Gruppe Monsun (Monsoon Group) was a force of German U-boats (submarines) that operated in the Pacific and Indian Oceans during the Second World War. Although similar naming conventions were used for temporary groupings of submarines in the Atlantic, the longer duration of Indian Ocean patrols caused the name to be permanently associated with the relatively small number of U-boats operating from Penang (primarily its capital, George Town).
After 1944, the U-boats of were placed under the authority of the Southeast Asia U-boat Region. The Indian Ocean was the only place where German and Japanese forces fought in the same theatre. To avoid incidents between Germans and Japanese, attacks on other submarines were forbidden. Altogether 41 U-boats of all types, including transports, were sent, many of these were lost and few returned to Europe.
Background
Indian Ocean trade routes
The Indian Ocean was strategically important, containing the British Raj (now Pakistan, India, Sri Lanka and Bangladesh). The region contained strategic raw materials and the shipping routes that the British needed for their economy. Early in the war, German merchant raiders and pocket battleships had sunk a number of merchant ships in the Indian Ocean. Later on, it became harder for them to operate in the area and by 1942 most were either sunk or dispersed.
From 1941, U-boats were considered for deployment to this area but due to the First Happy Time (July 1940 − October 1940/April 1941) and Second Happy Time (January−August 1942) it was decided that sending U-boats to the Indian Ocean would be an unnecessary diversion. There were also no foreign bases in which units could operate from and be supplied and would be operating at the limits of their range.
Japan's entry into the war in 1941 led to the capture of European south-east Asian colonies including British Malaya and the Dutch East Indies. In May and June 1942, Japanese submarines began operating in the Indian Ocean and engaged British forces in the Battle of Madagascar (5 May – 6 November 1942). The British had invaded the Vichy-controlled island to prevent it from falling into Japanese hands.
Eclipse of blockade running ships
By mid-1943, the prospects for Axis blockade-running by ships were poor. Ports on the Atlantic and Indian Ocean coasts had been reinforced, with ships and longer-range aircraft. The US Fourth Fleet (Vice-Admiral Jonas Ingram) had begun operating off the South American coast. No ship could evade air reconnaissance with radar, that penetrated night and bad weather, backed up with more ships to be guided to interceptions. In these circumstances bluff had lost its effect. In June 1943 the British began to use the Check-mate system, a daily forecast of the position of every independently-sailing merchant ship on earth. Hitherto, naval ships were informed of the ships they were likely to encounter and the information was revised through wireless signals. Despite this precaution, unexpected yet genuine ships were met, not reported to some commands. Only half of the British ships challenged answered with the secret signal they had been provided with and non-English-speaking merchant officers even less often. Ship names could be spelt wrong and not show in alphabetical lists that might not contain a name-change. Axis intelligence services informed blockade-running captains what ships to pose as in certain sea areas.
Using Check-mate, a maritime reconnaissance aircraft or a ship had a Merchant Ship Description Code and Merchant Ship Silhouette Characteristics Register, the merchant ship forecast and secret signal to identify a ship. If unsatisfactory, a signal in clear was sent to the local commander-in-chief, "EMERGENCY CHECK, ship signal letters, apparent name, position" that received a reply in a few minutes that was Mate True (the ship could be genuine) Mate False (could not be genuine) or Mate Doubtful (investigate further). On many occasions, genuine ships took no notice of challenges until a naval ship was in hailing distance that was dangerous to the naval ship in the case of a blockade-runner but a warning shot usually galvanised merchant captains. If a blockade-runner was uncovered, rapid fire was opened at the bridge and life boats to sink the ship, if necessary, Allied resources by then precluding the need to hijack the cargo. Rescues of survivors were permitted but there was a presumption that they would be left as bait, to induce Axis ships and U-boats to attempt a rescue. Aircraft were to summon help from ships. The only option for Axis ships was to be unseen and that meant using submarines to evade scrutiny.
Overland trade
The German invasion of the Soviet Union in 1941 closed the overland routes for the delivery of strategic materials from south-east Asia to Germany and few Axis blockade-running ships were able to avoid Allied patrols in the North Atlantic but the Empire of Japan wanted the exchange military technology with Germany to continue. Japanese submarines, designed for the Pacific, were better transports than the smaller German U-boats built for European waters.
