Pearl HarborAttack from Below
Naval History, December 1999
After careful analysis of a photograph
taken by a Japanese aviator over Pearl Harbor, a team
of experts is convinced that the 7 December 1941 attack
was more than an air raid.
For years historians
have relied on survivor and eyewitness accounts, material
testing, and, if available, declassified intelligence
reports to help gather evidence to piece together an
event. Advances in computer technology now have allowed
photo imagery analysts to enhance and sharpen images,
changing the way the world is viewed. For example, digital
photo imagery analysis techniques used to measure an
inward-bent hull plate have re-energized the debate
about the reasons for the destruction of the USS Maine
(see April 1998
ALL ILLUSTRATIONS COURTSEY OF
PETER K. HSU
History has recorded that five Imperial Japanese Type-
A-class midget submarines, deployed as an advance force
to attack the Pacific Fleet in Pearl Harbor, failed
to inflict any damage during the 1941 attack. Results
from digital imagery, engineering, and forensic analyses
of a single dramatic photograph taken that infamous
December morning by a Japanese aviator, however, now
refute that view.
Admiral Isoroku Yamamoto's Pearl Harbor operational
strike plan was one of combined arms, involving the
use of carrier-based aviation in concert with the fleet,
including the five midget-type submarines. The carrier
strike was to consist of two waves of torpedo and bomber
aircraft, with Zero fighters as escorts. The fleet submarines
would engage certain capital ships from Pearl Harbor,
and they would interdict the sea-lanes between Hawaii
and the U.S. mainland. The five midget submarines would
enter the harbor. Figure 1 depicts the approach routes
of the aerial strike and the operational areas assigned
to the nearly three dozen fleet and midget-type submarines.
According to Yamamoto's plan, the five Type-A-class
midgets were to launch from their mother submarines
in the early hours of 7 December. They were then to
proceed from their launch areas to Pearl Harbor before
the aerial strike (see Figure 2). Each midget would
enter the narrow (400 yards wide) and shallow (50 feet
deep in the middle) channel, and pass under the antitorpedo
nets that hung down to a depth of 35 feet. Once inside
Pearl Harbor, the midgets would wait for the aerial
attack to begin; then they would launch their torpedoes
against targets of opportunity (preferably battleships)
as the attack unfolded.
Captain Hanku Sasaki was in command of the special
attack force of the five mother-fleet submarines, each
carrying a single, two-man midget sub. The original
orders given to the 10 Ninja submariners on 14 November
1941 were specific: Attack the surviving capital ships
on the night of 7 December and escape. Yamamoto did
not want this portion of the operation to be suicidal.
Lieutenant Naoji Iwasa, the leader of the midget submarines,
asked permission to launch his attack immediately after
the air strike instead of waiting for night. Admiral
Yamamoto granted this permission.
Figure 1: Operation Hawaiithe
Japanese attack plans for 7 December 941 formulated
by Admiral Isoroku Yamamoto (right) and analyzed
by Commander Minoru Gendaincluded nearly
three dozen fleet and midget-type submarines.
For clarity, the midget submarines are not depicted
in this graphic.
As designed in 1938, the Type-A midget submarine was
a remarkable innovation. She was a two-man sub, consisting
of a junior officer who commanded the boat and a petty
officer who manipulated the control valves and ballast.
Figure 3 shows that the Type-A measured 80 feet in overall
length, and her hull measured 6 feet in diameter. She
displaced 46 tons when submerged. Two bolted hull joints
permitted the submarine to be separated into three sections.
She was propelled by a single 600-horsepower electric
motor, driving a single-geared shaft with a pair of
contra-rotating propellers. Acid-cell batteries supplied
power, with recharging accomplished by a mother submarine.
Top speed of this class was 23 knots on the surface
and 19 knots submerged. At top speed she had an endurance
of 55 minutes. At a submerged speed of 4 knots, however,
the midget had an effective range of 100 miles or 25
hours. Each carried two 18-inch torpedoes mounted one
above the other. The size of each torpedo warhead approximated
1,000 pounds of explosivemore than twice the destructive
firepower of those carried by the Japanese attacking
In 1993, while visiting Hawaii on business, coauthor
Rodgaard, a senior image analyst at Autometric, Inc.,
met Dan Martinez, historian of the National Park Service's
USS Arizona (BB-39) Memorial, and Burl Burlingame,
a noted Pearl Harbor historian and local journalist.
Martinez showed a copy of what many consider to be one
of the 20th century's most dramatic photographs. It
was taken from a Nakajima B5N2 “Kate” torpedo bomber
during the first attack wave. The Japanese government
distributed this aerial photograph to show the successful
attack to the world. But Martinez saw something intriguing
in it and asked Rodgaard if he could determine whether
a tiny object in the photograph was one of the Japanese
midget submarines (See Figure 4).
