Operation Tanne-Föhre

fir tree-Scots pine

'Tanne-Föhre' was the Axis oceanic rendezvous at which the U-boat U-530 and the Japanese submarine I-52 met to exchange high-priority trade goods and documents (23 June 1944).

The mission was part of U-530's sixth patrol. This German boat departed Lorient on 22 May ultimately for operations in the Trinidad area. On its outward voyage the boat was to make rendezvous with the Japanese submarine I-52 and supply the larger boat with a Naxos radar detector, a radar operator and a German navigator to help I-52 complete its journey.

On 10 March 1944, on its maiden voyage, I-52 (codenamed Momi or fir tree and possessing a submerged displacement of 3,644 tons) departed Kure via Sasebo for Singapore. Its cargo included 9.8 tons of molybdenum, 11 tons of tungsten, 2.2 tons of gold in 146 bars packed in 49 metal boxes, 3 tons of opium and 119 lb (54 kg) of caffeine. The gold was payment for German optical technology. The boat was also carrying 14 passengers, primarily Japanese technicians, who were to study German anti-aircraft artillery and torpedo boat engine technologies.

In Singapore the boat loaded another 120 tons of tin in ingots, 59.8 tons of raw rubber in bales and 3.3 tons of quinine, and headed through the Indian Ocean, to the Atlantic Ocean.

On 6 June, Rear Admiral Kojima Hideo, the Japanese naval attaché in Berlin, signalled the boat that the Allies had landed in Normandy, thus threatening its planned destination of Lorient on the north-west coast of France. The boat’s captain was advised to prepare to make instead for Norway, and also instructed to rendezvous with a German boat at 21.15 on 22 June in 15°N 40°W. I-52 responded with its position, which was currently 35° N 23° W, but the signal was intercepted and decoded by US intelligence, and I-52 had in fact been closely watched all the way from Singapore. Guided by the headquarters of the US 10th Fleet, a hunter/killer task force was directed toward the Japanese boat.

On the night of 22/23 June, 1944 about 980 miles (1575 km) to the west of the Cape Verde islands group off the coast of North-West Africa, I-52 rendezvoused with U-530, which provided the Japanese boat with fuel, and also transferred a Naxos FuMB 7 radar detector, an Enigma coding machine, electronic system operators and a liaison officer for the trip through the Bay of Biscay.

A US task force had meanwhile been assembled as a submarine hunter/killer group, comprising the escort carrier Bogue and the destroyer escorts Francis M. Robinson, Haverfield, Swenning, Willis and Janssen. The task force departed Casablanca on 15 June under the command of Captain Aurelius B. Vosseller, and Bogue carried nine General Motors FM-2 Wildcat fighters and 12 Grumman TBF-1C Avenger attack aircraft of the VC-69 squadron. On its passage from Hampton Roads to Casablanca, the task force had sunk another Japanese submarine, Ro-501 (formerly the German U-1224) on 13 May. This was a very effective force, sinking 13 German and Japanese submarines between February 1943 and July 1945.

Arriving in the area of the German and Japanese boats' rendezvous during the evening of 23 June, the carrier began launching flights of Avengers at around 23.00 to search for the submarines. U-530 escaped undetected and finally returned to the base at Flensburg after 133 days at sea.

At about 23.40, the radar operator in one of the Avenger aircraft, detected a surface contact on his only partially functioning radar, and the pilot immediately dropped flares to illuminate the area, and attacked. After his first pass he saw the depth charge explosions just to starboard of the boat as this dived. The pilot dropped a purple sonobuoy, a newly developed underwater microphone which floated on the surface, picked up underwater sounds and transmitted them to the aeroplane. A searching aeroplane usually dropped these in packs of five, code-named purple, orange, blue, red and yellow, and the operator was able to monitor each buoy in turn to listen for sounds emitted by the boat being targeted.

Hearing what sounded like submarine propeller sounds, the pilot began a torpedo attack, dropping a Mk 24 'Fido', the Allies' first type of acoustic torpedo. Developed by the Harvard Underwater Sound Laboratory, thus was designed to home on the sounds made by the boat. The 'Fido' was designed as a 'mission kill' weapon, which was meant to damage the boat so that it would have to surface and be captured, rather than to destroy it. Within minutes of the torpedo drop, the Avenger’s sonobuoys detected the sounds of an explosion and a hull breaking up.

The aeroplane commander and operators on Bogue believed that the the boat had been sunk, but after the first aeroplane had been relieved soon after 24.00 by another, this carrying a civilian underwater sound expert, at 01.00 the civilian export reported faint propeller noise in the area.

Vosseller ordered a second attack, and another 'Fido' was dropped. Nothing was heard for the rest of this aeroplane’s patrol, or during that of its replacement. After the break of day on 24 June, however, Janssen reached the site and her look-outs spotted flotsam in the form of a ton of raw rubber, a piece of silk and pieces of human flesh.