On 28 July 1943, Ro-103 transmitted a brief sighting report from somewhere north of New Georgia. Then nothing. She had been at sea sixteen days. She carried forty-three men. No survivors were ever found, no wreckage identified, no enemy claim convincingly matched her last known track. Eighty years later, the circumstances of her loss remain unresolved - one of dozens of Japanese submarines that simply ceased to transmit during the savage Solomons campaign and joined a long undersea roster of the missing.
Ro-103 was not one of the long-range hunters that harried the American coast. She was a coastal submarine of the Ro-100 class, derived from the earlier Kaichu type, built for island warfare rather than oceanic patrol. Sixty meters long, six meters in the beam, displacing just 601 tons on the surface, she could dive to seventy-five meters and carry eight torpedoes. Two 500-horsepower diesels pushed her along at 14 knots in daylight; submerged, her battery ran a pair of electric motors at a crawling eight knots. Her range was modest - 3,500 nautical miles surfaced, barely sixty underwater - but the war she was built for did not require more. The Solomon Islands, the Bismarck Sea, the approaches to New Guinea: this was her theater, measured in hundreds of miles rather than thousands.
Her keel was laid on 30 June 1941 at the Kure Naval Arsenal, still months before Japan's war became the world's war. She went into the water as Ro-103 on 6 December 1941 - the day before the attack on Pearl Harbor, though her builders had no way of knowing it. Commissioning came nearly a year later, on 21 October 1942, by which point the tide of the Pacific war was already beginning to shift. She ran workups with the Kure Submarine Squadron through the end of the year, then in January 1943 joined Submarine Squadron 7 of the Eighth Fleet. Her route south took her through Truk - the great Japanese fleet anchorage in the Carolines - and on to Rabaul, the fortress base on New Britain from which the southern offensive had been waged. She arrived on 8 February 1943.
Her war began badly. The first patrol east of Port Moresby turned up nothing. Then came the disaster of the Bismarck Sea, where Allied aircraft and PT boats annihilated a Japanese convoy carrying the 51st Division toward Lae, sinking all eight transports and half their destroyer escort. Ro-103 was ordered out from Rabaul on 7 March 1943 to rescue survivors. In the darkness of the following night, she ran onto an uncharted reef off Kiriwina in the Trobriand Islands. Her crew dumped food, supplies, and torpedoes overboard to lighten her. A strange destroyer passed in the distance and her commander ordered the code books thrown over the side. When that wasn't enough, they pumped the diesel fuel and the fresh water. She finally floated free on 11 March and limped back to Rabaul. The rescue mission had rescued no one.
Over the next three months, Ro-103 made routine patrols near Guadalcanal and east of the Solomons, all of them uneventful. Then on 23 June 1943, fifty nautical miles south of San Cristobal, she finally found what she had been built to kill. A convoy of three transports with three destroyer escorts crossed her scope. She attacked. One cargo ship went down. A second was so badly damaged that her own destroyer escort had to scuttle her. Six days later, surfaced after sunset to charge her batteries, Ro-103 spotted seven Allied ships south of Gatukai Island - almost certainly part of the invasion force massing for the New Georgia landings on 30 June. She did not attack. The moment passed. She returned to Rabaul on 4 July, her one successful patrol complete.
After a week in port, she sailed on her fifth patrol. Orders shifted mid-voyage from Rendova to Vanga Bay off Vangunu. Between 15 and 24 July she sighted Allied forces three times, never closing to firing position. On 28 July 1943 she transmitted a sighting report from north of New Georgia. That transmission was the last thing anyone heard from her. The Imperial Japanese Navy waited - two weeks, then three - and on 10 August declared Ro-103 presumed lost with all forty-three men aboard. On 1 November 1943 she was formally struck from the Navy list. No Allied anti-submarine unit ever filed a claim that matches her last reported position and date. She may have hit a mine, or struck another uncharted reef, or suffered some catastrophic failure below the surface. Forty-three men - most of them in their twenties - disappeared with her into the warm black water of the Solomon Sea.
Ro-103's last known position was approximately 8.33 degrees South, 150.75 degrees East - the Solomon Sea between the Trobriand Islands and the Louisiade Archipelago, east of Papua New Guinea. At cruising altitude above 10,000 feet on clear days the island chains appear as scattered green stepping stones across deep blue. Nearest airfields are Misima Island (AYMS) to the southeast and Gurney Airport at Alotau (AYGN) on the PNG mainland. The waters here remain unsurveyed in many places - the same reefs that snared Ro-103 still wait for unwary navigators.