'Heaven Shaker' Training

Plans were being concocted for a one-man midget which could be released while the mother submarine was submerged. During the winter of 1942-43 two naval officers, lieutenants Nishina and Kuroki, and a naval architect Hiroshi Suzukawa drafted a design based on the Type 93 Long Lance. All the major components of the original torpedo were retained, and the only major modification was the inclusion of an additional section between the warhead in the nose and the oxygen motor. This was the pilot's compartment, fitted with a periscope and a set of controls enabling a man to direct the torpedo run. By the spring of 1943 the designers had completed their drawings, and had calculated that their 'manned' torpedo, fitted with a 3,000 pound high explosive warhead, would have a range of forty nautical miles. The Long Lance had already proved it could break the back of a heavy cruiser with a man to direct it and a warhead three times more powerful there was every reason to suppose it could do the same to a battleship or an aircraft carrier.

Things had already started to go badly for the Imperial Navy and the Naval General Staff in Tokyo were looking for some way of changing the pattern of the Pacific war. The plans were presented for what the designers were now calling the kaiten. (The literal translation of kaiten is 'Heaven Shaker'. But in Japanese it means much more - suggesting a radical change in affairs.) But they were rejected as being too fantastic even But when the for consideration. Imperial Navy's attempt to smash the Americans at Saipan went awry the men in Tokyo began to have second thoughts about the kaiten. Nishina and Kuroki's persistent pleas to the Navy Ministry had culminated in a petition written in their own if this had much what undoubtedly caused the Naval General Staff to listen was what the Americans termed the 'Marianas Turkey Shoot.' when over blood. It is doubtful effect; 400 Japanese planes were lost. Thirteen months after they first sought it, permission was given for the construction of a prototype. But only on condition that it should have an escape hatch giving the kaiten pilot a chance to get away safely once he had put his weapon on a sure course to the tar- get. In February 1944 the prototype was approved and a base was set up on Otsujima Island near the Kure naval base, headquarters of Japan's Sixth Fleet and submarine force.

Strict security measures kept news of the project out of the public eye and only a few kaiten had been built by June 1944. But when the extent of the disaster which had overtaken the Imperial Navy in the Marianas finally dawned on them, the Naval General Staff sent out a frantic order for more to be produced on a crash priority. A second order called for volunteers to operate a Kyukoko heiki, a new national salvation weapon, on missions from which they were not expected to return alive. At first no mention was made of the nature of the weapon, but even when it was learned that their probable fate was an unseen death beneath the waves there were plenty of volunteers. Indeed it appears that the first ones were grateful at being accepted. Selection was supposedly based on three qualifications: physical and moral strength, evidence of strong sense of patriotism, and a minimum of family responsibilities. Married men were excluded and very few elder or only sons were chosen. The accent was on young fit men who would have little tendency to look over their shoulders.

At the kaiten depot on Otsushima, 'Base P', every effort was made to instill esprit de corps, Yamato damashii, Japanese spirit, in the volunteers. On arrival they were introduced to a prototype of their steel coffins before being shown to their quarters. The latter were, like their food, luxurious in comparison with what most of them had known in their previous training. But there were few recreation facilities - no cinema and no women. Nor were the men permitted leave of absence until they had completed their training, and were ready for the mission which was to be their finale.

Nishina and Kuroki organised the training of the first volunteers. But on 6th September, 1944, the kaiten claimed its first victim when Kuroki's torpedo stuck in the mud at the bottom of the placid waters of the Inland Sea. Six other lives were to be lost in training before the end of the war brought the demise of the kaiten. But, from September 1944 until the end of the Okinawa campaign, volunteers in groups which were given traditional names such as 'God's warriors', 'Group for the furtherance of the Samurai way', took kaiten courses at Otsushima. Lessons in the functions of the Type 93 torpedo were followed by simulated dry run missions to familiarise the pilots with the controls and accustom them to the confined space of their tiny cabin. Submerged practice drill against ships moored in Tokuyami Bay followed. Finally, when the pilots were considered proficient at these drills, the group was embarked on one of the fleet submarines for an operational dummy run.

Each of the I class submarines fitted to carry kaiten could take six of the weapons. During the approach to the target the kaiten pilots climbed into their tiny craft through a special hatch which was then sealed off. As the submarine closed on its victim, a telephone link between the submarine's conning tower and the kaiten enabled the captain to keep the pilots informed on the relative positions of the target. At the optimum moment the kaiten's engines were started and they were released at five-second intervals from the mother ship. Once in motion the pilot could observe the target through his own periscope, and make the necessary corrections to his course. Then at about 500 yards distance he would switch his craft on to automatic control for the final dash at full speed submerged to a depth of about twelve feet.

