008 - The Kate and the Type 91

Perfecting the Torpedo Bomber

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IOn December 7th at 7:55, two Nakajima B5N torpedo planes skimmed the surface over Pearl Harbor and let loose their 1700 pound Type 91 torpedoes. To the Japanese, these torpedoes were known as “gyorai”, or thunder fish. To the Americans, it meant thousands of sailors killed in action.

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In December 1940, just a year before the Japanese attacked Pearl Harbor, the British attempted a very similar feat against the Italian Navy at the Battle of Taranto. Yamamoto felt this type of attack would deal a decisive blow to the American fleet. With the help of his top pilots, Yamamoto pushed for the development of a perfect torpedo bomber.

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The B5N torpedo plane, known as the “Kate” to American forces, had problems with the shallow depth requirements of Pearl Harbor. Torpedoes would normally dive over 80’ deep. Pearl Harbor was only 40’ deep. The engineers developed a hydraulically-powered swing arm to deliver the torpedo closer to the waterline.

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Additionally, the engineers fixed the torpedo itself. They began by attaching special wooden fins on the tail to keep the torpedo from diving too deep. These fins also acted as dampers, keeping the torpedo from diving as it splashed into the water. Immediately after impact, the wooden fins broke off as the torpedo stabilized itself in the water.

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The engineers finally added a gyroscope and anti-rolling mechanism to the Type 91. The gyroscope helped to keep the torpedo upright. If the torpedo rolled one way, the anti-rolling mechanism corrected the roll by turning the rudders the opposite direction. The anti-rolling mechanism made the torpedoes run a straight course towards the target. These improved torpedo bombers proved devastating to the American Navy, especially at Pearl Harbor.

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