The strategy of the Imperial Japanese Navy was centred on one policy; attack. The concept of escorting convoys waiting for attack was contrary to official philosophy, and even when, on one occasion, the First Fleet was asked to escort a convoy as it moved from one zone to another, its commanders arrogantly declared that the First Fleet could not have its freedom of movement restricted. Contrast this with the famous Malta convoy, Operation Pedestal of August 1942, when no less than four aircraft carriers were allocated, and these were not escort carriers but fleet carriers, two of them modern, fast armoured ships. The Japanese were equally oblivious to the need for escort vessels, and it was not until late in the war that a class of escort vessel started to enter service, but by this time it was too few ships, too late.
Japan was at least as heavily dependent upon imported food, fuel and raw materials as the United Kingdom, and possibly even more so, despite the seeming ability to survive and fight on meagre rations. The need for oil and rubber had been among the reasons for the frantic expansion across Asia to take the Netherlands East Indies and its oil, the British territory of Malaya, with its rubber and tin, and Burma with yet more oil. This was no small military achievement. Yet Singapore was as far from Japan as Southampton was from New York, more than 3,000 miles, and while much of the distance was within range of land-based aircraft, it also left the Japanese merchant shipping as dangerously exposed to American submarines as Allied merchant shipping had been to the German U-boats.
Yet even when the Japanese did concede that a convoy would be necessary, the idea of a convoy would be a handful of merchant ships escorted by a destroyer or two. Strangely, the Germans and Italians also seemed incapable of grasping the degree of protection needed by a convoy, despite their success in creating havoc amongst Allied convoys. Across the Mediterranean in particular, from Italy to North Africa, convoys often consisted of small numbers of ships escorted by one or two warships. When British submarines posed a threat to an Italian convoy, the escorts did on at least one occasion include a cruiser, incapable of dealing with a submarine but presenting a welcome target! The small size of the Italian Mediterranean convoys could at least be excused on the grounds that the distance was short, and to wait for large convoys to gather could have interrupted supplies, while doubtless the waiting ships would have presented excellent targets for the British. Also, some of the ports in use at either end would have been incapable of turning a large number of ships around at once.
None of these excuses applied to the Japanese, as the distance was substantial. More significant, as former First World War allies of the British, they knew the dangers of submarine warfare and during the early post-war period had the benefit of a British naval mission to pass on wartime experience and the importance of naval aviation and submarine warfare.
In fact, the auxiliary carriers developed by Japan during the war years owed far more to the need to transport aircraft than to escort convoys, hence the use of the word ‘auxiliary’ rather than ‘escort’ for this article. As the Imperial Japanese Navy had aircraft carriers in plenty during the late 1930s as the intensity of the fighting in China grew, and these were used as transports on occasion, it is not surprising that the main driving force for the Japanese to convert merchant ships to carry aircraft was the Imperial Japanese Army, which needed to get its aircraft across the seas to combat zones.
We should not forget that Japan was a maritime nation and regarded itself as such. In common with the Germans, it felt that it needed Lebensraum, living room or space for an expanding population. With the exception of the United States, of the main combatants during the Second World War, none was self-sufficient in food, fuel or raw materials. Germany struck east looking for food and fuel. When German forces were pushed back by an increasingly confident and competent Red Army, Italy was first to suffer from the shortage of fuel as its German ally kept the available resources for its own needs.
The Japanese Auxiliaries
In common with the United States, the first Japanese aircraft carrier was the conversion of a non-combatant vessel. While the Langley had been a collier, Hosho was laid down as an oiler, but conversion started at an early stage during construction. The Imperial Japanese Navy benefited from its close liaison with the Royal Navy at the time. Nevertheless, the Japanese carriers developed a distinctive style of their own, usually failing to take advantage of the full length of the hull for the hangar and leaving the flight deck supported fore and aft on girders rather than plating up the hull to flight deck level.
