In late 1942, enemy raider activities in the Indian Ocean had virtually come to an end. The German raiders, once disrupting the shipping lines in these waters had all (with the exception of a few) been destroyed by the Royal Navy or had begun the long way home to the homeland in Europe. The Japanese were strategically not interested in sinking merchants, and had done little to disrupt Allied supply lines since their successful carrier sortie to Ceylon in April 1942.
But unknown to the allies, the Japanese Navy had decided to keep the pressure on the shipping lanes. Large and valuable tankers maintained a continuous stream of oil and other products from the Middle East to Australia and surrounding islands. The continuous pleas by the Germans will undoubtedly have had a large influence on this decision, as the Japanese were afraid the Germans would send more of their successful disguised raiders to the Indian Ocean, which the first considered "their backyard". The bond between the Japanese and Germans never grew as strong as the one between the Allied forces, and they distrusted each other from the very start of their alliance. To satisfy the Germans, the staff of the Imperial Japanese Navy decided to send their own raiders to these waters. Although raider warfare was not entirely strange to them, they could not build on the vast amount of experience as the Germans did, and as a result, their ships were far less effective.
In 1940, two passenger-cargovessels of the Osaka Shipping Line were requisitioned for conversion to Armed Merchant Cruisers, in anticipation of the likely thrust southward by the Japanese. The Aikoku Maru and Hokoku Maru, under construction for the route between Japan and South America, started their rebuilds in 1941, and by the time they were commissioned, they were armed to the teeth. Their specifications were as listed below:
With their heavy armament, they could overpower any smaller combatant or merchant, and their speed enabled them (in combination with their floatplanes) to search large areas of ocean. In service, they were attached together with Kiyozumi Maru to the 24th Special Cruiser Squadron under Rear Admiral Moriyoshi Takeda. Even though these ships were slightly more powerful than their German counterparts, they were less effective for several reasons. First, the Japanese had little experience in operating surface raiders, and it seemed to them that letting these ships operate in a pair reduced the risk of losing them. Second, the ships spent far less days at sea. They had played a modest role since December 1941, and their first contribution to the war effort came when they overpowered the American freighter Vincent on December 12, 1941, soon followed by the Malama. Their last operation was by far the most successful: they doubled as supply ships for the Japanese submarines operating in Mozambique Channel. These I-boats sank over 100.000 tons of shipping, and the Hokoku Maru and Aikoku Maru added additional ships to that tally. With these successes, they had sunk or captured 5  merchants within a year, totalling 31.303 tons. They left Singapore on November 5 on their fourth sortie, under overall command of Captain Imazato Hiroshi, commanding officer of Hokoku Maru. .
The Ondina was a modern tanker built for one of the shipping companies of Shell, La Corona. She was new, relatively fast and for contemporary standards well-armed with a 4-inch gun on her stern and several machine guns for use against aircraft. Under captain Willem Horsman, she was now ferried fuel between Australia and Abadan on the oil-rich shores of the Persian Gulf. On her journey to Abadan, she would only enjoy the protection of a small corvette. HMIS Bengal (Lt. Cdr. William Joseph Wilson, Royal Indian Navy Reserve, in command) was one of the R.A.N.-type Ballarat corvettes/minesweepers, with the exception that she and three sisterships were allocated to the Indian Navy. She only had one 3-inch gun (substituting a 4-inch gun which was in short supply), which made her firepower barely enough to protect the Ondina from submarines, let alone from enemy surface raiders. The two ships departed Fremantle on November 5, 1942, expecting a long but uneventful trip.
