The single greatest surprise the U.S. Navy suffered in the Solomon Islands campaign was the Japanese Type 93 torpedo. Fired by cruisers and destroyers, it was much more massive than U.S. torpedoes—but more important was its spectacular combination of long range and high speed. U.S. Navy captains were trained to zigzag their ships when they were in what they thought to be torpedo range. The Type 93 allowed the Japanese to hit ships that did not zigzag because their commanders believed them to be safe from torpedo fire. When the light cruiser USS Helena (CL-50) was sunk on 6 July 1943, it was assumed she had been hit by a Japanese submarine because of the distance between opposing forces (see “A Promise Kept,” April 2018, pp. 34–39). In fact, she had been hit by a Type 93.
The Type 93, called “Long Lance” by the U.S. Navy, burned ethanol or methanol, using as its oxidizer compressed oxygen rather than compressed air. (It is sometimes mistakenly described as “oxygen fueled.” Air is 78 percent nitrogen, producing a much-less efficient burn than pure oxygen—but alcohol is still the fuel.) The 93’s 24-inch diameter (compared to the U.S. Mark 15 torpedo’s 21 inches) gave it more efficient hydrodynamics. It had half-again the weight of the Mark 15—5,952 pounds versus 3,841—and was 25 percent longer—29 feet, 6 inches, against 24 feet for the U.S. torpedo.
It often has been claimed that the U.S. Navy was unaware of the potential for night torpedo attacks, but that was hardly the case. Like other sea services, the U.S. Navy had long seen the torpedo as a potent equalizer. In war games at the Naval War College, large numbers of destroyers often delivered massive torpedo salvoes, particularly at night. U.S. Navy strategists understood that Japan needed such equalizers, because the interwar naval treaties had left it on the short end of various tonnage ratios. One less-often noted equalizer was the Imperial Japanese Navy’s (IJN) superior night binoculars, which sometimes contributed more to situational awareness than early radars.
Uniquely among the world’s navies, Japan’s destroyers carried reload torpedoes with fast-reload mechanisms for the launchers. Its cruisers were equipped with heavy torpedo batteries, and just before the war the Japanese fleet converted some old light cruisers into specialized torpedo ships armed with 40 torpedoes.
The IJN considered the Type 93 its most important secret weapon—the best equalizer. Torpedo officers were forbidden to discuss torpedoes with officers not directly concerned with them, and speed and range settings—and the use of pure oxygen—were omitted from manuals used in torpedo schools. The night glasses that made long-range torpedo attacks practical must have been at least as secret.
The U.S. Navy received its first clear evidence of Japanese 24-inch torpedoes when the wreck of the destroyer Kikuzuki was examined in the summer of 1943 (she had been sunk at Tulagi in May 1942). The oxidizer was not discovered at that time, however, and U.S. Navy Bureau of Ordnance (BuOrd) estimates of performance were far below reality. BuOrd previously had rejected a prewar report by a U.S. naval attaché who had been told that the destroyer he visited carried 24-inch torpedoes.
BuOrd considered its own Mark 15 destroyer torpedo, which could travel 10,000 yards at 33.5 knots or 6,000 yards at 45 knots, as good as any. U.S. destroyers trained to make massed long-range “browning” attacks on enemy formations, the theory being that the total length of an enemy battle line would be such that hits were inevitable. (“Firing into the brown” means shooting at a flock of birds without aiming at any specific bird—hence, “browning.”) To that end, the Mark 15 had a low-speed, long-range setting: 15,000 yards at 26.5 knots. U.S. destroyer torpedoes could be set to turn after they entered the water, so a 16-tube destroyer could fire all its torpedoes in the same direction. The 15,000-yard range was the most that could be achieved in combination with a heavy enough warhead with the fuel space available on a Mark 15. Prewar training for coordinated mass attacks may explain why U.S. destroyers in the Solomons generally were not allowed to operate independently.
Given Japan’s close ties to Britain in the early 20th century, the Japanese must have been aware of British interest in longer-range torpedoes from 1916 on. The Battle of Jutland had convinced the Royal Navy that, until it fixed major flaws in its naval artillery shells, torpedoes were the best weapons available to the Grand Fleet. British torpedo developers discovered that larger diameters offered better hydrodynamics; that is why their postwar battleship torpedo (in the Nelson-class) had a 24.5-inch diameter. The British also experimented with oxygen, which they called “enriched air,” but they abandoned it because of problems during thunderstorms.
The Japanese did not. According to a March 1944 intelligence report, Japan acquired a British “enriched air” torpedo in 1924 or 1925 (all wartime reports refer to enriched air rather than oxygen). The IJN first tested the Type 93 in 1933, deploying it from 1938 on. Its secret was so well kept that, when in 1943 the Allies captured and translated a Japanese book on the torpedo lessons of the first phase of the war, references to the Type 93 did not attract attention.
The British did notice that the Japanese were firing from very long ranges, accepting low hit percentages. But the first indications of how they did it apparently came only in February–March 1944, by which time the Japanese no longer had much opportunity to practice night torpedo attacks. A February 1944 U.S. Navy survey of Japanese naval ordnance credited the Type 93 with a range of 22,400 yards at 50 knots and 33,000 yards at 32 knots. A March 1944 report based on prisoner-of-war interrogations and captured documents credited the Type 93, nearly correctly, with a range of 32,700 to 33,800 yards at 32 knots and 11,000 yards at 47 knots. The July 1944 edition of “Statistical Summary of the Japanese Navy” (ONI 222-J) described the Type 93 as “probably . . . the most radical and spectacular development in naval ordnance that has appeared in the Pacific Theatre,” crediting it with a 1,200-pound warhead and a range of 11,000 yards at more than 45 knots. (The night glasses did not figure in this handbook, although they had been captured and often were used by U.S. officers.)
Postwar data from Japanese sources showed a range of about 44,000 yards at 36 knots and about 22,000 yards at 48 knots with a 1,080-pound warhead. Performance of the 1943 version Type 93 was slightly less: 16,300 yards at 48 knots and 32,700 yards at 36 knots—but with a 1,720-pound warhead. U.S. Navy torpedo-protection systems were designed to resist about 800 to 1000 pounds of TNT. The Japanese meant the Type 93 to be a battleship killer. As the facts of the torpedo’s performance emerged, the U.S. Navy began to rethink what had happened in the Solomons night battles.
If the Naval War College typically included Japanese night torpedo attacks in the prewar exercises, why was the fleet surprised so badly in the Solomons? The answer appears to be that the Japanese managed to keep secret the range they had achieved—and their ability to exploit that range. U.S. commanders simply believed they were staying at a safe distance when in fact they were vulnerable. Sometimes secrets really can be kept until weapons are revealed in battle.