The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth. They displaced 11150NaN0 surfaced and 14470NaN0 submerged. The submarines were 80.5m (264.1feet) long, had a beam of 7m (23feet) and a draft of 4.07m (13.35feet). They had a diving depth of 80m (260feet).[1]
For surface running, the boats were powered by two 21000NaN0 diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600hp electric motor.[2] They could reach 19.75kn on the surface and underwater. On the surface, the K6s had a range of 11000nmi at ; submerged, they had a range of 45nmi at .
The boats were armed with four internal bow 53.3cm (21inches) torpedo tubes and carried a total of ten torpedoes. They were also armed with a single 76.2mm L/40 anti-aircraft gun and two single 251NaN1 AA guns.[3]
Ro-42 was laid down as Submarine No. 208 on 27 April 1942 by the Sasebo Navy Yard at Sasebo, Japan.[4] She was renamed Ro-42 on 25 September 1942 and was attached provisionally to the Maizuru Naval District that day.[4] She was launched on 25 October 1942 [4] and was completed and commissioned on 31 August 1943.[4]
Upon commissioning, Ro-42 was attached formally to the Maizuru Naval District and assigned to Submarine Squadron 11 for workups.[4] On 30 November 1943, she was reassigned to Submarine Division 34 in the 6th Fleet,[4] and on 4 December 1943 she departed Maizuru, Japan, bound for Truk, which she reached on 12 December 1943.[4]
Ro-40 got underway from Truk on 23 December 1943 to begin her first war patrol, assigned a patrol area off Espiritu Santo in the New Hebrides.[4] She was east of Espiritu Santo on 14 January 1944 when she torpedoed the 800-ton United States Navy fuel oil barge, which her commanding officer had mistaken for a 10,000-ton fleet oiler, at -15.45°N 199°W.[4] YO-159 suffered such serious damage that U.S. forces later scuttled her.[4] Ro-42 returned to Truk on 24 January 1944.[4] She took aboard torpedoes from the auxiliary submarine tender there on 28 January 1944.[4]
During Ro-42′s stay at Truk, U.S. Navy Task Force 58 conducted Operation Hailstone, a major attack on Truk by carrier aircraft supported by anti-shipping sweeps around the atoll by surface warships, on 17 and 18 February 1944.[4] Ro-42 and the submarines and put to sea on 17 February in an attempt to intercept the attacking ships.[4] Ro-42 was unsuccessful, and returned to Truk on 19 February 1944.[4]
Ro-42 began her second war patrol on 25 February 1944, departing Truk to head for a patrol area east of Kusaie.[4] On 1 March she received orders to conduct a reconnaissance of Kwajalein and then move to a new patrol area southeast of Kwajalein.[4] After she arrived off Kwajalein, she reported on 4 March 1944 that the island′s waters were too heavily patrolled by Allied patrol boats for her to approach it for a reconnaissance.[4] Ro-42 also began to suffer from malfunctioning equipment.[4] She was ordered to return to her patrol.[4]
Ro-42 was southwest of Mili Atoll in the Marshall Islands when she sighted a convoy of six transports at 08:00 on 6 March 1944.[4] WHile southwest of Mili on 15 March 1944, she reported sighting a battleship and five aircraft carriers of U.S. Navy Task Group 50.10, prompting the commander-in-chief of the 6th Fleet, Vice Admiral Takeo Takagi, to order the submarines Ro-36 and to intercept the ships.[4]
On 16 March 1944, Ro-42 received orders to move to the west-southwest,[4] and on 18 March to move another to the west.[4] She was south of Ponape on 21 March 1944 when she sighted a three-ship convoy.[4] On 23 March 1944, she and the submarines, Ro-36,,,, and were ordered to intercept a U.S. Navy task force that the submarine had sighted north of Jaluit Atoll that day.[4] She did not find the task force, and she returned to Truk on 28 March 1944.[4]
Ro-42 got underway from Truk in company with the submarines, Ro-36,, and on 12 April 1944 to intercept an Allied task force Japanese forces had sighted north of Kavieng.[4] She returned to Truk on 14 April.[4] On 23 April 1944 she departed Truk bound for Yokosuka, Japan, which she reached on 30 April 1944.[4]
On 15 May 1944, Ro-42 departed Yokosuka with the commander of Submarine Division 34 embarked and orders to conduct a reconnaissance of the American fleet anchorage at Majuro and then proceed to an area north-northeast of Kwajalein.[4] While she was at sea, she received orders to postpone the Majuro reconnaissance until 10 June 1944.[4] On 10 June 1944, she was operating in the Marshall Islands when she was ordered to reconnoiter both Majuro and Kwajalein.[4]
Ro-42 was on the surface east of Roi-Namur at 23:30 on 10 June 1944 when the destroyer escort detected her on radar.[4] After Bangust closed, made visual contact, and flashed a challenge, Ro-42 crash-dived.[4] Bangust searched for the submerged Ro-42 for the next eight hours and regained contact on sonar on the morning of 11 June 1944.[4] Bangust conducted three unsuccessful 24-projectile Hedgehog attacks as Ro-42 made evasive maneuvers such as frequent hard turns and sudden changes in speed.[4] However, her fourth Hedgehog attack resulted in a large underwater explosion that damaged Bangust′s hull.[4] Ro-42 tried to surface, but failed, and sank at 10.0833°N 190°W.[4] During the morning of 11 June, a large oil slick was sighted on the surface.[4]
On 13 June 1944 the Combined Fleet activated Operation A-Go for the defense of the Mariana Islands, and that day Vice Admiral Takagi ordered all available Japanese submarines to deploy east of the Marianas,[4] with Ro-42 ordered to head there at flank speed.[4] On 16 June 1944, she was assigned to Submarine Group B and ordered to move to an area southeast of the Marianas,[4] and on 22 June 1944 she was ordered to return to Truk.[4] She acknowledged none of the orders. On 12 July 1944, the Imperial Japanese Navy declared Ro-42 to be presumed lost with all 73 hands.[4] She was stricken from the Navy list on 10 August 1944.[4]
Some historians have identified a Japanese submarine the U.S. submarine sank on 16 September 1944 east of Japan as Ro-42, but the submarine Sea Devil sank probably was .[4]