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Type A ko-hyoteki

In describing the craft, all measurements for the submarine are reported with metric measurement and in the English system as many of the American and Australian analyses utilized that system.

The Type A ko-hyoteki is 23.90 m (78 ft. 5 in.) in length, with a maximum beam (in the center section) of 1.850 m (5 ft. 11 in.) and the maximum height, from the keel to the upper edge of the conning tower is 3.100 m (9 ft. 10 in.). The craft displaced 46 long tons submerged. The basic form of the submarine is round except at the bow, where the sides taper to form an oval, and at the stern, where the diameter decreases to form a point at the propeller shaft gland. The hull “had the form of an enlarged torpedo with a conning tower.” The external fittings of the boat were few; in addition to the propeller and torpedo guards and a 9.5 m long, (50 mm x 12 mm) was welded to the bottom of the hull, the only projections aside from the conning tower were mooring cleats cut from 12mm steel welded fore and aft.

The Royal Australian Navy analysis of the damaged HA-14 and HA-21 from the Sydney attack noted that the Type A submarine is “in general proportions…similar to a torpedo, i.e. overall length approximately twelve times the diameter; circular cross section for the greater part of its length; tail similar to British torpedo with vertical and horizontal fins and rudders, and right and left handed propellers.”

The conning tower, as described for HA-19 in 1941, contains a “small tube leading to hatch which can be opened from inside only,” a single periscope, a “vertical rubber covered radio antenna 32” high just forward of the conning tower hatch,” two “white lights, one forward (screened), one aft,” U-frame fairing periscope shears from “forward net cutting clearing line to top of sheers,” and the battery ventilation exhaust.

As designed and built, the Type A boats are single-hull craft constructed in three sections which bolted together. Each section was joined by 2-3/8 inch (60mm) flanges with threaded bolts 13/16-inches (20.6mm) in diameter. A rubber gasket separated each section between the flanges.

The three parts to each of the submarines are the 1) forward section (17ft/5.18m long), 2) the center (control) section (34 ft, 11-inch/10.64m long), and the 3) aft section (22 ft., 4-inch/6.8m long). The bow section’s primary function is to house the two torpedo tubes and the necessary equipment to fire the torpedoes. The center section contains the control compartment as well as two fore and aft battery compartments. The stern section contains the electric motor and gearing for the propeller and a free-flooding aft ballast tank. An inventory and description of each compartment was laid out by the U.S. Navy as part of the examination of HA-19:

  • FORWARD BATTERY -- contains:
    • H.P. air and oxygen flasks on port side
    • One fourth of entire battery
    • 90.5 gallon trim tank under battery
    • Air purification
    • 284 lead pigs on port side forward weighing 3133 lbs

  • CONTROL ROOM -- contains:
    • All depth and ship control instruments
    • Small crystal controlled radio
    • Periscope
    • Torpedo tube controls
    • Gyro compass
    • Electrically actuated directional gyro
    • Small electric trim pump
    • H.P. air manifold
    • Small regulator tank
    • Hydrogen detector

  • AFTER BATTERY -- contains:
    • 3/4 of entire battery (36 cells)
    • Sound equipment
    • Air conditioning apparatus
    • Air purification
    • 56.5 gallon trim tank under battery

  • MOTOR ROOM -- 24'10½" long to end of propeller hub, bolted to center section -- contains:
    • Motor control panels
    • Motor
    • Gear box
    • Small free flooding tank
    • Tail assembly

  • CONNING TOWER -- 5'6" high to top of periscope shears, mounted directly over control room, contains:

    • Small tube leading to hatch which can be opened from inside only
    • Periscope
    • Vertical rubber covered radio antenna 32" high just forward of conning tower hatch
    • Two white lights, one forward (screened), one aft
    • Telephone jack connection for outside communication
    • A U-Frame fairing periscope sheers from forward net cutting clearing line to top of shears
    • Battery ventilation exhaust
    • No bridge

As part of the attack on Pearl Harbor on December 7, 1941, the Imperial Japanese Navy sent an attack group of submarines to surround Oahu and sink ships attempting to flee. Five of the submarines carried top-secret "mini submarines." These submarines, each armed with two torpedoes and carrying two crew members, were to penetrate inside the harbor under cover of darkness before the attack began. They were to surface and fire their torpedoes during the aerial attack. Then, they would dive and escape the harbor, and rendezvous with their "mother submarines," again under cover of darkness the night of December 7.

While the aerial attack was devastating, the mini submarines failed in their mission. Only one made it into the harbor, and it was quickly sunk during the attack that morning. Another submarine washed ashore on the morning of December 8, and its surviving crew member, Kazuo Sakamaki, was captured along with his craft. The submarine was studied and then toured the U.S. to promote the sale of War Bonds. It is now on permanent display at the National Museum of the Pacific War in Fredericksburg, Texas.

At 8:30, inside the harbor, the minesweeper USS Zane (DMS-14) spotted what it reported as a “strange submarine” 200 yards aft of the moored USS Medusa (AR-1); Zane’s commanding officer ordered the ship’s No. 4 gun loaded and prepared to shoot at it, but the gun would not bear, and the nearby USS Perry (DMS-17) instead opened up with its gun. Medusa, meanwhile, had opened up with its antiaircraft guns at incoming Japanese planes. As the ship’s commanding officer noted in his after action report, his crew spotted the submarine near them, bearing some 1,000 yards off Medusa’s starboard quarter and about 500 yards astern of the seaplane tender USS Curtiss (AV-4).

