Kaiten, going down on April 27, 1945. These three vessels, measured against the 8 submarines and nearly 900 Japanese lives lost in the Kaiten program, make the enemy’s sacrifice seem fruitless.”

Moreover, there was a great cost in development and use, “from the time of the first Kaiten sortie on 20 November 1944 until the end of the Okinawa campaign, Kaiten took 7 lives in training and 48 in action, without achieving any significant confirmed results.”  Further: “The cause of the failure was not the weapon, but rather the manner in which it was employed.”

(“The End of the Imperial Japanese Navy” by Masanori Ito, page 196.)

One problem was that the Kaiten could only dive to 80 meters (264 feet) while the submarines carrying the Kaiten could dive to 100 meters (325 feet). That meant that the depth of the submarine was limited by the Kaiten.  Another was that the Kaiten was often unstable and training at 40 knots was “extremely difficult” and "accidents were not infrequent.”

(“The Japanese Submarine Force and World War II” by Carl Boyd and Akihito Yoshida, page 168-169.)

Although they may have had a long range for a torpedo, the range of the Kaiten was short compared to a submarine. They had a telephone connection to the mother submarine and a gyroscope, but lacked sonar, radio and navigation equipment. That meant it was difficult for the Kaiten to be truly effective. (“Submarines of the Imperial Japanese Navy” by Dorr Carpenter and Norman Polmar, page 137.)

There are indications that the pilots theoretically had an option to escape the Kaiten before it exploded. “They were allowed to build a prototype with the understanding that a means be provided for the pilot to be ejected before the torpedo struck.” (“Submarines of the Imperial Japanese Navy” by Dorr Carpenter and Norman Polmar, page 137.)

However, that may not have been practical. The Kaiten would have been traveling at 40 knots. It would have been very difficult, if not impossible for a pilot to escape. In any case, being as near to the target as the Kaiten pilot was expected to be, the explosion would likely have killed him even if he got free. For comparison, many survivors of sinking ships later died from the concussion of depth charges while they were in the water. The Kaiten warhead would have been much bigger than that of a depth charge leaving the pilot with an even smaller chance of survival. As a result, the Kaiten may not have been designed as a suicide submarine, but it almost certainly turned out to be so.

“The End of the Imperial Japanese Navy” by Masanori Ito; W.W. Norton & Co; New York; 1962.

“Imperial Japanese Navy Submarines 1941-45” by Mark Stille; Osprey Publishing Ltd.; Botley, Oxford, U.K.; 2007

“The Japanese Navy in World War II: In the Words of Former Japanese Naval Officers”, by David C. Evans, ed. and trans.; Naval Institute Press; Annapolis, MD; 1986

“The Japanese Submarine Force and World War II” by Carl Boyd and Akihito Yoshida; Naval Institute Press; Annapolis, MD; 1995

“The Pacific Campaign, the U.S.-Japanese Naval War 1941-45” by Dan van der Vat; Touchstone; New York; 1991

“Submarines of the Imperial Japanese Navy” by Dorr Carpenter and Norman Polmar; Naval Institute Press; Annapolis, MD; 1986

“Suicide Submarine” by Yutaka Yokota and Joseph D. Harrington, (formerly “The Kaiten Weapon”); Ballantine Books; New York; 1962

“Sunk; The Story of the Japanese Submarine Fleet 1941-1945” by Mochitsura Hashimoto; Cassell and Company, Ltd.; London; 1954

http://www.navweaps.com/Weapons/WTJAP_WWII.php;

Wikipedia entry for “Emperor Jimmu”