This was a German naval undertaking to secure the safe departure of the Japanese submarine I-8 from Brest in the north-western part of German-occupied France at the start of the second half of its blockade-running voyage from and back to Japan (5 October 1943).
This was one of a limited number of missions between the Axis powers for an exchange of strategic materials and manufactured goods between Germany, Italy and Japan. Initially, cargo ships made the exchanges, but when that became impractical in the face of the tightened Allied blockade, submarines were used. Only seven submarines attempted these long passages: the Japanese submarines I-30 (April 1942), I-8 (June 1943), I-34 (October 1943), I-29 (November 1943), I-52 (March 1944) and the German U-boats U-180 and U-511 (August 1943). Of these, I-30 was sunk by a mine, I-34 by the British submarine Taurus, I-29 by the US submarine Sawfish, and I-52 by US Navy aircraft.
Displacing 2,525 tons surfaced and 3,583 tons submerged, I-8 departed Kure in Japan on 1 June 1943, accompanied by I-10 and the submarine tender Hie Maru. The cargo included two of Japan’s excellent Type 95 'Long Lance' oxygen-propelled torpedoes, torpedo tubes, drawings of an automatic trim system and a new naval reconnaissance aeroplane, the Yokosuka E14Y. A supplementary crew of 48 men was also carried as the crew intended to bring the German 'Type IXC/40' class U-boat U-1224 back to Japan for reverse engineering.
Arriving in Singapore nine days later, I-8 also loaded quinine, tin and raw rubber before making for the Japanese base at Penang on the west coast of Malaya and thence across the Indian Ocean to pass round the Cape of Good Hope. On 21 July, I-8 entered the Atlantic, where it was met with fierce storms, but was able to continue to German-occupied France.
On 20 August I-8 made a pre-arranged rendezvous with U-161, which transferred to I-8 two German radio technicians and a FuMB 1 Metox 600A radar detector, which was installed on the Japanese boat’s bridge. As I-8 entered the Bay of Biscay on 29 August, the Luftwaffe sent Junkers Ju 88 heavy fighters to provide air cover, and the Japanese boat reached Brest safely two days later.
I-8 departed Brest on 5 October with a cargo of German equipment including machine guns, bomb sights, a Daimler-Benz torpedo boat engine, marine chronometers, radars, sonar equipment, anti-aircraft gun sights, electric torpedoes and penicillin. The submarine also transported Rear Admiral Yokoi, who had been the Japanese naval attaché to Berlin since 1940; Captain Hosoya, the naval attaché to France since December 1939; three German officers; and four radar and hydrophone technicians.
I-8 once again encountered adverse weather in the South Atlantic and off the Cape of Good Hope, which delayed her arrival in Singapore. She radioed her position to Germany, but the message was intercepted by the Allies, prompting an attack by anti-submarine aircraft, which failed. I-8 arrived in Singapore on 5 December, and finally returned to Kure on 21 December, after a voyage of some 34,500 miles (55525 km).