Curtiss’ commanding officer reported that his ship opened up on the submarine at 8:36 from a distance of 700 yards. The first shots missed the periscope, but as the submarine surfaced at 8:40, and the conning tower was visible, as well as part of the bow, Curtiss fired twice, reporting they hit the conning tower twice as the submarine fired one torpedo up the North Channel toward the oncoming destroyer USS Monaghan (DD-354).

Monaghan’s commanding officer, in his after action report, explained that as his destroyer approached the scene, he and his bridge crew spotted the conning tower of the submarine 200 to 300 yards off the starboard quarter of Curtiss, and that “vigorous fire” from Curtiss and USS Tangier (AV-8) was directed at it. The ship’s captain, W.P. Burford, ordered all engines ahead at flank speed and headed straight for the submarine to ram it, and at 8:43, the destroyer rammed and passed over the submarine while dropping two depth charges. A minute later, both charges exploded aft of Monaghan, and the submarine disappeared. Another of the ko-hyoteki had been destroyed.

The position of the submarine sunk by Monaghan was charted and Navy divers rigged it for recovery within a few weeks of the attack. A heavy lift crane and barge lifted the submarine from the bottom and placed it on a barge for inspection on December 21. Navy explosive ordnance disposal experts boarded the barge and found “the upper part of the front end (the torpedo tube section) was completely gone. There was nothing left of the tubes or their torpedoes. Below the tubes the hull seemed to have been blown outwards to where it was almost flat.”

The torpedoes, they surmised, had exploded in their tubes, “destroying the upper forward section of the midget submarine hull.” The examination noted other damage, including ramming damage to the aft hull; “it appeared that the sub had been rolled completely over and the rear end of the boat had been bent badly out of line. Overall the body of the boat showed the wrinkles and indentations typical of exposure to heavy explosions underwater.” The conning tower “clearly showed the path of a five-inch projectile completely through the conning tower.” With nothing to disarm, the EOD team left and the barge was moved to the sub base, where construction of a new pier provided a burial site for the craft and crew. Marine Cornelius Smith, Jr., visiting the base, saw the submarine there, and commented that “the conning tower is tiny, with room enough for one man to steer the craft. He’s still in there. One of our ships rammed him, broadside, and he’s caught, dead, with his legs hanging out; I guess they’ll have to cut him free with a blowtorch. I suppose the torpedoman inside is dead, too. “

After some items were recovered from the craft, including the still intact figure-eight torpedo tube guards, the crane barge “dumped the wreckage along the back edge of the pier where they were filling in to level the shoreward edge of the pier” after the two dead crew members were removed and buried in a nearby cemetery. In March 1947, a Japanese naval lieutenant’s shoulder patch was repatriated to Japan by the U.S. Navy and was said to have come from the dead submarine commander. The insignia suggests to the Japanese that this craft therefore was that of Lt. Naoji Iwasa, commander of the Special Attack Flotilla and whose midget was launched from submarine f I-22. That midget was crewed by Iwasa and Petty Officer 1st Class Naokichi Sasaki. The patch is now preserved at Yasukuni Shrine. The Monaghan midget resurfaced briefly in 1952 when workers improving the quay wall at the sub base hit the buried midget with a dragline while excavating landfill to drive new sheet piles. After digging a deeper trench alongside the exposed hull, they rolled the boat over and reburied it to clear the work area.

A third submarine was observed more than an hour before the attack trying to follow a U.S. ship into the harbor. It was quickly engaged and sunk by the crew of the destroyer USS Ward. The two other mini submarines disappeared.

The mystery of where the both missing midgets were was partly resolved in 1960 with the discovery of the fourth Pearl Harbor ko-hyoteki in the shallows of Ke'ehi Lagoon by U.S. Navy diver SK/1 C.F. Buhl during a long distance training dive on June 13, 1960. The sub was resting in 76 feet of water, torpedoes in their tubes, and as it was later learned, the hatch was undogged. The Navy salvage vessel USS Current (ARS-22) raised the submarine on July 13, 1960, where it was inspected and found to have experienced depth charging as evidenced by “bent piping, a door twisted off its hinges, her large electric motor torn from its mountings, and much shattered glass” (Stewart 1974:62). No human remains were encountered, but the demolition charge and the still live torpedoes gave naval officials pause. The bow section, with torpedoes, was unbolted, taken to sea and dumped.

The submarine was returned to Japan at the request of the consul general in Hawai'i, and it was transferred, minus the bow, to the Japanese LST Shiretoko on June 19, 1961. In Japan, the recovery of some artifacts, including a boot, a flying suit (the “uniform” of the midget crews) and tools, as well as a glove, led the Japanese Midget Submarine Association to determine it was likely I-18 tou, commanded by Lt. (jg)Shigemi Furuno and crewed by Petty Officer 1st Class Shigenori Yokoyama. A number of destroyers reported submarine contacts and actively depth charged them in this general area, and so it is probable that a damaged and out of condition I-18 tou was abandoned by its crew, who may have survived, but most likely died after leaving their flooded, sunken craft. Once in Japan, the battered ko-hyoteki was refurbished as a land-based exhibit with a mock up of its bow placed on it. Dedicated on March 15, 1962 in a special ceremony, the midget has been displayed ever since at the former Naval Academy at Eta Jima, outside of Hiroshima, in front of the Naval Tactical School.





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