The Type 91 was an aerial torpedo of the Imperial Japanese Navy. It was in service from 1931 to 1945. It was used in naval battles in World War II and was specially developed for attacks on ships in shallow harbours. The Type 91 aerial torpedo had two unique characteristics. Firstly, it used wooden stabilizers attached to the tail fins which were shed upon water entry. Secondly, it engaged an angular acceleration control system to control rolling movements, which was very advanced for its time. This system made it possible to release the Type 91 not only at a cruising speed of at an altitude of , but also in a power-glide torpedo-bombing run at the maximum speed of the Nakajima B5N or Kate, The Type 91 torpedo was in diameter. There were five models put into service, with high-explosive warheads weighing and having effective ranges of at . Since the Type 91 torpedo was the only practical aerial torpedo of the Imperial Japanese Navy, it was simply known as the Koku Gyorai, or aerial torpedo. Surface warships and submarines used other types of torpedoes, namely the Type 93 and Type 95 respectively, while the Type 97 torpedo was designed for use by midget submarines. Specifications The torpedo measured in length, with a diameter of , and weighed , with an explosive charge of . It had a range of and a speed of . A slight variant was used to sink HMS Prince of Wales and HMS Repulse, launched from Mitsubishi G4M "Betty" bombers in an action in the South China Sea three days after Pearl Harbor on 10 December 1941. Variants Below is the list of the series of Type91 aerial torpedo production models. The Type 91 (modification 2), was a shallow-water aerial torpedo that was designed for and used in the attack on Pearl Harbor in 1941. Wooden fins and a softwood breakaway nose cone were added to allow for launching into shallow water at low altitudes. There were two versions in the Type 91 warhead rev.3, differing in designed maximum launch speeds. Later, heavier models had a decreased range. Other Japanese aerial torpedoes In spring 1944, the Yokosuka air arsenal began development of the Shisei Gyorai M (trial model torpedo M), or simply the Two tonne torpedo. This was an enlarged version of the Type 91 aerial torpedo and was in diameter, long, weighing , and