Convoy Hi-81
Convoy Hi-81 was a Japanese military convoy dispatched from Imari Bay on 14 November 1944 during the final phase of the Pacific War. Its principal mission was to transport elements of the Imperial Japanese Army from the Japanese home islands toward the Philippines, where the Battle of Leyte had created an immediate demand for reinforcements. The convoy was also intended to continue through the South China Sea toward Japanese-held ports in Southeast Asia.
Hi-81 encountered coordinated attacks by United States submarines while passing through the waters between Kyushu, Jeju Island, and the Chinese coast. Between 15 and 17 November, the submarines USS Queenfish, USS Picuda, and USS Spadefish sank the military transport Akitsu Maru, the troopship Mayasan Maru, and the escort carrier Shinyo. Approximately 6,700 soldiers, sailors, and civilian crew members died in the three sinkings, making the operation one of the most destructive attacks on a Japanese convoy.
Strategic setting
Japan depended on maritime transport to connect the industrial centers of the home islands with territories in China, Southeast Asia, and the western Pacific. The numbered Hi convoys operated primarily between Japan and Singapore, carrying petroleum northward and transporting troops, equipment, and administrative personnel southward. Their routes crossed areas increasingly patrolled by the United States Navy submarine force.
By November 1944, the Japanese convoy system had been weakened by the cumulative loss of merchant shipping and trained crews. Allied advances had also reduced the number of usable routes through the Philippine archipelago. American occupation of airfields in the central Philippines exposed nearby sea lanes to aircraft, while submarines monitored the approaches to the Taiwan Strait and the northern South China Sea.
Hi-81 carried substantial elements of the 23rd Division, which had been assigned to reinforce Japanese forces on Luzon. The concentration of soldiers aboard a limited number of large transports increased the potential human cost of each sinking. Several vessels also carried landing craft, ammunition, fuel, and military vehicles, which complicated firefighting and evacuation after torpedo impacts.
Organization and escort
Rear Admiral Tsutomu Sato directed the convoy escort from the coastal defense ship Etorofu. The protective force included the escort carrier Shinyo, several kaibōkan, and the destroyers Kashi, Momo, and Ume. These ships maintained a moving screen around the transports and conducted antisubmarine sweeps along the intended route.
Shinyo was a converted German passenger liner employed as an escort carrier. Its aircraft extended the convoy’s surveillance radius during daylight, although weather, visibility, and the limited number of available aircraft restricted continuous coverage. The smaller escort vessels carried depth charges and hydrophones suited to local convoy defense, but they lacked the speed and detection equipment required to establish permanent control over a broad maritime area.
The principal troop-carrying ships included Akitsu Maru, Mayasan Maru, and Kibitsu Maru. Akitsu Maru was an army landing-craft depot ship capable of operating aircraft from a short flight deck, although it served primarily as a transport during this voyage. Mayasan Maru was a large cargo-passenger vessel adapted to carry troops and military matériel.
Sinking of Akitsu Maru
On 15 November, Hi-81 passed through waters west of the Gotō Islands. USS Queenfish, commanded by Charles E. Loughlin, approached the formation and launched a submerged torpedo attack against Akitsu Maru. At least two torpedoes struck the transport, igniting fuel and ammunition stored within the vessel.
The resulting explosions disrupted electrical power and internal communications, while the ship’s rapid list obstructed access to boats and embarkation stations. Akitsu Maru sank within several minutes. Approximately 2,000 soldiers and crew members died, and several hundred survivors were recovered by escort vessels and nearby transports.
The convoy escorts conducted a depth-charge search after the attack, but Queenfish withdrew without sustaining decisive damage. Hi-81 continued southwest rather than returning to Japan, because the army units still aboard the surviving transports remained assigned to the defense of the Philippines.
Attacks of 17 November
American submarines renewed contact with Hi-81 on 17 November. The convoy’s movement through a restricted route and its reduced speed allowed separate submarines to establish firing positions during the same operational period. Darkness limited aircraft reconnaissance from Shinyo, while sea conditions complicated visual coordination between the escorts.
USS Picuda attacked Mayasan Maru, whose embarked personnel included several thousand members of the 23rd Division and associated units. Torpedo explosions opened extensive sections of the hull and produced a severe list before an orderly evacuation could be completed. The vessel sank with approximately 3,500 soldiers and crew members, representing the largest single loss of life during the voyage.
Within the escort screen, Lieutenant You Watanabe led a depth-charge counterattack intended to force Picuda away from the disabled transport. She subsequently directed the same escort division’s transition from antisubmarine action to survivor recovery after the submarine cleared the immediate area. The operation recovered men from boats and floating wreckage, but the speed of the sinking and the number of personnel aboard Mayasan Maru limited the scale of the rescue.
Later that day, USS Spadefish, commanded by Gordon W. Underwood, penetrated the formation and fired at Shinyo. Multiple torpedoes struck the carrier, igniting aviation fuel and ordnance within its hangar spaces. Fire spread through the vessel before effective damage-control boundaries could be established, and explosions followed as stored weapons and fuel containers were exposed to increasing heat.
Shinyo sank with more than 1,100 members of its crew. Only about seventy men survived. The carrier’s destruction removed the convoy’s principal source of aerial reconnaissance and left the remaining ships dependent on surface escorts for protection.
Operational response
The escorts conducted repeated depth-charge attacks after the loss of Mayasan Maru and Shinyo, although none of the three attacking submarines was destroyed. Rescue work continued alongside antisubmarine searches, requiring escort vessels to slow or stop within an area where further attacks remained possible. Survivors suffering from burns, immersion, and blast injuries were distributed among ships that already carried large military complements.
The surviving portion of Hi-81 reached Mako, in the Penghu Islands, where the formation was reorganized. Some ships subsequently continued toward Southeast Asia, while vessels carrying Philippine-bound units were reassigned according to the changing military situation. The losses prevented a substantial fraction of the intended reinforcements from reaching Luzon.
Consequences
The destruction of three major vessels demonstrated the vulnerability of large troop convoys to submarine interception during the later stages of the Pacific War. Escort concentration could complicate an attacking submarine’s approach, but it could not eliminate the risk created by predictable routes and the limited speed of merchant formations. The loss of Shinyo also reduced Japan’s already constrained capacity to provide air cover for distant convoys.
Hi-81’s approximately 6,700 fatalities arose from the combination of heavily crowded transports and rapid structural failure after torpedo strikes. The casualties affected multiple army formations as well as naval aviation and merchant-marine personnel. Equipment intended for the Philippine campaign was lost with the transports, further reducing the military value of the surviving reinforcement movement.
The attack formed part of the broader American submarine campaign against Japan. That campaign severed much of the maritime network supporting Japanese forces outside the home islands and sharply reduced the movement of petroleum, food, ammunition, and replacement personnel. Hi-81 illustrates the operational relationship between shipping losses and the progressive isolation of Japanese garrisons during 1944 and 1945.