November 17th, 1944.
Waters northeast of Trrook Lagoon, Caroline Islands.
Lieutenant Commander No Ishikawa pressed his face against the attack periscope aboard I41, tracking the American escort carrier through the lens.
The flat topped vessel steamed placidly across calm seas, seemingly oblivious to the predator lurking 300 yd beneath the surface.
Ishiawa calculated the firing solution with practiced precision.
his finger hovering over the torpedo release.
In 45 seconds, everything he understood about submarine warfare would become obsolete.
The torpedo spread would miss.
But something else would find him.
Something that hunted submarines the way submarines hunted ships.
A weapon that turned the ocean itself into a death trap with no escape.
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The impossibility of what was about to happen should have been apparent from the intelligence reports filtering through Imperial Japanese Navy headquarters since August.
American aircraft were dropping torpedoes that chased submarines.
Not straight running torpedoes that could be evaded with a sharp turn, but weapons that listened for propeller noise and followed it relentlessly through the depths.
Japanese naval architects had dismissed these reports as exaggeration or propaganda.
Torpedoes could not think.
They could not hunt.
They simply ran straight until they hit something or ran out of fuel.
Yet here was I41 about to discover that American ingenuity had created something unprecedented in naval warfare.
A torpedo that actively pursued its target using sound itself as a tracking mechanism.
The Mark 24 mine called Fido by the Americans who developed it represented a technological revolution as significant as radar or the atomic bomb.
This 68-in long weapon weighed 680 lb and Contala carried a 92lb Torpex warhead capable of crushing a submarine’s pressure hull.
Unlike conventional torpedoes that ran at speeds exceeding 40 knots, Fido moved at just 12 knots, barely faster than a man could sprint.
This deliberate slowness was the key to its lethality.
At 12 knots, the torpedo’s electric motor produced minimal noise, allowing its passive acoustic receiver to detect the distinctive sound signature of submarine propellers from up to 1500 yd away.
Once it acquired a target, Fedo executed a circular search pattern, spiraling outward until its hydrophones picked up propeller noise, then turning toward the sound and pursuing it with mechanical patience that never tired, never lost concentration, never gave up.
The development of acoustic homing torpedoes began in 1941 at the Naval Torpedo Station in Newport, Rhode Island, and Harvard University’s underwater sound laboratory.
Scientists understood that submarines produce distinctive acoustic signatures, particularly the cavitation noise from propellers spinning through water.
If a torpedo could hear these sounds and steer toward them, it would eliminate the need for perfect aim.
The weapon would guide itself.
Early prototypes proved the concept, but suffered from technical challenges.
The torpedo had to distinguish between its own propeller noise and the target sound.
It needed electronics reliable enough to survive a drop from an aircraft flying at 200 ft.
It required a warhead powerful enough to destroy a submarine, but small enough to fit within the torpedo’s diameter.
Most critically, it needed a guidance system that could track erratic submarine maneuvers.
By May 1943, engineers had solved these problems.
The Mark 24 employed a passive acoustic receiver tuned to 24 kHz, the frequency range of submarine propeller cavitation.
A simple analog computer processed the incoming sound, determining whether the target lay to port or starboard.
Servo motors adjusted the torpedo’s rudder accordingly, keeping it pointed toward the loudest noise source.
The system was elegantly simple which made it reliable.
There were no complex calculations, no fragile components.
The torpedo simply followed the sound of its prey through the darkness.
The first operational deployment occurred on May 14th, 1943 when a TBF Avenger from the escort carrier USS Bogue dropped a Mark 24 against German submarine U456 in the North Atlantic.
The hubot had crash dived after being surprised on the surface.
The acoustic torpedo splashed into the water, activated its homing system, detected the submarine’s propellers, and detonated against the pressure hull 45 seconds later.
U456 sank with all hands.
The entire engagement, from aircraft detection to submarine destruction, took less than 3 minutes.
German survivors from other attacks reported hearing a torpedo circling their boat.
growing closer with each pass, stalking them like a mechanical shark.
The psychological impact was devastating.
The Imperial Japanese Navy entered the Pacific War with 29 fleet submarines and 33 smaller coastal boats.
Japanese submarine doctrine emphasized attacking enemy warships rather than merchant shipping, a strategic decision that would prove costly.