Rodgaard subsequently secured an original, higher-quality
duplicate image of the photograph from the U.S. Navy
Historical Center in Washington, D.C. Then he, together
with Carroll Lucas, Dewey Houck, and Tim Hosek of Autometric,
Inc., interpreted the image and measured the object
to determine whether evidence supported the premise
that a midget submarine was present at Pearl.
Figure 2: In an artist's conception
by coauthor Peter K. Hsu, Ensign Masaharu Yokoyama
and Petty Officer Second Class Tei Uyeda launch
in a Type-A midget from their mother sub in the
early morning of December 1941. Upon reaching
Pearl Harbor, they were to wait for the air attack
Figure 3: Prior to launch, all
but two of five tie-downs were removed, and the
midget was given positive buoyancy. The mother
took on water, achieving negative buoyancy, and
maintained 1 to 2 knots' forward speed. Using
a telephone connection, the midget skipper signaled
the mother, which then released the ties, sending
the midget in the opposite direction.
The Autometric team presented its findings to the USS
Arizona Memorial on 7 December 1994. In 1998,
Peter Hsu conducted a forensic study based on the 1994
findings. This study looked at underwater explosion
phenomena and shock-wave propagation, elaborating the
1994 findings. Hsu's results strengthened evidence that
a midget submarine fired two torpedoes at the battleships
USS West Virginia (BB-48) and USS Oklahoma
(BB-37) during the attack.
The 1994 photographic imagery analysis of the Japanese
aerial attack photograph consisted of five phases: The
first phase involved photoscientists who reproduced
the image and enhanced it digitally by applying various
computer algorithms and techniques. The second was to
establish the sequence of events within the scene. The
third was to search the photograph for observables that
lent themselves to the presence of a midget submarine.
The fourth involved precise photographic measurements
of the object that Martinez and Burlingame had thought
was the submarine. And the fifth was to compare the
size (length and height) of the object with the specifications
of the Japanese Type-A-class midget submarine. In 1999,
Hsu built on the 1994 findings and his 1998 analyses;
his underwater shock analysis provided a sixth phase.
The image was digitized into a software image file.
Next, an edge-sharpening algorithm was applied to the
overall image scene. The subject area, where the object
was said to exist, was magnified digitally, until the
image was about to lose its integrity. At this point,
measurements of the object were taken.
In the background of the attack photograph, smoke
obscuring most of Hickam Field Army Air Base indicates
that it already had been struck by the first wave of
Japanese bombers (see Figure 4). This indicates that
the photograph was taken after the initial attack run,
which occurred at 0755. Smoke also can be seen rising
from the USS Helena (CL-50) and the USS Oglala
(CM-4), indicating that at least one, if not both, ships
had been hit. The log of the Helena is specific
about the time (0758) she was struck by a torpedo. In
addition, history records that the first wave of torpedo
bombers attacked Battleship Row within minutes after
the attack on Hickam Field. Photo evidence shows that
the first torpedo strike against the battleships USS
California (BB-44), Oklahoma, and West
Virginia had occurred. The California was
struck by one torpedo, and she was leaking oil. The
Oklahoma had been torpedoed and was listing to
port. The West Virginia had been hit at least
once, as indicated by several wakes.
The first Battleship Row attack occurred at 0758.
The photo shows torpedo entry points and wakes originating
from the Southeast Loch and located south-southeast
of their targets. Concussion waves on the surface indicated
that the West Virginia and the Oklahoma
had been struck before the photograph was taken (See
Figure 5). Water spouts associated with the torpedo
detonation against the side of the ship already had
dissipated. Considering this evidence, including calculations
from shadow analysis, the best estimate for the time
was between 0801 and 0803.
The image shows a rectangular, black object present
in the suspect area. This smaller object appears to
be sitting atop a dark linear object. The composite
shape is surrounded by the white-water area. Numerous
concussion waves also are visible in the image. The
concussion waves appear to have affected the object.
Three distinct rooster-tail sprays appear behind the
composite shape. Each tail seems to coincide with the
passing of a concussion wave. The height characteristics
of the tail sprays grow toward the composite shape with
the smallest tail, or the oldest, having occurred first.
The sprays have a distinct characteristic shape different
from the splashes one would see from torpedoes entering
the water from the air, or from exploding ordnance.