Yanagi missions
The Japanese submarine (2,198 GRT) had an aircraft hangar forward of the conning tower and a range of at on internal fuel. The submarine began the submerged transport of strategic materials between the Japanese empire and Nazi Germany on 20 April 1942 (the birthday of Adolf Hitler). The commerce raiders Aikoku Maru and Hōkoku Maru accompanied and fuelled that reconnoitred ports in the Indian Ocean and then passed the Cape of Good Hope for the voyage to Europe. Escorted by German minesweepers in Unternehmen Kirschblüte}} (Operation Cherry Blossom) docked at Lorient on 5 August 1942, delivering of mica and of shellac. brought cargo back to the Indian Ocean and arrived at Penang on 9 September 1942, called at Singapore on 11 October but hit a mine two days later outside the port.
sailed, disguised as , from Lorient on 10 May 1943 on a Yanagi mission, to be transferred to the Imperial Japanese Navy, carrying cargo and six passengers, one being Admiral Naokuni Nomura. On 27 June, the U-boat sank , an un-escorted, 7,194 GRT, US Liberty ship. On 9 July, the 7,176 GRT US Liberty ship , carrying of ammunition and other cargo was sunk with no survivors. Refuelling took place from and the boat arrived at Penang on 17 July. With a Japanese pilot and interpreter, the boat arrived at Japan on 16 September, where it was re-named and became a training boat after the Japanese found it too complicated to copy. The crew returned by ship to Penang where a former British base was being constructed for Italian cargo boats and German cargo and operational U-boats.
The Japanese submarine (2,231 GRT) departed from Penang on the night of 26/27 June 1943, with its usual complement of 100 men plus 48 more to take over under construction in Hamburg. The submarine rendezvoused with to fuel and to fit a German radar-warning set. was eased on its journey through the Bay of Biscay by British patrols keeping their distance against German glider-bomb attacks. The torpedo boats , and escorted into Lorient in late August. transferred to Brest on 5 September 1943 to load anti-aircraft guns, torpedo and aircraft engines and ten German technicians. A powerful escort force (Operation Lilac) escorted the submarine into deep water where it submerged to begin its voyage. survived an air attack in the Atlantic and an air and sea search from 10 to 14 November off South Africa, being tracked by its wireless signals by Huff-Duff. evaded the hunt by remaining under water during daylight, reaching Singapore on 5 December 1943.
Japanese plans
The Indian Ocean appeared to be a promising area of attack for the German U-boat arm, where the British lacked bases. In December 1942 the Japanese had offered the the use of bases at Penang and Sabang. On 28 December, the (Naval War Command) accepted the Japanese offer but until the defeats in the Atlantic in May 1943 German enthusiasm for such a venture was tepid. As no supplies were available at either location, the idea was turned down, although a number of U-boats operated around the Cape of Good Hope.
A few days after reached the East Indies, became the first U-boat to complete the voyage. This boat carried the Japanese naval attache Admiral Naokuni Nomura from Berlin to Kure. The boat was given to Japan as ; its German crew returned to Penang to provide replacement personnel for the main submarine base being established at a former British seaplane station, on the west coast of the Malayan Peninsula. A second base was established at Kobe and small repair bases were located at Singapore, Batavia (now Jakarta) and Surabaya. As part of the dispersal of U-boat operations following the losses in the North Atlantic in early 1943, Wilhelm Dommes was ordered to sail from his operating area off South Africa to assume command at Penang.
Transport submarines Submarines had been used as transports since 1940 and the had used bigger U-minelayers as improvised tankers and supply vessels to keep the smaller Type VII U-boats at sea for longer. New construction was begun for specialist U-boats, the Type VIIF carried a cargo of 25 torpedoes, two Type IXD1 boats were fitted with high-speed diesel engines that were not a success, ten Type XIV boats carried of fuel. The Type XV and Type XVI were mammoth in size but were never built and Type XXI variants were proposed.
The Japanese Navy used submarines to supply distant garrisons, with landing craft, inflatable boats and tanks on deck for carrying stores to the shore. Twelve I361-class boats weer built that carried of cargo internally and externally. The submarine carried including of petrol and the Ha-101-class also carried of stores. The Japanese army built their own submarines, 24 Yu-1-class, ten Yu-1001-class and two Yu-2001-class. While not intended as merchant ships the Japanese Navy demonstrated the utility of submarine transport between Asia and Europe.
the large submarines of the (Italian Royal Navy) had proved ineffective for convoy attacks in the Atlantic. The Italians converted , , , (renamed in May 1943) , , and built between 1936 and 1941 into transport submarines. The Italians intended to sail them from from BETASOM (Bordeaux) with trade goods for Japan, rendezvousing with merchant ships south-east of Madagascar, the normal maximum range for U-boats operating off the South African coast, having refuelled from milch cow tankers, there to transfer their cargoes and return to Europe.
Prelude
Wolfpack
In August 1943, Admiral Karl Dönitz, the (BdU, commander U-boats) concluded that operations around South Africa had run their course. In just under a year, U-boats had sunk 102 vessels within of the South African coast for the loss of two U-boats. The quantity of sinkings had diminished since the first attacks in October 1942 and Allied anti-submarine warfare operations in the region had increased in extent and effectiveness. The Allied victory in North Africa had opened the Mediterranean for merchant ships, drastically reducing the importance of the Cape route to the Middle East. Dönitz had been defeated in the North Atlantic and resorted to harassing tactics, wherever inadequacies of the Allied anti-submarine effort could be found, until better U-boats, weapons and equipment, allowed a resumption of the Atlantic campaign.
Wolfpack