While German hubot nearly strangled Britain by sinking cargo vessels, Japanese submarines focused on hunting American battleships and carriers, difficult targets that traveled with extensive escorts.
This doctrinal choice meant Japanese submarines achieved far less strategic impact despite operating in waters rich with vulnerable supply ships.
The type B submarines that formed the backbone of Japan’s undersea fleet were formidable vessels by any standard.
I41 commissioned on November 15th, 1944, displaced 1,780 tons surfaced and 2,215 tons submerged.
She stretched 356 ft from bow to stern and could dive to depths exceeding 300 ft.
Her diesel engines generated 4700 horsepower on the surface, pushing her to 17 1/2 knots.
Submerged electric motors provided 6 1/2 knots for 1 hour or 3 knots for extended periods.
She carried 17 torpedoes for her six tubes and mounted a 5-in deck gun for surface engagements.
By the standards of 1944, she represented sophisticated naval engineering, but sophistication meant nothing against a weapon that could hunt her through the depths.
Japanese submarine tactics in late 1944 reflected desperation born from three years of catastrophic losses.
The Imperial Navy had lost 63 submarines by the end of 1943, many to American destroyers equipped with increasingly effective sonar and depth charges.
Submarine commanders received orders to avoid unnecessary risks, to strike only when success seemed certain, to preserve their vessels for the final defense of the home islands.
This cautious approach reduced effectiveness further.
Submarines that attacked infrequently sank few ships.
Yet aggressive tactics invited destruction from the growing American anti-ubmarine forces that patrolled every major sealane in the Pacific.
The fundamental problem facing Japanese submarines was the same challenge that had destroyed Germany’s yubot force.
The compression of survival time through technological advancement.
In 1942, a submarine surprised on the surface had perhaps 2 to 3 minutes of warning before aircraft arrived overhead.
Lookouts could spot approaching planes at 5 to 8 miles on clear days.
The submarine could crash dive, reaching safe depth before bombs or depth charges arrived.
By 1944, American carrier aircraft equipped with advanced radar could detect surfaced submarines at 15 mi and attack at speeds exceeding 250 mph.
The window between detection and attack compressed to less than 1 minute.
Even worse, the introduction of acoustic torpedoes meant that successful crash dives no longer guaranteed survival.
A submarine that escaped to the depths could still be hunted and killed by a weapon specifically designed to find it there.
The TBF Avenger became the primary delivery platform for acoustic torpedoes in the Pacific Theater.
This single engine torpedo bomber manufactured by Grumman Aircraft Engineering Corporation entered service in 1942 and quickly proved itself as one of the war’s most versatile naval aircraft.
The Avenger measured 40 ft in length with a 54 ft wingspan and could carry a 2,000 lb torpedo or equivalent weight in bombs.
Powered by a right R2600 cyclone engine producing 1,700 horsepower, the aircraft reached maximum speeds of 275 mph and cruised at 145 mph.
Most importantly for anti-ubmarine work, it could remain airborne for extended periods, loitering over suspected submarine positions while searching with radar.
The typical anti-ubmarine patrol pattern involved TBF Avengers flying from escort carriers in coordinated sweeps across designated search zones.
Each aircraft carried an ASV radar capable of detecting surfaced submarines at ranges up to 15 mi in good conditions.
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When radar contact was established, the Avenger would close at maximum speed, using the radar to guide it to within visual range.
The pilot would then identify the target visually, ensuring it was indeed a submarine rather than a whale, floating debris, or friendly vessel.
If confirmed as an enemy submarine, the attack sequence began immediately.
The attack profile for acoustic torpedo delivery differed significantly from conventional torpedo drops.
Standard torpedoes required precise speed, altitude, and angle calculations.
The pilot had to release at exactly the right moment for the torpedo to enter the water properly and run toward the target.
Acoustic torpedoes simplified this dramatically.
The pilot only needed to drop the weapon reasonably close to where the submarine had submerged.
Fedo would handle the rest, spiraling outward until its hydrophones detected propeller noise, then pursuing the target autonomously.
This forgave errors in aim and timing that would cause conventional torpedoes to miss entirely.
The procedure called for the Avenger to approach at 150 ft altitude and 130 knots air speed.