Six torpedo wakes can be seen in the photograph. Four
appear erased partially by concussion waves. The broadest
wakes represent greater degradation. No torpedo wakes
are visible beyond the easternmost splash points. The
two narrowest (newest) wakes (numbers five and six)
converge at a point behind the rooster tails. The evidence
concluded that a line of convergence can thus be traced
through the dark rectangular object, the rooster tails,
and the intersection of the most narrow torpedo wakes.
Figure 4: According to photographic
imagery and marine forensic and photogrammetric
analysis this combat phototaken during the first
wave of the attack from a carrier-based Nakajima
B5N2 "Kate" torpedo bomberverifies
the presence of midget submarines in Pearl Harbor.
Splash points and torpedo tracks have been highlighted
Figure 5: An enlargement of
the area in question indicates the presence
of black, rectangular object sitting atop a
dark linear structure, followed by "rooster
tail" plumes. The analysis have determined
these images to be a midget submarine, its conning
tower and contra-rotating propellers exposed
by the concussion of a torpedo hit on the U.S.
battleship West Virginia.
Two objectives had to be achieved. First, it was necessary
to locate the object geographically and position it
relative to the battleships in the field of view. The
second objective was to determine the object's height
and size. To achieve these objectives, a least-square
photogrammetric triangulation determined the position
and attitude of the camera used by the Japanese.
Two aerial stereo photographs taken by a U.S. Army
Air Corps mapping camera on 26 November 1941 and known
ground points served as a control base for triangulation.
A measurement of the object's pixel position on the
digital photograph established its location in the harbor.
Several objects with known locations were also measured
to establish the validity of the triangulation resection.
The geographic coordinates of the object were calculated
as 21 degrees 21 minutes and 4.2 seconds North latitude,
by 157 degrees 57 minutes and 12.5 seconds West longitude.
The minimum height and length dimensions of the object
were thus determined by the same techniques and calculated
to be 1.25 meters high and 18.2 meters long.
Comparing the physical dimensions of the Type-A-class
midget submarine with the photogrammetric measurements,
the vertical height and length measurements of the object
coincide with those derived from the analysis.
In order to establish a methodology to conduct forensic
analysis of the shock-wave effects of the underwater
explosion phenomena associated with the torpedo attack,
the research team postulated a chronology of images
observed and an activity scenario.
History documented that the first attack wave occurred
at 0758. Based on previous rudimentary shadow analysis,
the team verified that the combat photo was taken between
0801-0803, thus establishing the latest time at about
0803. In the absence of both a huge water spout associated
with torpedo detonations against the side of the ship
and torpedo bombers flying over the battleships, the
forensics team concluded that the attack group had completed
its attack run and observed the following (See Figure
1. The distance between the splash points of the aerial
torpedoes aimed at (wakes 2 and 4) the West Virginia
is 450 meters. Assuming the average torpedo speed was
40 knots, the running time is estimated to have been
about 22 seconds.
2. Huge water spouts observed from another photo taken
during the attack measure about as high as two ships'
lengths. A water mass falling from this altitude would
take an estimated 30 seconds, with approximately another
30 seconds for the dense mist to dissipate partially.
3. In Figure 4, a water spout has formed next to the
port hull plating of the West Virginia. In the
absence of any “Kate” or “Val” bombers over the battleships,
the hypothesis of torpedoes being fired from a submarine
becomes viable. In the photo, the first torpedo has
exploded, and the second has just hit the Oklahoma.
Since the midget torpedoes were Type 97, which contained
a 1,000-pound warhead, and the air-drop torpedoes were
Type 91, which had a 452-pound warhead, the explosive
energy to be considered in the underwater explosion
analysis ranges from 452 to 1,000 pounds.
4. White-water disturbance surrounds the suspect submarine.
5. Three triangle-shaped water sprays, south and in
line with the submarine, exhibit a typical spray pattern
associated with contra-rotating propellers striking
the approaching surface concussion waves.
6. The concussion waves were propagated radially from
the West Virginia. The distance between each
wave ring appears to be substantially large, migrating
with significant momentum. (Reticule measurements indicate
each concussion wave ring was about 2 to 3 feet high).
7. With regard to the West Virginia image in
Figures 4 and 5 and in the explanation of the water
spout in item 3 above, the forensics team performed
an analysis of the relationship of the torpedo detonation
and its generated cavitation region.
During an underwater explosion event, the chemical
reaction of the solid explosive yields gaseous products
and exerts high explosive pressure on the surrounding
water. The disturbance (compression shock wave) near
the charge propagates radially at 3 to 5 times the speed
of sound. The surrounding water is compressed and attains
an extremely high radial velocity. The compression shock
wave reflecting from the free surface results in a tensile
reflected wave, causing a region of water to cavitate,
thus forming a bulk cavitation zone. The water refraction
near the surface wave has a vertical velocity. The velocity
of a water particle is the vector sum of the velocities
produced by the associated pressure. Thus, the midget
submarine is within the cavitation zone and was buffeted
by a vertical motion, exposing the conning tower and
its contra-rotating propellers.