The first Japanese escort carrier, or auxiliary carrier, was prompted by the need to provide aircraft transports for the Imperial Japanese Army, as the Japanese Army Air Force needed to bridge the long distances across the Pacific over which ferrying aircraft was seldom a practical option. This was so important that despite the growing shortage of merchant shipping, two passenger cargo liners were requisitioned, the Akitsu Maru, 9,186grt, and the similar but slightly larger Nigitsu Maru, 9,547grt, which together became known as the Akitsu Maru-class after conversion, giving a tonnage of 11,800 tons. Conversion was kept as simple as possible, with the superstructure left largely intact and the flight deck running up to the level of the boat deck, so that the boiler uptakes were simply redirected to the starboard side and a simple island was also provided for navigation. The space below the flight deck not taken up by the superstructure was left open and used for aircraft storage, with a simple lift at the after end of the flight deck to bring aircraft up. A pair of derricks, possibly belonging to the ships pre-conversion but moved aft and which would have made landing on impossible, was provided. Up to twenty aircraft could be carried, or small landing craft could be substituted for these. Anti-aircraft armament was limited to two 3-in guns, while another ten 3-in field guns were modified, possibly to provide some protection if threatened by an American warship.
Built by the Harima Shipyard, Akitsu Maru was commissioned on 30 January 1942, and the Nigitsu Maru followed in March 1943. Little further is known about the way in which these ships were used, other than that the Nigitsu Maru was torpedoed by the submarine USS Hake on 12 January 1944 while south of Okinawa, and her sister ship sank after being torpedoed by the submarine USS Queenfish off Kyushu on 15 November 1944.
Although the conversions seem more in line with those of the British and Dutch MAC-ships, it is clear that neither ship would have been able to operate as a convoy escort, partly because of the position of the derricks but in addition the height of the flight deck above the waterline on a narrow merchant hull could have given problems with stability in open waters.
Nevertheless, the Imperial Japanese Navy did convert a number of merchantmen as the desperate need for additional flight decks became clear, even before the losses of 1942. The first stage was the acquisition of three 17,100 ton passenger liners still under construction which had a maximum speed of around 21 knots; the Kasuga Maru, Yawata Maru and Nitta Maru. The Kasuga Maru was converted at the Sasebo Navy Yard and joined the fleet on 15 September 1941 as the class-leader Taiyo, the only one to enter service before Japan attacked the United States. The other two ships were both converted at the Kure Navy Yard, with Yawata Maru joining the fleet on 31 May 1942 as the Unyo and Nitta Maru on 25 November 1942 as the Chuyo. After conversion, all three resembled escort carriers, although at 17,830 tons they were larger and faster than their British and American equivalents. The boiler uptakes were re-routed to starboard with the usual Japanese downward pointing funnel, but no island was created leaving navigation from a wheelhouse at the forward end of the single 300 feet hangar built above the main deck and below the forward end of the flight deck. Although sponsons were provided on both sides for AA armament, the perceived need for additional armament at the bows and stern meant that the flight deck was foreshortened, and no arrester wires or catapults were fitted, which must have made life interesting for Japanese Navy Air Force pilots. Two lifts were provided at the hangar ends. AA armament seems to have been found at the back of the navy yard stores, consisting of redundant 4.7-in guns augmented by some 25-mm weapons, although on Unyo and Chuyo eight 5-in weapons were added later. Lacking an island, radar had to be mounted on the starboard deck edge, which must have limited its effectiveness considerably.
Despite their size, with their ability to accommodate up to twenty-seven aircraft, none of these ships was used on offensive operations or indeed as an escort carrier, all of them being used as aircraft transports and on training duties. The nearest they came to combat was when Taiyo accompanied the battleship Yamato during the campaign in the Eastern Solomons, but here too her role was that of support. All three met their ends in encounters with American submarines. Last to enter service, Chuyo was first to go, being torpedoed and sunk by the USS Sailfish off Yokosuka on 4 December 1943. Taiyo survived until 18 August 1944, when the Rasher found her off Luzon in the Philippines, and the following month Unyo was torpedoed and sunk by the Barb in the East China Sea off Hong Kong on 16 September, and far from the carrier acting as an escort, the tanker accompanying her was also sunk by the same submarine.
In late 1941, the passenger liner Argentine Maru, 12,755grt, was requisitioned by the Imperial Japanese Navy as a troopship, doubtless with the advance through south-east Asia and Indonesia in mind, but the following year other needs were more pressing and the decision was taken to convert her to an aircraft carrier. She joined the fleet as the Kaiyo on 23 November 1943, at just 16, 483 tons and her aircraft capacity was around twenty-four. Her original diesel engines were replaced by steam turbines, giving the relatively high speed for a conversion of 24 knots. Once again, there was no island and the trunking for the boiler uptakes was directed to a starboard side downward pointing smokestack. There was a simple box-like hangar with two lifts to the wooden flight deck, which seems to have had arrester wires but no catapult. Once again, radar was fitted to the edge of the flight deck. Her sister ship Brazil Maru was lost before she could be converted, but after serving as an aircraft transport and training carrier, Kaiyo was crippled by aircraft from HMS Formidable, Indefatigable and Victorious while off Kyushu on 24 July 1945.
