WWII Anti-Vessel Ordinance

The Magnetic Mine

Napoleon once said that he preferred marshals with luck. Somebody else said, “Luck is a matter of planning.” The story of defeating the magnetic mine, which to the British was a bad surprise, shows how one side’s poor planning was the other side’s luck.

Toward the end of 1939, some ships entering and exiting British ports were damaged by underwater explosions that hit their lower hulls. The damage usually was not fatal, but in many cases bottom plates were torn, rivets popped out, and internal machinery and propeller shafts dislodged. Many of these ships had to be written off or at best put into dry dock for repair.

An investigation confirmed that these ships were not hit by conventional sea mines. (Such a mine is usually placed at low depth and anchored to the bottom by a cable so that it will be positioned a few feet below the surface.) The investigation of the ships that managed to stagger into port pointed to an explosion beneath the ship but at a distance from it. This led to the conclusion that the damage was caused by a so-called “influence mine,” which was laid on the bottom and was activated by the propeller noise, the pressure wave of the approaching ship, or the effect of the ship’s metal hull on the local magnetic field of the earth. The experts tended to assume that these were magnetic mines, because already in World War I such mines were developed although never used. The trouble was that no effective countermeasures could be devised and employed without knowing the exact characteristics of the detonation mechanism, and finding one became a priority undertaking. But how do you identify and recover a mine lying somewhere on the sea floor? Here Lady Luck smiled on the British—and not once but twice.

A German aircraft dropping such mines made a navigational error at night. At high tide, the area flown over by the airplane was covered with water, and the pilot (or navigator) probably thought he was in the right position, but when the tide receded the mine was observed lying in the mud next to a British military base. The mine was moved into a workshop, and the experts (who already suspected it to be a magnetic mine) manufactured a set of bronze (nonmagnetic) tools, disassembled it, and learned how it worked. Here luck played a role again. The mine contained an antimotion device to protect against tampering if dropped on land. This device was to be deactivated by water entering it, if dropped at sea. The short time the mine spent in the water rendered it safe for handling.

The British developed three ways to counter the mine. The one that finally became standard, because it was the cheapest and did not require sailing through “cleared” corridors, was the “degaussing” of the ships. By dragging charged electrical cables over the hulls, the ships became nonmagnetic. This took about half an hour, although the process had to be repeated every six months. The technology of the magnetic mine was not really new, and the Germans chose a well-suited weapon to use. Without better information, the British might have groped in the dark for a long time, spending time and effort trying to deduce the exact nature of the mechanism. Navigational carelessness negated all the work the Germans invested.

The Acoustic Torpedo

An acoustic torpedo, which homes in on the noise the target produces, was thought of during World War I but, because of technical limitations, was never developed. The Germans were later the first to produce one designed to home in on the propeller noise of surface ships. A first variant was introduced in July 1943 but quickly superseded by a faster variant (the Zaunkoenig), which was used with moderate success. It had a major problem that the Germans were apparently unaware of: it sometimes exploded just when entering the turbulent wake behind the target. The Allies for some time suspected such a German development, because the Americans were busy developing their own acoustic torpedo and concurrently thought of potential countermeasures. So within sixteen days of the appearance of the Zaunkoenig, they introduced the Foxer, a towed noisemaker that caused the torpedoes to detonate prematurely (Macksey 2000, 143).

The Germans distributed this torpedo sparingly, and submarine crews were instructed to use it only against escort vessels and not merchantmen (Gannon 1996, 99–100). Later, when several such torpedoes were captured by the Allies, it was found that they could home in only on ships moving at twelve to nineteen knots (Gannon 1996, 101). It is not clear if the Germans were aware of this limitation or that the torpedo was designed from the start to attack escort ships as first priority.

The Americans advanced the homing technology much further. They had no need to attack merchantmen or escort vessels in the Atlantic but were acutely aware of the need to attack submarines. (The German submarine force was deemed of higher priority than the Japanese merchant fleet and its escorts.) From 1943, the ocean was regularly scanned by aircraft that took off from Iceland or Greenland and from convoys’ escort carriers. When such an airplane discovered a submarine, it would attack using bombs or depth charges and report the position to a Combat Information Center, which then decided whether to send a surface vessel (if one was available) or aircraft, which would force the submarine to stay submerged until the arrival of surface vessels.

But depth charges were of a limited efficacy. To explode near the submarine, the attacker had to follow the underwater maneuvering of the submarine and stay more or less above it. This remained true even after the next generations of forward-firing projectors—starting with the Hedgehog—were developed. More important, depth charges were set before firing to explode at a given depth. While this did not totally depend on guesswork, it was nearly so. Obviously, something better was needed.