The magnitude of the vertical velocity and the motion
history of the water particles behind the shock front
is dependent on its location relative to the charge,
the free-surface effect, and the bottom reflection,
which was compounded by explosive gas pulsation, especially
in shallow water. In general, such underwater explosion
energy will lift an object vertically in the cavitation
Extrapolations of the bulk cavitation zone calculated
from selected charge size and attack geometry indicated
that the midget submarine was within the cavitation
region. The 40-foot shallow draft of Pearl Harbor, with
its bottom reflection potential and the gas bubble pulsation
would have exaggerated the cavitation range, causing
the submarine to be launched upward by the explosion.
As the submarine broached near the surface, it increased
speed suddenly and headed into the oncoming waves caused
by the concussion. Water crashed into the propeller
disk, forming three distinct rooster-tail sprays synchronous
with each passing wave. The third smallest rooster tail
had dissipated somewhat by the time the photo was taken.
This underwater explosion analysis has strengthened
evidence of the existence of a midget submarine through
a positive correlation between the motion history of
the midget submarine and the observed surface disturbance
Photographic evidence indicates that a Type-A-class
midget submarine penetrated Pearl Harbor on 7 December
1941. She positioned herself in the Southeast Loch and
waited to launch her torpedoes at the capital ships
on Battleship Row. These ships were identified as the
USS Oklahoma and the USS West Virginia.
When the aerial strike started, the midget fired her
first torpedo at the West Virginia, then turned
to port, fired her second torpedo, and increased speed.
With her second torpedo heading toward the Oklahoma,
the midget began her run toward the harbor entrance
to escape. During this time, the submarine was struck
by a series of concussion waves struck the submarine;
because of a loss of trim owing to the release of her
torpedoes, the effects of her speed, and the oncoming
concussions, the midget broached the surface as she
was buffeted by cavitation effects. She pitched vertically
in the cavitation field generated by the underwater
explosion, which exposed her rudder and contra-rotating
propellers and caused rooster tail sprays to form. An
aerial photograph captured the entire event.
Six torpedo wakes are evident in the photo. Four appear
partially obliterated by the concentric concussion waves
emanating from the recent strikes on the battleships.
The remaining two are thinner and affected less by the
wave action. These two wakes terminate in the midst
of the water disturbance where the suspect object is
located. Two of the broader aerial torpedo wakes also
terminate at these ships, with the third broader wake
aligned in the direction of the California. The
broader torpedo wakes could indicate greater wave action
degradation, thus making them older than the narrower
wakes. The two narrower wakes converge at another area
of water disturbance nearly in line with the aerial
torpedo entry points but nearer to Battleship Row. At
the point of convergence, a thin, dark object can be
seen projecting above the surface of the disturbed water
surrounding it. Slightly to the rear of this object,
three spray splashes appear, with the largest nearest
the object and the smallest the farthest away. Each
spray plume is aligned with the concentric concussion
waves that have just reached the area.
Figure 6: An artist's conception
by coauthor Hsu depicts a midget submarine scoring
a torpedo hit on the West Virginia at
a depth of 10-15 feet beneath the water surface,
or below the ship's armor. Torpedo wake numbers
5 and 6 coincide with the calculations of the
research team, which contends that the West
Virginia and the Oklahoma were attacked
The observed torpedo track (see Figure 4, Track No.
5) indicated alignment with the water plume on the port
side of the West Virginia. Because of the water's
optical reflection and parallax effect, the research
team determined that the torpedo was at a depth of about
10-15 feet beneath the water surface. This alignment
triangulates the impact point to be beneath the ship's
armor but where the vertical plane intersects with the
observed plume (see Figure 6).
The final disposition of this midget submarine is
still unknown. Photographic and forensic engineering
evidence, however, indicates that she entered Pearl
Harbor successfully on 7 December 1941 and positioned
herself in the Southeast Loch. When the aerial attack
against Battleship Row was under way, she fired both
of her torpedoes against the West Virginia and
The authors all are marine forensic and
imaging specialists. Commander Rodgaard is a senior
image analyst for Autometric, Inc.; Mr. Hsu is a technical
director for Techmatics, Inc., and is a member of the
Marine Forensic Panel (SD-7) of the Society of Naval
Architects and Marine Engineers; Dr. Lucas is a staff
member of CL Imagery Exploitation Services, and Captain
Biache is an independent photographic imagery specialist.
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