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Type 91, Japanese Torpedo


Photograph of Type 91 torpedoes
Wikipedia Commons


Type 91 Model 2

Specifications:


Dimensions     
17.7" by 18'0"
45cm by 5.486m
Weight
1841 lbs
935 kg
Range 2200 yards (2000m) at 42 knots
Warhead 452 lbs Type 97 explosive
205 kg Type 97 explosive
Propulsion       Kerosene-air two-row eight-cylinder radial wet heater (210 bhp) driving contra-rotating four-blade propellers
Launch speed
260 knots
Variants

The Model 1, produced up to April 1941, was slightly shorter and had only a 331 lb (150 kg) charge.

The Model 3, produced after October 1941, increased the charge to 529 lbs (240 kg).

The Model 3 Improved, produced from 1943, could be launched at 300 knots.

The Model 3 Strong, introduced in 1944, sacrificed range (1640 yards or 1500m at 42 knots) for a launch speed of 350 knots.

The Model 4 Strong, introduced later in 1944, increased the charge to 538 lbs (244 kg).

The Model 7 Strong, introduced in late 1944, increased the charge to 926 lb (420 kg) at the cost of a knot in speed.


The Type 91 was the principal aircraft torpedo of the Japanese Navy. Unlike most other Japanese torpedoes, it did not use pure oxygen. Although the Japanese had experimented with a pure oxygen aircraft torpedo (Type 94) the complications and hazards involved were eventually judged not worth the increase in performance.

This torpedo was nonetheless a good weapon, reliable and with a high maximum launch speed. However, contrary to Japanese expectations, a single hit was not capable of penetrating the torpedo protection of American battleships. In the attack on Pearl Harbor, Nevada was hit by a single Type 91 Mod 2 that split seams and caused leaks, but did not penetrate the holding bulkhead. California was hit by two Type 91 Mod 2 torpedoes that deflected the holding bulkhead inwards but left it intact. California sank only because the hatches to the void spaces of the underwater protection system had been left open for an upcoming inspection, allowing extensive flooding that would not have taken place had the ship beenin watertight condition.

The warhead was fitted with a 0.4" (1 cm) protective rubber sheath which shattered on impact with the water. The Japanese believed this to be superior to the wooden nose shroud used on American Mark 13 torpedoes. However, the Japanese did use plywood tail appendages to help stabilize the torpedo after it was dropped. These broke away on impact with the water. The earliest models of the Type 91 were stabilized against rolling by leaving the warhead only partially filled, and much of the increase in explosive charge in later models came from improved anti-roll flippers that allowed the warhead to be completely filled.

When dropped from low altitude at very high speed, the Type 91 sometimes buckled around the engine chamber. The Strong models corrected this problem and permitted a greater launch speed. Branfill-Cook (2014) describes a Type 4 torpedo, which may be identical from the Type 91 Mod 4 Strong, which could be launched at up to 400 knots.

The Japanese experimented with a shaped charge warhead, the "V" warhead, for the Type 91. Tests demonstrated that this could punch completely through the underwater protection of a Colorado-class battleship. However, only three saw use in combat. The Japanese also developed and began deploying an antisubmarine variant of the Type 91, the QR, that was set to circle and descend after being dropped. The diameter of the circle was 300 yards (290m) and the maximum depth was 320' (100m). Only fifty were produced and there is no record of operational use.

References

Branfill-Cook (2014)

Campbell (1985)

Zimm (2011)



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