When positioned approximately 400 yd from the submarine’s last known position, the pilot toggled the release.
The torpedo splashed into the water, its impact fuse armed.
A water activated battery powered up the electric motor and acoustic guidance system.
Within seconds, Fido began its deadly search pattern.
The entire sequence from aircraft sighting to weapon activation took approximately 45 seconds, exactly the time a Japanese submarine needed to reach crush depth and attempt evasive maneuvers.
Commander Nouo Ishiawa understood the mathematics of submarine survival with intimate precision.
He had graduated from the Imperial Japanese Naval Academy at Itajima in 1933, ranking 17th in his class of 192 midshipman.
His early career had been unremarkable, serving aboard destroyers and cruisers before transferring to the submarine service in 1938.
By 1944, with 11 years of naval experience, including two war patrols as executive officer, Ishikawa had developed the cautious competence that kept submarines alive in hostile waters.
He knew that aggressive action invited destruction, but excessive caution achieved nothing.
The balance between these extremes defined successful submarine command.
I41’s mission orders for November 1944 were straightforward.
patrol northeast of Trrook.
Interdict American supply lines.
Attack targets of opportunity.
Avoid unnecessary combat with escorts.
The orders reflected Japan’s deteriorating strategic position.
With the Marana Islands lost and American forces advancing toward the Philippines, Japanese submarines could no longer operate freely, even in waters that had been secure just months earlier.
Every patrol became an exercise in survival, as much as combat operations.
The sighting of the American escort carrier on November 17th seemed like a gift.
Ishiawa had positioned I41 along a predicted convoy route, running submerged during daylight hours, surfacing only at night to recharge batteries and ventilate the boat.
The appearance of a flat top steaming without apparent escort defied probability.
Escort carriers never traveled alone.
They operated as centerpieces of hunter killer groups surrounded by destroyers and destroyer escorts specifically configured for anti-ubmarine warfare.
Yet here one cruised across calm seas as though the ocean belonged solely to America.
Ishiawa’s tactical training warned him that situations too good to be true usually were.
An unescorted carrier might be bait deliberately exposed to lure submarines into attacking.
Japanese intelligence had reported American task groups specifically organized to hunt submarines with carriers launching continuous air patrols while destroyers waited to pounce on any submarine foolish enough to reveal its position.
Attacking such a group meant certain death.
Yet the carrier before him showed no sign of escorts.
Perhaps it had become separated from its group.
Perhaps damage had forced it to proceed independently to the nearest port.
Or perhaps Ishiawa was about to make a fatal mistake.
He spent 12 minutes observing through the attack periscope, keeping the observation periods brief to minimize the chance of being spotted.
The carrier maintained steady course and speed, making no suspicious zigzags or alterations.
No destroyers appeared on the horizon.
The sky remained clear of aircraft except for what appeared to be a single plane circling lazily miles away.
Everything suggested a legitimate opportunity for a successful attack.
Ishikawa made his decision.
They would attack.
The firing solution was textbook perfect.
Range 3,200 yd.
Target speed 16 knots.
Target bearing steady.
I41 carried type 95 torpedoes.
Oxygenfueled weapons with 49 knots speed and range exceeding 12,000 yd.
Ishiawa ordered a spread of four torpedoes, calculating that at least two should strike the carrier’s hull.
The submarine’s torpedo officer repeated the firing data, confirming each setting.
The crew fell silent, as they always did before torpedo launch, every man aware that the next moments would determine whether they became heroes or casualties.
Fire one.
Fire two.
Fire three.
Fire four.
The torpedoes hissed from their tubes in precise sequence.
Four deadly fish racing through the water toward their target.
Ishiawa kept the periscope raised, watching for the explosions that should occur in approximately 90 seconds.
The crew began counting silently, each man calculating the time until impact.
90 seconds became 2 minutes, then three.
No explosions.
The torpedoes had missed or malfunctioned.
Ishiawa ordered the periscope lowered, preparing to maneuver away from the area before American escorts could triangulate their position from the torpedo wakes.
Then the lookout’s voice crackled through the intercom with words that turned every submariner’s blood cold.
Aircraft bearing 270, range closing rapidly.
The TBF Avenger had been waiting.
The apparently unescorted carrier was indeed bait and I41 had taken it.
