The Mark 13 torpedo was the U.S. Navy's most common aerial torpedo of World War II. It was the first American torpedo to be originally designed for launching from aircraft only. They were also used on PT boats. Design Originating in a 1925 design study, the Mark 13 was subject to changing USN requirements through its early years with resulting on-and-off development. Early models—even when dropped low to the water at slow speeds—were prone to running on the surface, or not running at all. By late 1944, the design had been modified to allow reliable drops from as high as , at speeds up to . The final Mark 13 weighed ; of this was the high explosive Torpex. The Mark 13 was designed with unusually squat dimensions for its type: diameter was and length . In the water, the Mark 13 could reach a speed of for up to . The Mark 13 ran slower than the Mark 14 torpedo, a characteristic which, along with a lesser mass, lesser negative buoyancy and the lack of a magnetic influence feature in its Mark IV exploder, meant that it did not suffer from some of the same problems as its larger siblings. 17,000 were produced during the war. Wartime development By 1942, poor combat performance had made it apparent that there were problems with the Mark 13, as 35 out of 41 torpedo bombers were lost at the Battle of Midway without scoring a single hit: The Committee assigned the California Institute of Technology to undertake the first systematic study of the dynamics of aerial launched torpedoes. Tank tests using scale models revealed that the "low and slow" approach that had been presumed necessary for a successful drop was actually counterproductive: striking the water at a flat angle frequently caused the after body of the torpedo to “slap”, damaging the mechanism. Full scale testing simulated aerial torpedo drops under controlled conditions by pneumatically launching full size torpedoes down a slide on California's Morris Dam into a mountain lake known for its clarity, allowing all aspects of the water entry to be examined utilizing high-speed photography. Fragile or vulnerable components were improved, tested, refined, and tested again. Improved components were shipped to Newport Rhode Island for air drop testing – 4,300 drops in all. The Caltech study led to the development of "drag rings" that slowed and stabilized the torpedo in flight