After a long dry spell (including four months without home internet access during which I was like an addict going through withdrawals), I'm back with this little offering.
as completed in 1945.
The Sen-Ohka boats were originally planned to be additional Sen-Toku-class submarines, but the design of the Ohka bomb caused several designers to consider the possibility of launching them from a ship. As such, the hulls of seven Sen-Toku-class submarines were transferred to the new class (named I-500
) and construction proceeded.
The new submarines were truly massive: the aft deck gun had been removed, providing for the elongation of the aircraft hangar, and the conning tower that had originally been offset to port was widened and extended the width of the hangar, but the access hatch was still in its original location. The widening of the conning tower actually stabilized the submarine, for the AM- and Sen-Toku-class boats, whose conning towers remained offset, were notorious for instability in rough seas. The original design provided for a walkway around the conning tower on the roof of the hangar, but submarine I-503
's design deleted the walkway; her conning tower and hangar were all one piece, with a small access hatch forward for access to the forward triple-mount 25-mm. antiaircraft gun.
as completed in 1946. Note the modified conning tower.
The elongated hangar now had space to carry seven Yokosuka MX7Y Model 43 Ohkas. Launch crews for the suicide weapons began training in January 1945 on simulated launch areas at the Kaiten depots. The Ohkas were brought out of the submarine's hangar on carts and rolled into position in front of the catapult like the Sen-Toku-class boat's Seiran aircraft; after their wooden wings were folded down into launch position and a final check-off, the hangar door was shut and sealed to protect the crews from the blast effects of the rocket engines, and the Ohka was launched.
The first boat of the class to be commissioned was I-501
on 5 May 1945. However, the giant submarines were not sent out on patrols right away; Combined Fleet HQ ordered them held back from combat to receive more training so that when the time came for the 'Decisive Battle' they would be ready. That time came on 28 May 1946 with the aftermath of the Battle off Northern Honshu. Four of the huge submarines (I-504
was still in training and I-505
had only been commissioned that month, so neither participated in the battle) positioned themselves in different areas and launched their suicide weapons against the U.S. Third Fleet the day after the Americans had all but annihilated the Japanese surface fleet.
The Ohkas came in at nearly wave-top level, and were so fast that patrolling U.S. fighters had little chance to intercept them. The first Ohka, piloted by Lt (j.g.) Nishizumi Hiromatsu from Cdr Hashimoto Mochitsura's I-503
, streaked in from the north and headed straight for the heart of the U.S. fleet. As personnel aboard various ships watched in horror, Nishizumi's 2,000-pound rocket-powered bomb slammed into the bridge of the carrier Franklin D. Roosevelt
, sending up a gigantic fireball. Commander Task Force 58 Vice Admiral Ralph Davison, his staff, and most of the carrier's senior officers were killed instantly. Things only got worse as two more Ohkas from I-503
roared in; the second Ohka pilot, seeing the flames on the Roosevelt
, headed for the same ship, slamming his aircraft into the forward-most starboard 5-inch gun mount and starting another massive fire. The third Ohka was even more devastating: its pilot used his aircraft like a giant torpedo, coming in at wave-top level and striking the Roosevelt
abreast her starboard engine room. Secondary explosions touched off one of the nearby avgas tanks, turning the stern of the ship into an inferno. Shattered, Roosevelt
capsized to starboard and sank at 1528 with severe casualties. The chaos caused by the three suicide weapons allowed for an Ohka from I-501
to sneak in; the pilot of this bomb apparently decided - correctly - that the Roosevelt
was finished and so chose a different target. The effect of a 2,000-pound rocket-propelled bomb on a carrier was bad enough; on a heavy cruiser the effect was nothing short of devastating. The fourth Ohka slammed into the deck of heavy cruiser Pittsburgh
abreast her forward turrets with enough force to penetrate to the magazine, and the Baltimore
-class vessel disintegrated.
The news of the sinkings of Roosevelt
fell upon the flagship of Third Fleet, battleship Georgia
like a ton of bricks. FADM Halsey ordered his ships into tighter formation and for the escorting destroyers of both the surface and carrier groups to keep "...an eagle eye out for the (expletive) (racial slur) buzz bombs!" Not ten minutes after he'd said that, his own group of ships got introduced to the chaos of an Ohka attack. An Ohka from I-502
screamed in from the south, zipping through the formation and straight into the port quarter of light carrier Monterey
. The bomb ripped straight through the carrier's thin armor, setting off her aft magazine and producing an explosion that made the death throes of her late sister ship Princeton
look tame in comparison. Within twenty minutes of the hit, Monterey
pointed her bow skyward and sank stern-first. Shortly thereafter, I-500
made its appearance known in a big way; its first Ohka attacked Halsey's flagship Georgia
, slamming into the port side of the battleship's No. 3 turret. The turret was badly damaged and shrapnel went everywhere, but Georgia maintained station. Light cruiser Topeka
wasn't as lucky; another of I-502
's Ohkas dove on her, but its pilot misjudged his angle and hit the water alongside instead. Even so, the force of impact was enough to set off his bomb, which blew a twenty-foot hole in the cruiser's side, the underwater shock wave severely straining her keel. She went dead in the water, and Halsey was forced to detach her with three destroyers back to Ulithi for repairs.
