The air dropped, Mk 13 torpedo was probably best of the US torpedoes of the early ww2, reliability-wise. Its main problems were low allowable drop speed and height, forcing the A/C carrying it to slow down too much, turning those into sitting ducks. In case the A/C were above the speed altitude limits, the torpedo would display a host of problems. The speed and altitude limits were increased by mounting the wooden 'noses' that acted as air brakes, and would also receive a good deal of stress on themselves, once the torp hit the water. Also the wooden frame was attached around the tail. Another problem was the low speed of the torpedo itself, making possible for most of the targeted ships to simply turn away and run from it.
People might want to check out this link, for an overview of this and other US ww2 torpedoes.
Here's what navweaps has to say about the Mk 13:
A total of 156 Mark 13 Mod 0 torpedoes were produced which was enough to provide two loads for each of the four 18-plane torpedo squadrons assigned to the pre-war carrier fleet plus a dozen spares. Mod 0 differed from later mods by having a rail-type tail in which the propellers were in front of the rudders. This was the only US torpedo to ever have this feature. The Newport Torpedo Station was unhappy with arrangement for reasons unknown and the Mod 1 entered service in 1940 with a conventional propeller arrangement, as can be seen in the photographs above. Unfortunately and unlike the Mod 0, the Mod 1 proved to be an unreliable weapon, with only one of ten torpedoes dropped by VT-6 during an exercise in July 1941 having a hot, straight and normal run. Of the others, four sank and could not be recovered while the other five experienced erratic runs.
These problems continued into the early war years, with a mid-1943 analysis of 105 torpedoes dropped at speeds in excess of 150 knots found that 36 percent ran cold (did not start), 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. The total exceeds 100 percent as many torpedoes had more than one defect. The early models were further handicapped by the need to drop them low and slow - typically 50 feet (15 m) and 110 knots - which made the torpedo planes carrying them vulnerable to attack.
These problems were greatly reduced by the latter years of the war. Torpedoes had fin stabilizers, nose drag rings and tail shroud rings added, all of which worked to slow the torpedo after it was dropped so that it struck the water nose-first and at an acceptable speed. These improved the drop characteristics such that the recommended aircraft maximum launch parameters were increased to a height of 2,400 feet (730 m) and a speed of 410 knots.
A lanyard was attached to the tail of the torpedo. When dropped, the lanyard tripped a starting lever, but a water trip delay valve prevented the combustion flask from lighting off until the torpedo had entered the water. When dropped at 150 knots or more, the torpedo would enter the water at an angle of between 26 and 30 degrees. The water needed to be at least 150 feet deep (45 m) and the torpedo assumed its preset running depth after water travel of 300 yards (275 m). The exploder mechanism was armed after water travel of 200 yards (180 m). Depth could be set up to 50 feet (15 m).
The addition of the nose drag ring improved aerodynamic performance by stabilizing the torpedo in flight and reduced air speed by about 40 percent. It also acted as a shock absorber when the torpedo struck the water. The tail shroud ring improved the water run by reducing hooks and broaches and by eliminating much of the water roll which had characterized the earlier Mark 13s. Hot, straight and normal runs now approached 100 percent. To speed availability of the much improved torpedo, the Bureau of Ordnance had tail assemblies built with the shroud ring attached and then shipped these to the fleet for upgrading the existing inventory. By the fall of 1944, the modified torpedo was in general use by the front-line carrier units which were enthusiastic in their praise. On one occasion in early 1945, six torpedoes were dropped from altitudes between 5,000 and 7,000 feet (1,500 to 2,100 m). Five out of the six were observed to make their runs hot, straight and normal. By the end of the war, the USN considered the Mark 13 to be the best aircraft torpedo produced by any nation and it remained in service until 1951.
USA Torpedoes of World War II
However, the RN and IJN had some very capable airborne torpedoes by 1945, so I have to take the last sentence with a grain of salt.
