Submarine tender
After Heian Maru returned to Japan in August 1941, NYK was informed that due to rising tension between Japan and the United States, the liner would be converted to military use. On 3 October the ship was formally requisitioned by the Imperial Japanese Navy, and designated an auxiliary submarine tender with the Yokosuka Naval District. Two weeks later conversion was begun at Mitsubishi Heavy Industries in Kobe. Amongst numerous other alterations, four 15 cm/50 41st Year Type naval guns, two dual-mount 13 mm AA guns, two searchlights, and a rangefinder were installed.[1]
The outbreak of World War II in the Pacific on 7 December 1941 (8 December in Japan) found Heian Maru still being refitted, but by the end of the month she was on her way to Kwajalein to take up a new posting with IJN 6th Fleet. In early February 1942, while at Kwajalein, Heian Maru's crew got their first experience in combat during raids launched from the American aircraft carrier USS Enterprise (CV-6).
Throughout 1942 and into the early months of 1943, the Heian Maru shuttled between Japanese bases at Truk (later known as Chuuk Atoll), Rabaul in the Solomon Islands, and Yokosuka and Kure, in Japan. She performed her designated task of supplying the dozen submarines of the IJN 6th Fleet with torpedoes, provisions, spare parts, and replacement crewmen, but, with her capacious holds, was also used as a troop and general cargo transport. At Rabaul in January 1943 Heian Maru was caught in two major Allied aerial attacks, during which she narrowly avoided bomb hits.[1]
On 2 June 1943 Heian Maru was reassigned to the IJN 5th Fleet and arrived at Paramushiro in the Kuril Islands to support operations in the Aleutian Islands. She was used as a floating command post for the secret successful withdrawal of 5,000 Japanese troops from the island of Kiska, then returned to Yokosuka on 14 August and was returned to the IJN 6th Fleet.
Over the next several months, Heian Maru was busy transporting troops, vehicles, and other supplies of the IJA 17th Infantry Division from Shanghai to Truk and Rabaul. During a brief refit at the Yokosuka Naval Arsenal, her four 152 mm guns were replaced by two Type 10 120 mm guns, two Type 96 anti-aircraft guns and a Type 2 sonar. She was repainted in a dazzle camouflage pattern. Afterwards, she ferried torpedoes, distilled water, and other cargo to Truk, and, on 19 November, had a tense encounter with the American submarine USS Dace (SS-247) which tested her new commander, Captain Tamaki Toshiharu. She spent December 1943 and January 1944 disbursing supplies to submarines and other ships of the Combined Fleet at Truk lagoon.
The Japanese naval base at Truk was a large, sprawling complex, with hundreds of vessels anchored among dozens of islands, surrounded by a protective coral reef. The islands were studded with airfields, hospitals, repair shops, storage sheds, fuel depots, and command facilities. It was defended by coastal guns in concrete casemates, hundreds of fighter planes, and hundreds of anti-aircraft guns of all types, both on ship and shore.[10] Heian Maru was moored next to her sister ship Hikawa Maru (in wartime service as a hospital ship) on the leeward side of Dublon Island when, on the morning of 17 February 1944, the Americans launched Operation Hailstone.
Carried out by the US Navy's Task Force 58, with nine aircraft carriers, under the command of Vice Admiral Marc A. Mitscher, Hailstone was a massive two-day combined air-surface-submarine raid. Although the IJN had moved its aircraft carriers and battleships from Truk a short time earlier, their defenses were unprepared for the scale of the attack, and the remaining navy and merchant vessels were devastated by wave after wave of American warplanes. Heian Maru quickly put to sea and went into evasive maneuvers north of Dublon Island - with Vice Admiral Takeo Takagi and his Sixth Fleet staff on board - but as one of the largest targets in the lagoon, enemy attacks were relentless. At mid-morning two bombs fell close astern, damaging one of her propeller shafts and flooding an aft hold. The crew managed to correct the trim by pumping fuel to her bow tanks, and after sunset Heian Maru returned to Dublon, where Admiral Takagi and some of the ship's cargo of Type 95 torpedoes were offloaded.[1]
Early the following morning, 18 February 1944, Heian Maru got underway as the American aerial attacks resumed. Shortly after 0300 she was struck, in quick succession, by two pairs of bombs; fire engulfed the bridge and threatened the hold containing the remaining torpedoes. The wounded ship began sinking, and at about 0500 Captain Tamaki gave the order to abandon ship. Most of the crew, including Tamaki, reached the shore safely, but a total of 18 men were killed, and 25 wounded.[1]
At 0900, Grumman TBF Avenger torpedo bombers from USS Bunker Hill (CV-17) attacked the still-burning Heian Maru, a torpedo striking her amidships on the port side. She sank soon after, coming to a rest on her port side in about 110 feet of water.
On 31 March 1944 Heian Maru was officially removed from the navy list.[1]