Inside the kaiten even a small man was cramped. And, although the controls were simple, considerable skill needed to operate the craft Under his feet was a tiny box of emergency rations and a small flask of Japanese whiskey. Neither was intended for operational missions. Directly in front of the pilot's face was the viewing glass of the short, stubby periscope which was raised or lowered by a crank on the right. Also on the right but above the pilot's head was the valve regulating the oxygen flow to the motor immediately behind him. Overhead on the left was a lever connected with the kaiten's diving planes, which controlled the rate of descent or climb underwater. Below efficiently. was a valve for letting in sea water. This was necessary to maintain stability as the oxygen fuel was used up. Finally, there was the rudder control lever which steered the weapon right or left and which was the last control to be touched by the pilot when he set his final course for an enemy ship. To operate the kaiten efficiently a man really needed six hands. And about the same number of eyes for watching the control panel. Apart from the periscope there was a gyrocompass, a clock, and depth and fuel gauges. Any sudden change in the controls or contact with an underwater obstacle invariably resulted in the pilot banging his head on one or other of the instruments. In consequence, bandaged heads were a frequent sight on Otsushima.

On an operational mission the captain of the mother submarine would align his ship with the target and this lever man would check his compass bearing. In the conning tower each kaiten the attack course of each individual kaiten would then be plotted and relayed by telephone. For example a typical order might be 'Go right thirty degrees on leaving. Speed twenty-five knots for twelve minutes and thirty seconds.' These instructions were designed to bring the kaiten to within 500 yards of his target, at which point the pilot was expected to raise his periscope and set the controls for his dash for the enemy ship's vitals at the top speed of forty knots.

Training finished with the successful completion of an operational dummy run. The kaiten men were then entitled to a few days leave before assignment to an operational mission. On this leave they were not expected to reveal the fact that they were now committed to a suicide operation. Nevertheless many of the families of such men appear to have guessed the reason for the special leave even if they were not told. Any suspicions they may have had would often be confirmed by little luxuries with which their relative was laden when he arrived. When the leave was over it was not considered good taste to mention that the next meeting would probably be at Yasukuni. But no doubt the thought was there.

'Heaven Shaker' Action

News that the Americans had seized Ulithi Atoll in the Carolines, where a deep water anchorage would provide an ideal fleet base, prompted the Japanese to launch their first kaiten attack. Twelve of the newly trained would-be suicides were selected for the strike. Among' them was one of the two inventors of the weapon. Lieutenant Sekio Nishina. Determined to show the worth of his innovation Nishina was taking along a box containing the ashes of his deceased co-inventor. This would ensure that both would go to Kudan and be enshrined at Yasukuni together. A dedication ceremony was conducted at the Otsujima base during the afternoon of 7th November 1944. Vice-Admiral Shigeyoshi Miwa, commander of the Imperial Sixth Fleet, supervised the proceedings and explained the forthcoming operation to the kaiten pilots. Three fleet sub marines, the I-36, I-37, and I-47, which were in the bay nearby, would transport four kaiten each to the vicinity of Ulithi where large numbers of American ships were reported to be concentrating. The kaiten pilots were to sink the biggest ships they could. A presentation of short swords and hachimakis followed, and that night there was a party for the twelve doomed men. Next morning they 9am the I-36 led the three I-class submarines out of the harbour. As they steamed slowly up the channel the crews on other ships lined the rails shouting 'banzai' and waving their caps in a farewell gesture.

The three submarines parted company not long after leaving port. I-37 was to proceed towards Kossol Passage in the Palaus, to attack Allied shipping there. I-36 and I-47 meanwhile would head straight for Ulithi. Their mission was to attack the American invasion fleet at anchor, launching their kaiten at through two different entrances to the atoll's giant lagoon. But I-37 was fated not to reach her destination. Despite having six lookouts on the bridge whenever she surfaced she was spotted by the American destroyer Nicholas on 12th November. In a sudden and unexpected attack the I-37 was caught before she could dive and take evasive action.

