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.
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 . 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:
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 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 8 May 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:
left|thumb|Mark 13 Model 6 with shroud ring