Lieutenant James McCarthy, piloting Avenger number 23 from escort carrier USS Anzio, had been circling at 8,000 ft, far too high for visual detection from a submarine, but perfectly positioned to monitor his radar scope.
When I41 launched her torpedoes, the disturbance in the water showed clearly on his ASV radar.
McCarthy pushed his throttle forward, diving toward the target area at maximum speed.
His radio man, aviation radioman Secondass Robert Chen, armed the Mark 24 torpedo in the Bombay.
They had perhaps 45 seconds before the submarine reached safe depth.
Ishiawa didn’t hesitate.
Dive, dive, all ahead full.
Take her to 90 m.
The claxon shrieked through I41’s compartments, sending every crew member racing to diving stations.
The main ballast tanks flooded with roaring fury.
The bow planes tilted to maximum down angle.
Men scrambled forward to add weight to the bow, speeding the dive.
The submarine’s nose dropped sharply, angling toward the depths that might save them.
But 45 seconds was not enough time.
McCarthy’s Avenger reached the target area 30 seconds after I41 began her crash dive.
The submarine’s conning tower had just disappeared beneath the surface, leaving swirling water and a spreading oil slick marking her position.
McCarthy flew directly over the disturbance at 150 ft altitude, toggling the torpedo release.
The Mark 24 dropped cleanly from the bomb bay, splashing into the water 400 yd from where I-41 had submerged.
The acoustic torpedo activated immediately upon water entry.
Its electric motor hummed to life, spinning the single propeller that would drive it through the depths at 12 knots.
The passive acoustic receiver powered up, its hydrophones extending to detect sound in all directions.
For 3 seconds, Fedo ran straight, clearing the surface turbulence.
Then its guidance system activated, beginning the spiral search pattern that would either find a target or exhaust its battery in approximately 15 minutes.
Aboard I41, racing toward depth at emergency speed, the crew heard nothing unusual at first.
The roar of flooding ballast tanks and the wine of electric motors at maximum power drowned out external sounds, Ishikawa focused entirely on reaching 90 m as quickly as possible.
Knowing that American depth charges were set for shallower detonations, if they could reach depth fast enough, survive the initial attack, they might escape by running silent and deep while the aircraft exhausted its ordinance.
The submarine passed through 50 m, still diving steeply.
60 m.
The pressure hull creaked from the strain of rapid submergence.
70 m.
They might make it.
80 m.
And then the sonar operator’s voice cut through the controlled chaos.
[Music] The announcement made no sense.
They had fired the torpedoes.
The Americans shouldn’t have torpedoes in the water unless another submarine was attacking, which seemed impossible given the aircraft overhead.
But the sonar operator insisted, “Trpedo screws confirmed.
” Not our fish.
Enemy torpedo approaching from a stern.
Ishikawa’s mind raced through the tactical situation trying to understand what was happening.
An American submarine would have announced itself with a spread of torpedoes already striking I41’s hull.
This single torpedo approaching from the direction of the aircraft made no tactical sense unless unless the aircraft had dropped it.
But aircraft torpedoes ran straight.
They couldn’t chase a submarine through the depths.
Yet the sonar operator’s next report confirmed the impossible.
Torpedo is following us.
Bearing constant range 800 meters and closing, it’s tracking our movement.
For the first time in his naval career, Nou Ishiawa felt pure terror, a torpedo that followed, a weapon that hunted.
The intelligence reports had been true.
The Americans had created something that transformed submarines from predators into prey, and I41 had less than 2 minutes before that weapon found them.
All stop.
Emergency silence.
Ishiawa barked the orders, hoping against probability that stopping the propellers would save them.
If the torpedo tracked sound, perhaps silence would cause it to lose contact.
The electric motors shut down immediately.
The submarine’s forward momentum carried her deeper, passing 90 m and continuing toward 100.
Inside the pressure hull, absolute silence descended.
No one moved, no one spoke.
Every crew member held his breath, listening for the sound of approaching death.
The sonar operator whispered into his microphone so softly that Ishiawa could barely hear.
Torpedo screws fading, range increasing.
It’s moving away from us.
A collective exhalation swept through the control room.
The weapon had lost them.
Stopping the propellers had worked.
They were safe.
Ishiawa allowed himself a moment of relief before beginning to plan their escape.