War in Europe caught at least one major German merchant ship unawares and too far from home to be safely repatriated. This was the Norddeutscher Line Scharnhorst, an 18,184grt passenger liner, in port at Kobe in September 1939. Early in 1942, the ship was requisitioned by the Imperial Japanese navy as a troop transport, but as the losses of both carriers and carrier-qualified aircrew mounted that year, the decision was taken to convert her into an aircraft carrier. While the simplest form of conversion was sought, including retaining the original German steam turbines, the fact that this was a completed ship rather than a conversion partway through construction presented many problems, as did the pronounced sheer lines in her hull. The result was that the hangar deck was far higher than would have been ideal and naturally enough the flight deck was even higher, so that the ship became inherently unstable and large bulges had to be fitted to the hull. Otherwise similar to the naval conversions already mentioned, a flight control position was installed on the edge of the starboard flight deck with a lift mechanism so that it could be raised and lowered as required. The initial AA armament of eight 5-in and thirty 25-mm guns was raised to fifty 25-mm guns in mid-1944. After commissioning as Shinyo on 15 December 1943, she was employed on training duties until she was caught by the submarine USS Spadefish in the Yellow Sea on 17 November 1944, torpedoed and sunk.
Despite the priority being accorded rebuilding the carrier fleet, time was running out for the Japanese. The growing aerial supremacy of the Allies was more than matched by the activities of the US Navy’s submariners once early problems with poor torpedo reliability had been rectified. As with Kaiyo, the passenger liner Chichiba Maru had been acquired in 1941 as a hospital ship and troop transport, but then in 1942 she was earmarked for conversion to an aircraft carrier but sunk by the submarine Gudgeon in April 1943 before work could start. When the conversion of the cruiser Ibuki was ordered in 1943, while she was still under construction, a growing shortage of materials meant that work was delayed and was finally abandoned in March 1945 when it was clear that it could not be completed. This was not really an auxiliary or escort carrier and would have been a more ambitious conversion than some of those already mentioned, with a starboard island and rocket launchers, although the complement of 1,015 seems to have been high for a ship of just 12,500 tons. Speed could have been a problem with half the boilers removed to accommodate extra fuel.
It is an indictment of the poor strategic vision of the Imperial Japanese Navy that the only two ships that could fairly be described as escort carriers were ordered by the Imperial Japanese Army. These were the two tankers Yamishiro Maru and Chigusa Maru, both of 10,100grt. These were in fact similar to MAC-ships in concept, having no hangar and therefore no lifts, and were intended to operate eight Kawanishi Ki-44 lightweight fighters of the Japanese Army Air Force to provide air cover, but once again nothing seemed to be in mind for protection against submarines other than an anti-submarine depth charge projector on the forecastle. Despite the fighters, the first ship, Yamishiro Maru, was sunk in an American air raid on Yokahama on 17 February 1945, just three weeks after she had entered service on 27 January. Her sister ship was never completed.
The last aircraft carrying ship to be completed for the Imperial Japanese Army was the Kumano Maru, which took the concept of the Akitsu Maru a stage further by having a ramp at the stern down which landing craft could be launched, but unlike the earlier ship was based on a cargo vessel rather than a passenger cargo liner. Although commissioned on 30 March 1945, she is believed never to have seen active service – by this time the Japanese armed forces were going nowhere!
Finally, in late 1944, the Imperial Japanese Navy decided that escort carriers would be useful and ordered the conversion of two tankers, the Shimane Maru and Otakisan Maru. While the original hulls were not modified, these ships differed from the MAC-ships and the Yamishiro Maru in having a hangar built over the hull, and while there was no island, once again the starboard side down sloping smokestack was part of the design. The design provided a single lift and up to twelve aircraft could be carried on a ship of 11,800 tons. Commissioned on 28 February 1945, Shimane Maru was sunk by American aircraft off Takamatsu on the island of Shikoku on 24 July 1945 before she entered service. Her sister ship was never completed and had the misfortune to drift onto a mine at Kobe on 25 August, later being scrapped. Had not the war ended, and had materials been available, another two ships would have been converted, although by this time the Japanese had no convoys to escort!