In the fall of 1942, the U.S. Navy developed the sonobuoy. This device parachuted to the water, listened for anomalous sounds, and broadcast them to an airplane. It succeeded in detecting submarine propellers up to three and a half miles away. In order to fully exploit this capability, the United States then developed an acoustic torpedo that could home in on the submarine’s propellers, and specifically on cavitation noises. This torpedo, the Mk-24 (referred to as the Mk-24 Mine to hide its true nature, and nicknamed FIDO), entered service in the beginning of 1943 and was meant to be kept in production only until the end of the year. It was assumed that by that time the Germans would figure out its characteristics and its usefulness would be over (Price 1980, 110). To delay this possibility, the Allies introduced some strict rules. One of these said that this torpedo was not to be dropped against a submerged submarine when surfaced submarines were in the vicinity. By that time, the Allies controlled the air to such an extent that they could force even groups of submarines to submerge and then attack (Price 1980, 181). This torpedo also exploited the basic instinct of any submarine’s commander: when detected, dive as fast as possible. But running the motors at highest power caused cavitation, which was his undoing. In fact, if he had just shut down his motors, the torpedo would have lost its lock-on, but as pointed out, this was against the basic instincts of submariners. The secret of the Mk-24 torpedo was not compromised until the end of the war (Price 1980, 225n1).

Due to the combination of advanced technology and good secret keeping, this torpedo achieved a high success rate of nearly 20 percent sinkings and 9 percent damaged submarines, compared with 9 percent for depth charges.

“Long Lance”[1] Type 93 Torpedo

The modern torpedo, initially intended to be fired from surface ships, was developed by Robert Whitehead, a British engineer who lived in Italy (then under Austrian rule) and operated there a successful factory for marine engines. In 1848, Whitehead observed Austrian troops in Milan suppressing a popular uprising. He was horrified by what he saw and became a pacifist. He then thought of developing naval weapon so dreadful it would prevent future wars. His occupation with marine engines and his belief that naval warfare was the key to victory (in this, he anticipated Admiral Alfred Mahan) no doubt lay behind this conclusion. In 1860, he saw a demonstration of a remotely controlled explosive-carrying boat, but he thought that an underwater vehicle would be better and sat down to develop one. In 1870, he demonstrated his “torpedo,” and the Austrian navy, which at the time controlled part of the Adriatic Sea coast, was the first to buy it. The Royal Navy, the strongest naval power of the time, was the second, and in a few short years all the world’s navies were equipped with torpedoes. One of the torpedo’s main advantages was that even small boats could pack a punch comparable to big ships, which led to the development of a new class of ships—the “torpedo boat destroyer”—which eventually became the “destroyer.” The Royal Navy was the first to fire a torpedo in anger, in 1877, against some Peruvian rebels. It missed, but it was enough to scare the rebels away.

Toward the end of the nineteenth century, the torpedo was improved. Its original source of propulsive power, compressed air, was replaced by an internal combustion engine that received oxygen from a tank of compressed air. This was a major improvement but had a major drawback: Beside oxygen, air consists of 80 percent nitrogen, which does not contribute to the combustion and thus is exhausted as a visible wake of bubbles. This sometimes enabled a ship to avoid the torpedo by a quick maneuver. Everybody was looking for something better.

Replacing the air in the tank with pure oxygen, or high-concentration peroxide (H2O2), which the Germans tried, would have solved two problems. It would have increased the amount of oxygen in a given air tank, and since all combustion products were water-soluble, the bubbles would have been eliminated. However, the proximity of pure oxygen to grease and moving parts is an invitation for uncontrolled combustion, especially on surface ships engaged in combat.

Experimentation with oxygen was undertaken by several navies, and on the entrance of the United States into World War II, such torpedoes were at various stages of testing. However, Admiral King, the U.S. Navy’s chief of naval operations, believed such research would interfere with the production of standard torpedoes and assigned it the lowest priority (Blair 1975, 279–80).

The Japanese, in their effort to achieve excellence, were aware of the dangers but decided that the advantages of oxygen technology surpassed its disadvantages. They developed several versions of this torpedo, to be launched from surface ships, submarines, and aircraft. Thanks to the use of oxygen, these torpedoes were faster, had more than double the range, and carried a heavier warhead than any comparable Western torpedo. After the war, the Japanese also reported that they had no shipboard accident with these torpedoes (Blair 1975, 279–80).

