When Admiral Yamamoto became commander of the Imperial Japanese Combined Fleet, he brought a true appreciation of the importance of airpower. However, he knew that carriers had some limitations. Any attack on the American west coast by his carriers would be highly risky. They would too far removed from Japanese controlled waters and any ship significantly damaged would probably not reach the safety of home waters. To be able to attack those targets far removed from the range of Japanese air power of land based or carrier based assets, Yamato proposed large submarines carrying aircraft. One pet project was to use them for attacks on the locks of the Panama Canal . The aircraft would not only use bombs but torpedoes as well to destroy the gates of the locks, although using a torpedo for a target that small would be problematical.

In the 1930s the IJN had built a few aircraft carrying submarines in the Junsen or I-1 type. These were derivations of the large submarines designed for long range reconnaissance. These variants could carrying only one small floatplane and the wings would have to be added to the fuselage after the submarine surfaced in preparation for launch. Clearly these would be insufficient for a significant attack In 1938 a larger submarine the Type A, which could carry one aircraft but with wings folded and which could be accessed while submerged. Therefore launch preparation could be started before the boat surfaced. This was still a development of the long-range reconnaissance mission. For an air attack mission more aircraft were needed.

Two types of large submarines were developed for this purpose. One type was a modified Type A, which reworked the initial Type A to provide for two aircraft. However, the largest submarines of World War Two were designed from the keel up for the long-range aerial attack mission. This was the Sen-Toku Type. Two overlapping cylinders were used for the large pressure hull, which gave the design a figure-8 cross section. The design was 400 feet long and displaced 5,223-tons surfaced and 6,560-tons submerged. The long hangar could carry three Aichi M6A1 Seiran float planes to be launched from the catapult on the bow. The design provided for a phenomenal range of 37,500nm at 14 knots surfaced. Additional armament consisted of eight 21-inch (533mm) torpedo tubes with 20 torpedoes. Deck guns consisted of one 5/5-inch/50 (140mm) gun and ten 25mm AA guns with three triple and one single mounts.

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Seventeen units were planned but with the death of Admiral Yamamoto in 1943, the construction program was placed on the back burner. Only three were completed in 1944 and 1945. I400, I401 and I402 never saw operations in their intended role and were inactive after completion. I402 was subsequently modified to serve for fuel supply but again never employed in this mission. All three were captured at the end of the war. Construction on I404 was suspended in March 1945 when almost complete. I403 and the twelve boats of the 1942 program were cancelled.

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Aoshima has now produced a 1:700 scale model of this remarkable design. The box contains five sprues of parts plus decals. One sprue has just the waterline base, while another has the two hull halves. A third sprue contains the decks and aircraft. There are parts for the main deck, conning tower deck and two aircraft. The aircraft come with one version with wings extended, ready for launch, and the other with wings folded. Since the hangar access door is a separate part and can be assembled in an open position, the folded wings aircraft can be shown inside the hangar, which has a detailed interior. Separate float parts are included. One half of the upper superstructure is also on this sprue. A forth sprue concentrates on the conning tower with both halves of the hangar, one of which includes the upper superstructure. Also included on this sprue are the 5.5-inch gun, single 25mm gun, periscope arrangement, propeller guards, hangar door and snorkel. The last sprue is a standard armaments sprue from which only the triple 25mm AA guns are used, with a lot of other weapons and equipment for the spares bin. Detail on the parts is good but not spectacular with deck access hatches and knock out panels. It is a nice touch to have hangar interior ribbing, which enhances the look of the boat with hangar door open and folded wing stored aircraft. You may want to replace the 25mm guns with photo-etch pieces, as the plastic parts are thick.

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Where do you go to get your long-range strike boats for your attack on the US Pacific coast? Well of course the answer is simple. For Pacific coast attacks, go to Pacific Front where Bill Gruner can outfit your 1:700 scale IJN with this unique boat.