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.
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.
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:
Despite the complications that were attending the other phases of torpedo development, the Bureau of Ordnance considered the aircraft torpedo problem so important that it was assigned the highest priority at the Newport Station. The improvements and modifications of 1942 and 1943 still left the weapon unpopular, however, and production problems were as great as those stemming from incomplete development. In mid-1943 an analysis of 105 torpedoes dropped at speeds in excess of showed clearly why aviators distrusted the Mark 13: 36 percent ran cold, 20 percent sank, 20 percent had poor deflection performance, 18 percent gave unsatisfactory depth performance, 2 percent ran on the surface, and only 31 percent gave a satisfactory run.