I-47 was under command of Lieutenant-Commander Zenji Orita, one of Japan's ace submarine captains. He steamed slowly for his destination, making twenty knots on the surface, until he came within range of the American patrol planes. He then submerged by day, surfacing at night to charge his ship's batteries and to pick up radio reports from Sixth Fleet headquarters at Kure. His ship and I-36 were working in close cooperation with reconnaissance planes from Truk. They would provide reports on American shipping at Ulithi.

On 17th November the I-17's radio picked up a message relayed by Tokyo reporting that one of the reconnaissance planes had seen a vast concentration of American ships at Ulithi on the previous day. According to the pilot they appeared to be anchored in three groups, and he had seen battleships and carriers among them. Next day, fifty miles west of Ulithi, Captain Orita surfaced so that the Kaiten could be given a final check. All four were found to be in good working order. By noon on the 19th the sub- marine had closed to within a mile of the southern entrance of the Ulithi lagoon, and at midnight the four Kaiten pilots began making their final preparations. Last minute messages were written and handed to Orita together with their wills; finally all four men wound their hachimakis round their heads.

Ensigns Akira Sato and Kozo Watanabe climbed into their kaiten at midnight while the submarine wallowed quietly on the surface. Lieutenants Nishina and Fukuda were able to defer their entry, because there were access tubes to their weapons from the submarine. (Access tubes to all kaiten were provided on later sorties, so that the submarine could remain submerged.) When the lids of their weapons had clanged shut, Orita dipped I-47 beneath the waves and then edged the submarine stealthily forward to the very en- trance of the lagoon. This manoeuv- ring occupied three hours, during which Sato and Watanabe sat in their kaiten - their only contact with the world outside being two telephone cables. At 3am Nishina and Fukuda struggled through the access tubes to their kaiten, Numbers One and Two. All was now set for the attack. Four cables bound each kaiten to the submarine's deck during the voyage. Two of these had been loosened when the I-47 surfaced at midnight; the other two could be released from inside the submarine. At 4am Captain Orita, guided by the twinkle of welding torches on the US ships which he could see in his periscope, declared that he was in the firing position. Over the telephone lines the four kaiten men reported they were ready for action.

'Kaiten Number One stand by, start your engine!' ordered Orita.
'Standing by', came Lieutenant Nishina's soft voice over the circuit. The third cable on Number One kaiten was loosened.
'Start your engine!' said Orita.
Inside the submarine, a motor sound could be heard.
'Engine started'
'Ready?'
'Ready!'
'Go!'

The fourth cable was loosened. It was 4.15am, 20th November 1944. Captain Orita, peering through his periscope, could see just a trace of bubbling water for a moment, as Nishina's kaiten moved off. Final checks of position, depth and the course Nishima was to follow had been made. He was now on his run-in, under orders to penetrate as deep into the anchorage as he could before raising his periscope and selecting a target for attack.

Ensign Sato left at 4.20, followed by Watanabe and Fukuda at five minute intervals. The second and the third kaiten were to get inside, then move off to the right and left, respectively. Fukuda was to attack when just inside the lagoon. This, it was hoped, would throw the Americans into a panic, when ships began exploding at widely separated points. The last words heard from kaiten pilots in I-47's conning tower were Fukunda's, 'Tenno heika banzai!'. Long Emperor!

The four kaiten forged towards their targets at about thirty knots. Mean- time the barely submerged I-47, live the suddenly freed of twelve tons of metal, lurched towards the surface. Orita submerged again to periscope depth and headed south-east. He had wanted to be well away from the of the anchorage when the kaiten completed their mission. He also wanted a clear view of what happened to take back to Japan. Thus at 5am, the I-47 surfaced again. It was pre-dawn twilight and the crew was edgy, for daylight comes quickly in the South Pacific. The minutes ticked past. Then, at 5.07, an orange flash blossomed over Ulithi, and there was a distinct boom from well within the lagoon where Nishina was supposed to hit a target.

At 5.11 another flash set the submarine's crew banzaiing. The appearance of an American destroyer soon stopped that, however. Orita dived, but when the absence of depth charges suggested the submarine had not been spotted he surfaced again. The sun was now up and the destroyer could be seen threading its way through the entrance to the Ulithi anchorage. At 5.52 the dull thud of another explosion was reported by I-47's sonar as coming from the atoll. It seemed that at least three of the kaiten had scored hits on something.