News of I-500
's attack on battleship Georgia
was received with mixed emotions; disappointed he had seemingly not even damaged the battleship, Cdr Tanabe Yahachi decided to break off and search for easier prey. His persistence paid off when I-500
picked up a message from an I-402
search plane, reporting a task group of six escort carriers with support ships within striking range of the suicide weapons. Tanabe immediately surfaced and began launching his Ohkas. The attack upon the escort carrier group was nothing short of devastating, and just what the Japanese dreamed the suicide weapons would do: escort carriers Block Island
, Kula Gulf
, Salerno Bay
, and Sicily
were all hit and sunk, the thinly-armored vessels no match for the powerful bombs.
The Ohka attacks continued throughout the day; Hashimoto's I-503
sent in three more bombs to attack Task Force 58, two of which hit and mortally wounded the carrier Leyte
, while the third struck the light cruiser Duluth
, causing explosions which tore her apart. The submarine's last-remaining Ohka failed to launch when its engine refused to start. Unwilling to risk his boat by taking the time to push the weapon back into the hangar, Tanabe ordered it dropped over the side via the collapsible crane before departing for Japan. I-502
launched a further two Ohkas against Halsey's fleet, damaging battleships Kentucky
and New Hampshire
, but neither ship was sunk. I-501
waited until sunset to attack, launching four Ohkas that caught one Task Group in the midst of refueling. An Ohka slammed into fleet oiler Pecos
amidships, and the unarmored vessel disintegrated when her full load of fuel went up in a titanic fireball. The explosion also severely damaged carrier Randolph
, whom she was alongside refueling. Two more Ohkas hit Randolph
herself, and when combined with the damage from the exploding oiler, mortally wounded the ship. The last Ohka struck destroyer Lewis Hancock
amidships, blowing her in half. The horrors of the day were still not over; I-502
crept in close to the U.S. fleet and sighted her previous Ohka target, the crippled Topeka
. Three conventional torpedoes finished off the limping light cruiser, but her escorting destroyers were quick to pin the big submarine with sonar and blow her apart with depth-charges. Early the following morning, I-501
surfaced and launched her two remaining Ohkas for a dawn strike on the U.S. fleet, but was surprised by a flight of three Avengers, one of which dropped an acoustic homing torpedo and scored a fatal blow. Her two Ohkas found the U.S. fleet, but due to limited visibility attacked destroyers instead of higher-value targets. The two ships hit, destroyers Twiggs
, were both sunk.
The submarine-based Ohka attack came to be known in the U.S. Navy as the "Darkest Day of May". Halsey, livid at how badly his fleet had suffered, ordered the 'buzz-bomb boats' found and sunk at all costs. But even though Combined Fleet Headquarters had envisioned great plans for the Sen-Ohka boats, the war was rapidly drawing to a close, and aside from this one action, the Sen-Ohka boats contributed little else to the outcome of the war. I-504
was sighted by the submarine Besugo
coming out of the Inland Sea on her first combat sortie on 29 June 1946. The Japanese submarine's conning tower and hangar structure seemed so large, Besugo
's CO thought the submarine was a troop transport. Two torpedoes blew I-504
to pieces. I-505
deployed on her first sortie on 24 July 1946, only to be sunk two days later by U.S. aircraft as she was recharging her batteries. The last unit of the class, I-506
- the only other boat of the class to be completed to the same conning tower design as I-503
- was commissioned on the same day her sister ship I-505 was sunk, 26 July 1946. She was never used operationally; on 13 August, she was caught in a massive U.S. air raid on Sasebo and hit amidships by an aerial torpedo, causing her to roll onto her port side and sink in the harbor. Her wreck was scrapped post-war.
The two survivors of the class, I-500
, surrendered in Kure. Both submarines served in the post-war fleet as squadron flagships, their Ohka hangars remodeled into staff headquarters and sleeping areas. Both were decommissioned and scrapped in 1965. The conning tower tarps and battle flags of both submarines are preserved; I-500
's are on display at the Yokosuka Fleet Museum, while I-503
's are preserved in Kyoto, birthplace of her CO, the late VADM Hashimoto.