On November 11 at 11.58 in the morning, the naval authorities in Fremantle received a S.O.S.-signal sent out by the Bengal, reporting that she and the Ondina were under attack by two enemy raiders, identified as Japanese, in position 19.45 S - 92.40 E. The battle started when a lookout aboard Ondina sighted an unknown vessel at about 12.000 metres away, bearing 270 degrees, followed by ship of similar size. As no allied ships were reported in the vicinity, they could only assume they were hostile and for some time these ships were even identified as Japanese carriers. On the Bengal, the lookouts saw the two warships a few minutes later, bearing 290. The ships both made a quick 90 degrees turn to starboard away from the enemy to a NNW course. Bengal then turned to engage, thus hoping to buy enough time for the Ondina to escape. She opened fire at 12.12 hours from 3200 metres away, soon followed by the Ondina 8000 metres away. The sensible thing to do for the Ondina was to follow the order to escape, but the captain decided to stay, as his ship, armed with a 4-inch gun, still carried a heavier punch than her smaller companion. In addition, the Ondina could only sustain a speed of 12 knots versus 21 of the Japanese ships.The Aikoku Maru (Captain Oishi Tamotsu) and Hokoku Maru (Captain Imazato Hiroshi) commenced firing at 12.12 hours, and soon straddled the Ondina with their cruiser-armament. The first hit on Ondina ripped off a part of the main mast, leaving only a stump standing. The Ondina herself had her answer ready: the third shell fired was a hit in the superstructure of Hokoku Maru, but apparently did little to effect her speed or armament. Content with the hit, the officer in charge then ordered the gunners to concentrate their fire on the stern. Only a few moments later, a lucky hit on the starboard torpedo mount turned the Hokoku Maru in a ball of red and yellow flames, and as the ship emerged from the smoke, she was listing heavily to starboard, and simultaneously started to settle by the stern. The explosion ripped off the stern and threw her two floatplanes overboard, while massive fires raged in the superstructure. Hokoku Maru was not built as a warship, and therefore didn't have a sufficient number of watertight bulkheads. Shells fell from their lockers as a result of the increasing list and threw sailors overboard. Men, covered with burns and blood tried to fight the flames. Reports came in indicating large fires in the engine-room and the loss of all electricity. There was little hope of salvaging the Hokoku Maru, and Captain Imazato could do little else than to order "abandon ship". The raider finally sank in a massive explosion at 13.12
Meanwhile, the Aikoku Maru hotly engaged the Bengal and Ondina, scoring several hits on the latter. Fortunately, shells and torpedoes have little effect on empty tankers, as the large number of watertight tanks keeps them afloat under the most difficult circumstances. Aikoku Maru also fired at the Bengal, which had shortened the distance to about 2200 metres. One shell hit her in the forecastle at 1220, luckily doing little damage. After smoke floats failed to work and the supply of ammunition had almost been depleted , the Bengal turned away at 1240 and made smoke. During her retreat, the Bengal was hit in her stern at 12.41, which caused a fire in the officers' baggage room. Last the men aboard Bengal saw was the Ondina trying to evade the shells, continuously straddled by the Aikoku Maru. A shell was seen hitting her aft of the bridge at 13.08. By 13.45, the sea was empty.
After Bengal had disappeared behind the horizon, Ondina was still steaming around at full speed. Not built as a warship, she had only a small stock of ammunition. Aikoku Maru closed the range to 3500 metres, and placed several hits in the following minutes, one of which was observed by the Bengal. Ondina herself had only 12 shells left, four of which she fired at the Hokoku Maru, the rest at Aikoku Maru, apparently without placing a hit. A last attempt to escape by dumping smoke-buoys overboard failed, and the captain ordered the crew to abandon ship to avoid further bloodshed. The engines were stopped, the lifeboats lowered and a white flag was hoisted, all under continuous fire from the Aikoku Maru. A few moments later, Captain Horsman was killed by a piece of shrapnel from a shell hitting the bridge. Two lifeboats and two rafts were lowered into the water and later, another lifeboat was in the water with the remainder of the crew. Most of the crew (with the exception of officers and guncrew) consisted of Chinese, and they had been troublesome during the whole action, refusing any assistance that might help save the ship.
Aikoku Maru approached Ondina to about 400 metres, and fired two torpedoes to finish her off. Both blew large holes in the starboard side, but did little to sink the ship itself. These tanks had been empty and the ship remained afloat on the other, undamaged fuel tanks, despite a 30 degree list. Then Aikoku Maru changed course and the Japanese gunners opened fire on the drifting lifeboats , killing the 1st engineer and three Chinese stokers. One wounded was a young British sailor named Henry, originally assigned to the Bengal. Satisfied with the results, Aikoku Maru then steamed away to pick up survivors from Hokoku Maru . Later, the Aikoku Maru came back one more time, firing a torpedo which missed the tanker. She paid little attention to the survivors and steamed further, convinced the Ondina was doomed. 
Meanwhile, the men in the lifeboats had given the deceased a seaman's burial, and then exchanged thoughts about what to do next. The first officer Rehwinkel wanted to return to the tanker, but only one man of the guncrew was willing to go with him. Most of the others were convinced the Ondina was about to go down. Not without trouble, Rehwinkel managed to assemble a small skeleton crew and returned to the ship, where counterflooding reduced the list. Inspection revealed that her engines were also still intact. The small fires were extinguished and the last crewmembers in the lifeboats were taken aboard, after they had been convinced there was no danger of sinking. Now the long leg back to Fremantle began. The lifeboats were patched up as good as possible, in case the Aikoku Maru came back. The British sailor Henry was however in very bad shape. He had a crushed leg and after two days the first officer was forced to send out an uncoded signal for help. Uncoded, because the codebooks had all been thrown overboard when "abandon ship" was ordered. This unexpected signal caused a shock in Colombo, as the Ondina had been reported sunk by Ondina and logically, the British thought the Japanese were playing a trick on them. A signal went out from Fremantle to report her position. Expecting a Japanese trap, the Ondina naturally didn't reply. Without medical attention the Ondina steamed towards Fremantle. Fortunately, on the 17th an Australian Catalina flying boat was sighted, about 200 miles northwest of Fremantle. The lookouts had reported a ship some time earlier, and the Catalina was asked if that ship could provide the much needed help. The unknown ship proved to be a hospital-ship , where doctors immediately began a series of blood-transfusions which saved Henry's life.