Whether their missions were successful or not Orita concluded that all four pilots were now dead, and at 6am he ordered a silent minute of prayer for their souls. Then he dipped his ship beneath the waters and headed for home. I-36 was not so lucky. Lieutenant- Commander Teramoto, the captain, shut Ensigns Taichi Imanishi and Yoshihiko Kudo into their kaiten from the deck shortly after midnight. At 3am Lieutenants Kentaro Yoshimoto and Kazuihisa Touozumi climbed into their craft through the access tubes. Everything seemed to be going well until I-36 reached the point designated for launching, just off the eastern entrance to the Ulithi lagoon. There, at the very moment set for firing kaiten Numbers One and Two were found to be stuck fast in their racks. They could not be freed after their engines had started. Then the pilot of kaiten Number Four reported that his craft was leaking badly. The only weapon that could be despatched was Ensign Imanishi in Number Three, who was launched at 4.54am.

Yoshimoto and Toyozumi returned to the submarine through their access tubes, and the I-36 surfaced briefly to take in Kudo. At this point the captain decided no more could be achieved, and when the I-36 submerged he turned her bow towards the open sea. Shutting off all the motors the crew listened intently. At 5.45am an explosion was heard, and at 6.05 another. Soon afterwards a pattern of depth charges rocked the I-36 and Teramoto decided it would be wise to get away from the area.

But the I-36 was compelled to stay submerged while American destroyers overhead methodically searched the area for the submarine which they thought had fired conventional torpedoes from the eastern entrance. Nineteen hours passed. By that time the air in the submarine was foul with fumes, and the crew was exhausted. No depth charges had been heard for more than an hour, and Teramoto decided that he would have to surface to get fresh air and charge his batteries. Shortly before midnight the tanks were blown and the vessel surfaced. It was dark night and as there was no sign of American ships Teramoto took a risk. Running north on the surface at maximum speed, he cleared the area without further incident.

I-36 and I-47 both got back to Kure on 30th November. On 2nd December a special conference was held on board the Tsukushi Maru, flagship of the Sixth Fleet, to consider Orita and Teramoto's reports on the kaiten attacks. Over 200 staff officers and specialists attended, and there was a lot of discussion before the results were summarized by a staff officer of the Sixth Fleet. Men on board I-47 had seen two fires, he said. And the crew of I-36 had heard explosions. Photographs of Ulithi taken by a reconnaissance plane from Truk, on 23rd November, three days after the kaiten operation, were then produced. 'From these', declared the speaker, 'we can estimate that Lieutenant Nishina sunk an aircraft carrier, as did Lieuttenant Fukuda and Ensign Imanishi. Ensigns Sato and Watanabe sank a battleship apiece!'

This was the conclusion the audience wanted to hear, and there was a great outburst of banzais. The Japanese high command had ordered kaiten to be produced in quantity, and news that the first strike had been an outstanding success was a great boost to the morale of the scores of young men in training. 'Die for the Emperor, but not in vain' was a good motto. Every embryo Kaiten pilot was positively looking forward to his death-dealing mission, when the news was circulated. The Japanese estimate of ships destroyed was a complete fabrication. The only ship sunk in the operation was the US tanker Mississinewa.

The Formidable Power of One-Hit Sinking

Damaged William D. Porter listing heavily. Landing Craft Support ships LCS(L)(3)-86 and LCS(L)(3)-122 (behind) are assisting.

On 28 May, Japan’s Navy Ministry for the first time made public the operations of the Thunder Gods, extolling them for their fighting spirit and “the formidable power of one-hit sinking.” Newspaper accounts also carried the names of 332 Thunder God pilots who had already sacrificed their lives. Despite the public adulation, most ka pilots now went about their duties under a cloud of despondency, often ignoring the frequent air raid alarms, instead staying in their quarters. Increasingly, petty officers were even sneaking off base to carouse at local inns.

For Ryuji Nagatsuka, meanwhile, May’s end marked the completion of suicide tactical training for Jun-no Special Attack Corps. Nagatsuka received his promotion to flying officer and now was in line for a posthumous promotion. Credible war news was sparse, but conditions were undoubtedly desperate. Each time flights of American Grummans headed for their base northwest of Tokyo, the pilots flew to safer airspace. Machine guns had been removed from their planes and the primary objective was to preserve them for tokko missions.

The rainy season was in full swing, and the only possible bright spot was a brief visit from his mother and two of his sisters. But even this reunion was awkward. Candor about the future went unspoken in the presence of the young girls. When Nagatsuka left the three of them at a nearby train station, he knew he was seeing them for the last time.

