and cushioned its impact with the water and "shroud rings" (also known as the "ring tail") that reinforced the vulnerable tail fins. They also tested and developed a box-shaped wooden tail that stabilized the torpedo in flight and absorbed energy as it was stripped off as the torpedo entered the water, based on the Kyoban series of similar aerodynamic tails, first developed in 1936 by the Japanese for their Type 91 torpedo used at the attack on Pearl Harbor, but first observed at the Battle of the Coral Sea on May 8, 1942. Experiment soon revealed that optimum water entry angles were approximately 22-32 degrees relative to the plane of the surface: the torpedo might plunge as deep as but it would return to its set depth and bearing if the mechanism was undamaged. This enabled the US Navy to develop a series of attack profiles that varied the combination of speed and altitude to produce the ideal 22-32 degree water entry angle. For the Grumman TBF Avenger torpedo bomber this meant drop altitudes as high as and drop speeds as high as which the Avenger could achieve by diving to the release point. Multiple attack profile options also allowed strike planners to de-conflict attack routes by assigning each torpedo squadron a different attack profile, greatly reducing the risk of mid-air collision over the target. Finally, there was the added benefit of increased range, as the torpedo traveled a significant distance in the air before entering the water (up to when released at and ). Combined with radar that delivered the exact range to the target, the results proved to be remarkable: Deployment on PT boats In 1942, US Navy Patrol Torpedo (PT) boats operating in the South Pacific were experiencing shortages of 21 inch (53 cm) Mark 8 and Mark 10 torpedoes and confronting large numbers of Japanese Daihatsu barges, which were generally too shallow to attack with torpedoes. Installing larger batteries of heavy machine guns and cannon on PT boats to deal with the barges was attempted. Such installations caused weight and stability problems, and torpedoes were still needed to counter larger Japanese vessels such as destroyers and cruisers. One solution, implemented in 1943, was to replace each of the PT Boat's two to four Mark 8 torpedoes, and their Mark 18 torpedo tubes, with the significantly lighter Mark 13, carried in lightweight Mark 1 launching racks, at