The Japanese were very careful to make sure that no such torpedo fell into the wrong hands. This policy sometimes caused large numbers of ships to search for lost practice torpedoes, which were supposed to surface after their run (Lowry and Wellham 2000, 38). Nevertheless, their security sometimes failed. Luckily for them, the Americans did not notice.

In 1934, the U.S. Office of Naval Intelligence (ONI) translated a Japanese article that stated “our latest torpedoes ran with practically no track.” One of the officers who read that passage highlighted it, but there is no evidence that ONI pursued the matter further (Mahnken 2002, 70). A worse security leak occurred several years later.

At the end of 1939 or the beginning of 1940, the American naval attaché in Tokyo was approached in his tennis club by a local medical student who turned out to be Chinese. The man, angered by Japanese atrocities in China, told the American that the Japanese navy organized tours for students in order to encourage a national spirit and increase recruitment. The American asked some specific questions, and on their next meeting the man told him that the Japanese had developed an oxygen-propelled torpedo and cited its performance, which surpassed anything available in the West (Mahnken 2002, 70–71). The naval attaché forwarded a report to Washington, and although the range was understated by the Chinese student, it still caused a stir at ONI. A copy was forwarded to the Bureau of Ordnance, but they declared that such a weapon was impossible (Mahnken 2002, 71). They probably understood that to obtain such performance the torpedo had to utilize oxygen technology, as the Tokyo report clearly stated. But since the United States and Britain were struggling with this technology, they assumed the Japanese could not have perfected it on their own. The Bureau of Ordnance experts preferred to consider the report a mistake rather than face the spectre of Japanese technological superiority. Ironically, the Japanese developed this technology because of a mistaken belief that the British had already mastered it (Mahnken 2002, 71n101).

Armed with the judgment of the Bureau of Ordnance, ONI filed away all reports about oxygen-powered torpedoes and abandoned pursuing any further “rumors” about advanced Japanese torpedoes.

In response to the Guadalcanal landing and in an attempt to hit American supply ships in the area, the Japanese sent in a task force of cruisers and destroyers. In a night battle (the Savo Island Battle), it attacked and defeated a similarly sized American force in what was later described as the worst defeat in battle of the U.S. Navy, which lost four cruisers and a destroyer against no losses and only slight damage to the Japanese. It was the first in a series of night battles in which the Japanese fired long-range torpedoes at ranges far longer than the range of their or American guns.

In the beginning of 1943, such a torpedo, called the Long Lance, washed ashore at Cape Esperance on Guadalcanal, was taken apart, and its data was sent to Pacific Fleet intelligence, but nothing except rumors filtered back. In a meeting preparatory to one of these battles (Kula Gulf), the captain of one American cruiser who had heard the “rumors” warned the presiding admiral not to approach the Japanese to less than ten thousand yards. The admiral, who believed that a submarine sank one of his ships in a previous engagement, dismissed the story as “scuttlebutt” (Morison 1949, 196). In the ensuing battle, this captain’s ship, in addition to a destroyer, was sunk.

The U.S. Navy was aware of Japanese emphasis on night fighting, which reduced the advantages of American material superiority (Mahnken 1996, 435). This possibility was already exercised in 1933 in an American war game in which the American force was defeated by a torpedo attack, nine years before a Japanese admiral actually did this for real. (A night gun battle could not be efficient, let alone decisive, without radar.) Surprisingly, the Americans did not ask themselves whether the real-life Japanese (not those in the war game) would look for other means to circumvent their inferiority in radar technology.

And there was another failure, that of not realizing that the enemy thinks in a different way. In the United States, it was thought that radar developments would enable gun battles at night, and this might have led to the implicit assumption that when the Japanese would catch up in radar technology, naval battles would revert to gunnery, including at night. But apparently the Japanese understood early the advantage the Long Lance conferred on them. Their doctrine thus called for a night battle, initiated by torpedoes fired from cruisers and destroyers, and a daylight mopping up by guns. For this purpose, they equipped many destroyers and cruisers with large numbers of these torpedoes, and they even converted two cruisers to “torpedo cruisers,” which carried dozens of them (Mahnken 1996, 435).

[1] The Type 93, designated for Imperial Japanese calendar year 2593) was a 61 cm (24 in)-diameter torpedo of the Imperial Japanese Navy (IJN), launched from surface ships. It is commonly referred to as the Long Lance by most modern English-language naval historians, a nickname given it after the war by Samuel Eliot Morison, the chief historian of the U.S. Navy, who spent much of the war in the Pacific Theater. In Japanese references, the term Sanso gyorai, lit. “oxygen torpedo”) is also used, in reference to its propulsion system. It was the most advanced naval torpedo in the world at the time.


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