The horrid weather, while it curtailed American air attacks, also delayed launch of Kikusui No. 9. An announcement trumpeting the assault went out each morning only to be rescinded by afternoon. Finally, on 3 June, a break in the weather set Kikusui No. 9 in motion. The operation’s buildup vastly overshadowed its substance: in a scattered series of sorties, barely fifty suicide aircraft flew south toward Okinawa, most without escorts.

These handfuls of kamikazes were having a harder time sneaking through, and their attacks seemed to be odd sideshows. Though not any less chaotic or dangerous, the air-sea duels involved many fewer planes and ships.

On 6 June, eight bogeys set upon DMs J. William Ditter and Harry F. Bauer on patrol southeast of Nakagusuku Wan. One attacker’s wing clipped Ditter’s after stack and tore open a long strip of shell plating on the port side, flooding the after fire room and forward engine room.

A plane also crashed close to Bauer’s starboard beam, tearing a twelve-foot gash in her side. Bauer’s damage seemed to be limited to flooding, but crewmen also spotted a large hunk of metal submerged near the forward fireroom and worried it might be a bomb. After taking a look, a bomb disposal expert dispatched from Wiseman’s Cove assured Bauer’s XO Robert Morgenthau it might be the plane’s engine or its landing gear, but was no bomb.

Destroyer William D. Porter’s time off Okinawa did much to erase the stigma that plagued her CO and crew since the accidental but near disastrous torpedo shot at battleship Iowa. But then, on 10 June, bad luck caught up with Porter on RP15 when an undetected kamikaze Val dove at her through a low overcast. The plane struck only a glancing blow to Porter’s radar mast, but when its bomb exploded in the water nearby, the blast tore up the after half of Porter’s hull and unleashed uncontrollable flooding. Even the pumps on LCSs dispatched to help Porter could not stay ahead of the rising water, made worse by the explosion of several jettisoned depth charges. Porter’s sailors were finally evacuated to CO Richard McCool’s LCS-122. Lined up along LCS-122’s railings, Porter’s men watched their hard-luck ship sink at 1119.

On RP15 at dusk the next day, it was LCS-122’s turn, but almost a different kind of turn. After escaping a near miss crash by one Val, LCS-122 took a direct hit to its conning tower by a second Val. The crash and explosion killed 11 men and seriously wounded another 29, including McCool. Despite his wounds, with 122 on fire and sinking, McCool somehow managed to exit the conning tower, jumping first to the gun deck and finally the main deck. McCool rallied his crew to fight fires, hauled one man to safety, and helped rescue several others before 122 had to be abandoned.

This was to be the last kamikaze blow for a week—though by no means the last off Okinawa or the last of the war. Still it was almost a showcase—an attack that occurred in focus and isolation, instead of the thudding, relentless blur of April’s and May’s attacks (and, earlier, the attacks in the Philippines). The LCS-122’s casualties (over half the crew) and the actions of the survivors and the rookie CO somehow symbolized all the suffering, determination, and instinctive heroism displayed by thousands of men through the seemingly unending days of eight long months.

Lieutenant Richard M. McCool, captain of the landing ship LCS(L)(3)-122, received the Medal of Honor in part for assisting in rescue of survivors of William D. Porter.

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Experimental Midget Submarines

Besides the midget submarines described above, Japanese designers produced a number of experimental models which did not achieve operational status. Had they done so, they would have been deployed for the final defence of Japan – as suicidal weapons.

The only model to go into production – about 14 were said to be on hand at Kure in August 1945 – was the boat known only by the codename of U-Kanamono (“U-Type Metal Fittings”). This crude weapon was strongly reminiscent of the Confederate semi-submersibles of the War Between the States: it was an awash-boat controlled by a two-man crew housed in a squat turret on a cigar-shaped hull, some 46ft (14m) long and 6.5ft (1.98m) in beam. Displacing c.15 tons (15.24 tonnes), it was powered by a single-shaft compressed-air torpedo motor, giving a maximum speed of only c.3kt (3.45mph, 5.5kmh) and a very limited action radius. Its armament consisted of one 17.7in (450mm) bow torpedo tube.

