Japanese submarine Ro-40

The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth.[1] They displaced 1,133 tonnes (1,115 long tons) surfaced and 1,470 tonnes (1,447 long tons) submerged. The submarines were 80.5 meters (264 ft 1 in) long, had a beam of 7 meters (23 ft 0 in) and a draft of 4.07 meters (13 ft 4 in). They had a diving depth of 80 meters (260 ft).[2]

For surface running, the boats were powered by two 2,100-brake-horsepower (1,566 kW) diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600-horsepower (447 kW) electric motor.[3] They could reach 19.75 knots (36.58 km/h; 22.73 mph) on the surface and 8 knots (15 km/h; 9.2 mph) underwater. On the surface, the K6s had a range of 11,000 nautical miles (20,000 km; 13,000 mi) at 12 knots (22 km/h; 14 mph); submerged, they had a range of 45 nmi (83 km; 52 mi) at 5 knots (9.3 km/h; 5.8 mph).[1]

The boats were armed with four internal bow 53.3 cm (21.0 in) torpedo tubes and carried a total of ten torpedoes. They were also armed with a single 76.2 mm (3.00 in) L/40 anti-aircraft gun and two single 25 mm (1.0 in) AA guns.[1]

Ro-40 was laid down on 8 August 1942 by Mitsubishi at Kobe, Japan, with the name Submarine No. 206.[4] She was renamed Ro-40 on 5 February 1943 and was attached provisionally to the Maizuru Naval District that day.[4] She was launched on 6 March 1943[4] and completed and commissioned on 28 September 1943.[4]

Upon commissioning, Ro-40 was attached formally to the Maizuru Naval District and assigned to Submarine Squadron 11 for workups.[4] During a training cruise, she collided with the sailing vessel Okaki Maru in the Seto Inland Sea 3 nautical miles (5.6 km; 3.5 mi) off Murozumi Lighthouse on 5 October 1943, with both ships suffering minor damage.[4] She called at Tokuyama to refuel from 2 to 4 December 1943.[4]

As of 1 January 1944, Ro-40 was assigned to Submarine Division 11 in Submarine Squadron 7 along with the submarines I-42, I-43, I-52, I-183, I-184, Ro-41, Ro-43, Ro-113, and Ro-114.[4] On 15 January 1944, she was reassigned to Submarine Division 34.[4] She departed Maizuru on 20 January 1944 bound for Truk, which she reached on 29 January 1944.[4]

Ro-40 got underway from Truk on 12 February 1944 to begin her first war patrol, ordered to operate in the Marshall Islands and then proceed to a patrol area east of the Gilbert Islands in the vicinity of Makin Island.[4] The Japanese never heard from her again.[4]

On 16 February 1944, the United States Navy destroyer USS Phelps (DD-360) was 45 nautical miles (83 km; 52 mi) northwest of Kwajalein when she made sonar contact at a range of 1,700 yards (1,600 m) on a submerged submarine approaching the convoy she was screening.[4] After Phelps dropped a pattern of 13 depth charges, the destroyer USS MacDonough (DD-351) and minesweeper USS Sage (AM-111) also depth-charged the submarine, sinking it at 09°50′N 166°35′E.[4]

The submarine Phelps, MacDonough, and Sage sank probably was Ro-40.[4] The commander-in-chief of the 6th Fleet, Vice Admiral Takeo Takagi, ordered her to a new patrol area between Kwajalein and Eniwetok on 20 February 1944[4] and ordered her to return to Truk on 4 March 1944, but she did not acknowledge either order.[4] On 28 March 1944, the Imperial Japanese Navy declared her to be presumed lost in the Gilbert Islands area with all 61 hands.[4] She was stricken from the Navy list on 30 April 1944.[4]

• Bagnasco, Erminio (1977). Submarines of World War Two. Annapolis, Maryland: Naval Institute Press. ISBN 0-87021-962-6.
• Carpenter, Dorr B. & Polmar, Norman (1986). Submarines of the Imperial Japanese Navy 1904–1945. London: Conway Maritime Press. ISBN 0-85177-396-6.
• Chesneau, Roger, ed. (1980). Conway's All the World's Fighting Ships 1922–1946. Greenwich, UK: Conway Maritime Press. ISBN 0-85177-146-7.
• Hackett, Bob; Kingsepp, Sander (2017). "IJN Submarine RO-40: Tabular Record of Movement". SENSUIKAN! Stories and Battle Histories of the IJN's Submarines. Combinedfleet.com. Retrieved 20 September 2020.
• Hackett, Bob; Sander Kingsepp (2003). "Kaichu Type". Sensuikan!. Combinedfleet.com. Retrieved 20 September 2020.
• Hashimoto, Mochitsura (1954). Sunk: The Story of the Japanese Submarine Fleet 1942 – 1945. Colegrave, E.H.M. (translator). London: Cassell and Company. ASIN B000QSM3L0.