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.
