The Type91's tail fins were fitted with Kyoban wooden aerodynamic stabilizer plates. Introduced in 1936 these stabilized the torpedo in flight and helped to ensure the proper angle for water entry. The plates were designed to shear off on water entry, absorbing energy and preventing the torpedo from diving too deep. The aerodynamic wooden plates stabilized the torpedo in both the vertical and horizontal axes and provided drag to ensure the torpedo struck the water at or near the proper water entry angle despite the inevitable variations in drop altitudes and air speeds encountered in combat. The structure was simple and worked well, as seen at Pearl Harbor, which was generally considered too shallow for aerial torpedoes prior to the attack. The Kyoban was so effective the US Navy copied it for their Mark13 torpedo after observing it in action at the Battle of the Coral Sea.