The smallest of Japan’s midgets, the two-man, 11.5 ton (11.7 tonne) Shinkai (“Sea Vibrator”), was intended for suicidal expeditions into Allied anchorages in operations somewhat resembling those of the Italian “Pig” and British “Chariot”. It was a shallow-draught (3.9ft, 1.2m) submersible armed with a detachable, magnetic, 2,000lb (907kg) warhead, to be affixed to the hull of an enemy ship. The only unit built, codenamed 9-Kanamono (“Type 9 Metal Fittings”) and completed at Ourazaki in August 1944, was powered by a 20shp electric motor giving a maximum 9kt (10.3mph, 16.6kmh) submerged. The 41ft (12.5m) craft was both unstable and plagued by mechanical failure.

Also intended for attacks on Allied anchorages was the Type C variant known as M-Kanamono (“Type M Metal Fittings”), built at Ourazaki in late 1944. Very little larger than the Type C, and with the same engines, it had no torpedo tubes and instead carried four mines. The single unit completed is said to have been equipped with tracks for crawling along the sea bottom.

The last of the experimental midgets was the Maru-Se (“SE boat”), of which one prototype was built by Kawasaki in 1944 for the Imperial Japanese Army. This craft, of which few details survive, was powered by a Walter high-test peroxide motor, a German-developed unit similar to the hydrogen- peroxide/hydrazine engine used in the experimental Kaiten II. This gave a submerged speed of c.15–20kt (17–23mph, 28–37kmh). It was to be armed with two electrically-driven torpedoes, then under development.

“Fukuryu”: the Suicide Frogmen

Suicide divers (Fukuryu "Crouching dragons") were a part of the Special Attack Units prepared to resist the invasion of the Home islands by Allied forces. They were armed with a mine containing 15 kilograms of explosive, fitted to a 5 meter bamboo pole. They would dive and stick the pole into the hull of an enemy ship, destroying themselves in the process. They were equipped with a diving jacket and trousers, diving shoes, and a diving helmet fixed by four bolts. They were typically weighed down with 9 kg of lead, and had two bottles of compressed air at 150 bars. They were expected to be able to walk at a depth of 5 to 7 meters, for about 6 hours. 

It was estimated that the mass onslaught would destroy some 35–50 per cent of the Allied armada before any troops could be put ashore. Offshore, a last line of maritime defence would be provided by the least-known of the “special attack” forces: the demolition frogmen called fukuryu (“crouching dragons”).

Their training had begun at Kawatana in November 1944 (see here), although the IJN had employed teams of swimmers on hazardous missions since early in the war; notably at Hong Kong, where skindivers defuzed Allied mines to prepare a way for landing craft. A Japanese prisoner taken at Peleliu, Palaus, late in 1944, claimed that he belonged to a 22-strong Kaiyu unit of swimmers trained to attack landing craft. Each swimmer was armed with three grenades, a knife and a simple demolition charge: a wooden box of c.160in3 (2620cm3) packed with trinitrophenol (Lyddite) with a fuze cut to the required length. But the kaiyu units, credited with damaging an LCI in the Palaus and a DE and an attack transport at Okinawa, were surface swimmers rather than frogmen.

The fukuryu appear never to have been deployed outside the home islands. Their role in the final defence would have been suicidal – as was, to some extent, their training. Their equipment – a loosely-fitting wet suit; a clumsy helmet not unlike that of a deep-sea diver; bulky air circulation and purification tanks strapped to chest and back and linked by a tangle of hoses – was most unsatisfactory. “There were very many [fatal] accidents during the training of fukuryu”, a Japanese veteran told me, “because the twin-tank oxygen re-breathing equipment was no good – but nothing better was available”. Nevertheless, some 1,200 fukuryu graduated from Kawatana and Yokosuka Mine School by the war’s end, when 2,800 were still in training.

To destroy inshore landing craft, each fukuryu was armed with a 22lb (10kg) impact-fuzed charge, incorporating a flotation tank, mounted on a stout pole (much like the anti-tank “lunge mine” described above). If his equipment functioned perfectly, the frogman could stay at an optimum depth of 50ft (15m) for up to 10 hours, sustained by a container of liquid food. Construction was begun of underwater pillboxes, concrete with steel doors, in which fukuryu would shelter from a pre-landing bombardment while awaiting their opportunity to sally forth and thrust their explosive lances against the bottoms of landing craft.

The fukuryu would form part of a network of beach defence. Farthest from the beach were moored mines, electrically detonated from ashore; then three lines of fukuryu deployed so that each man guarded an area of c.470sq yds (390sq m); then lines of magnetic mines; and finally beach mines. Capt K. Shintani, commanding the fukuryu, was somewhat optimistic in hoping that his men might “cause as much damage as the kamikaze aircraft”.