On 18 November, the Ondina entered Fremantle after a journey only a few ships had experienced, let alone lived to tell about it. The corvette Bengal had entered Diego Garcia the day before. Ondina remained in Australia as depot-ship until 1943, when she was finally repaired. Both Bengal and Ondina survived the war.
Very few questions remain concerning this clash, but the most important is who fired the fatal shot? Answering this question is difficult, as both the Ondina and Bengal claimed to have scored the fatal hit and this mystery may never be solved. The Japanese themselves thought it was the Ondina. According to them, her shell hit the starboard torpedo mount, causing the torpedoes to explode. At the time, the Bengal was given the benefit of the doubt, according to the author of my main source as an attempt to use this battle for propaganda in India, where the British had a lot of trouble keeping the population under control. . Apparently, the British themselves didn't know, as a footnote to the article in London Gazette mentions: "...Ondina also claimed this hit with the fifth round from her 4-inch gun at 8,000 yards range. It is not possible to adjudicate between these two claims...".
In retrospect, this battle not only was a tactical success for the Allies, but also it also had strategical implications. The loss of the Hokoku Maru led to the abandoning of raider warfare by the IJN, and never (with one exception) tried to severe the lifeline again.
Ondina was given a rare Dutch distinction, the Koninklijke Vermelding by Dagorder, issued on July 9 1948. Captain W. Horsman became Ridder in de Militaire Willemsorde der 4de Klasse posthumously and was Mentioned in Dispatches, while gunner Hammond received the Distinguished Service Medal and the Bronzen Kruis. The captain of the Bengal, Lieutenant-Commander Wilson, received the Distinguished Service Order, while others of his crew were also awarded.
 Eiichi Nakajima "Hokoku Maru - the unknown Q-ship" lists the number of victims as 8. I have been unable to identify the other three, if there were any more. Their results were as follows:
Malama (USA, 3275 tons), bombed and sunk by floatplane January 1 1942
Genota (Dutch, 7987 tons), captured on May 9 1942
Elysia (British, 6757 tons), shelled June 5 1942. Wreck was sunk by the submarine I-18 (Cdr. Otani) on the 9th.
Hauraki (New Zealand, 7112 tons), captured July 12 1942
 According to Warship International, she carried only 40 shells
 Captain Oishi Tamotsu of the Aikoku Maru was born in Kochi in 1900, and graduated from the Japanese Naval College in 1920. He then specialized in navigation, and had some ship assignments until his graduation from the advanced course of the Navigation Naval School in 1926. He received his first command in 1938 (gunboat Saga). In October 1940, he was appointed senior staff officer of the 1st carrier-division. From following April, he had the same function with the 1st Air Fleet. As staff officer, he participated in all carrier operations from Pearl Harbor to Midway, and then took command of the Aikoku Maru in August 1942. On April 3 1943, he was relieved from his command and held several positions until the end of the war. He died on February 13 1946, and was promoted to rear-admiral posthumously. Apparently, he was never tried for machinegunning the survivors of Ondina.
 The Aikoku Maru picked up a total of 278 survivors of a crew of 354. Captain Imazato was one of the 76 killed during the action. I haven't found any mention of damage or casualties aboard the Aikoku Maru
 Aikoku Maru became a high-speed transport and was sunk in February 1944 during operation Hailstone, the bombardment by American aircraft of the Japanese base at Truk. The wreck is still very popular with divers.
 This was probably the Australian hospitalship Wanganella (9576 gross tons, built 1932) with homeport Melbourne. It reportedly arrived in Fremantle on the 17th.
 In retrospect, political considerations may not have been the reason for crediting the Hokoku Maru to the Bengal. A report by a British naval officer at the Public Records Office, the content of which was forwarded to me by Mr. Chris Amano-Langtree, indicates the British authorities had the impression that Ondina's first mate exaggerated this ship's role in the action.
 The following men of the gun crew of Ondina received the Netherlands Bronze Cross (information from London Gazette, August 3, 1943):
Acting Able Seaman Raymond Henry Bayliss, C/JX.249728
Acting Able Seaman Henry Charles Boyce, C/JX.312193
Acting Able Seaman Henry Alfred Brooklyn, P/JX.313497
Bombardier William Nicol, 2882212, 1st Battery, Maritime Royal Artillery Regiment
Bombardier Frank Ryan, 4192090, 1st Battery, Maritime Royal Artillery Regiment
1st engineer J.J.F. Niekerk
Chinese stoker Soo Fuiy
Chinese Stoker Fung Kam
Chinese stoker Ho Chung
K.W.L. Bezemer "Verdreven doch niet verslagen"
Eiichi Nakajima "Hokoku Maru - the unknown Q-ship (an extract was kindly provided by Mr. Sander Kingsepp)
Warship International No 4, 1994
Thanks to Roel Zwama, Anne Niemantsverdriet, Bert Kossen, Jean-François Masson and Shigure for providing additional details