They would remain silent for at least 30 minutes, allowing the aircraft to exhaust its search and depart.
Then they would creep away at minimal speed, putting distance between themselves and this deadly hunting ground.
But the sonar operator’s voice returned, now edged with panic.
Torpedo screws increasing again.
It’s coming back.
Bearing 345, range 600 m.
The Mark 24 had executed exactly as designed.
When I41’s propellers stopped, Fido lost its acoustic target and began a wider spiral search pattern.
The torpedo circled through the depths, its hydrophones searching for any sound that might indicate a submarine.
For nearly 1 minute, it detected nothing.
Then, faintly, it heard something.
The creaking of I41’s pressure hull adjusting to depth.
The whisper of water moving past the submarine’s stationary form, perhaps even the breathing and heartbeats of 70 terrified men packed into a steel tube.
The sounds were subtle, barely distinguishable from ambient ocean noise.
But they were enough.
Fido turned toward the source and resumed its patient pursuit.
Ishikawa understood they had seconds to make a decision that would determine whether his crew lived or died.
If they remained silent, the torpedo might lose them again, but it would continue searching, spiraling closer with each pass.
If they restarted the motors and attempted to flee, they would provide a clear acoustic target for the weapon to track.
Either choice could be fatal.
He chose action over helplessness.
All ahead, full rightful rudder.
Take her to 120 m.
The submarine lurched forward as her electric motors roared back to life.
The sudden acceleration threw men against bulkheads.
Ishiawa ordered a sharp turn, hoping to throw off the torpedo’s pursuit.
I41 healed over at a steep angle, turning 90° to starboard while diving deeper.
Perhaps the change in bearing combined with greater depth would confuse the weapons tracking system.
The sonar operator’s voice became a continuous stream of updates.
Torpedo tracking our turn.
Range 400 m.
It’s matching our maneuvers.
Range 300 m.
Speed unchanged.
Range 200 m.
The weapon was getting closer.
No evasive maneuver worked.
I41 turned again, this time to port.
The torpedo followed.
The submarine dove steeper, approaching dangerous depths where the pressure hull might fail.
The torpedo stayed with them, closing relentlessly at 12 knots, while I41 fled at 6 1/2 knots, the maximum her batteries could sustain.
Simple mathematics doomed them.
The torpedo was twice as fast.
It would catch them.
Range 100 m, 50 m.
The sonar operator abandoned any pretense of calm.
Brace for impact.
The Mark 24’s proximity fuse detected the mass of I41’s hull at a range of 15 ft and triggered the detonator.
92 pounds of torpex explosive erupted against the submarine’s portside aft of the conning tower, creating a pressure wave that traveled through water at nearly 5,000 ft pers.
The explosion punched a hole 8 ft in diameter through the pressure hull, instantly flooding three compartments.
The force of the blast snapped the submarine’s keel, breaking her back.
I41 went from a functioning warship to a sinking wreck in less than 1 second.
Commander Nouo Ishikawa died instantly when the explosion crushed the control room.
So did 47 of his 71 crew members.
The 24 men who survived the initial blast found themselves trapped in the submarine’s forward section, which flooded rapidly through the massive hull breach.
Three men managed to reach escape lungs and began emergency ascent procedures, blowing air into their lungs until their bodies adapted to decreasing pressure.
Two of them reached the surface alive but died from the bends before American destroyers could reach them.
The third man, Petty Officer Secondass Teeshi Yamamoto, survived the ascent and was rescued by destroyer escort USS England.
He spent the rest of the war in a prisoner camp in Hawaii.
One of only 41 Japanese submariners to survive their submarines destruction and be captured alive during the entire Pacific War.
The destruction of I41 on November 17th, 1944 represented more than a single submarine lost.
It marked the culmination of a technological revolution that had been building since August 1943 when the first Mark 24 acoustic torpedoes arrived in the Pacific theater.
In the 16 months between August 1943 and December 1944, these weapons would sink or damage 37 Japanese submarines, fundamentally altering the strategic balance in the Pacific and accelerating Japan’s march toward defeat.
The Mark 24 mine, despite its innocuous designation as a mine for security purposes, represented one of the war’s most closely guarded secrets.
The weapons development had assigned the highest priority classification with information compartmentalized even within the Navy.
Pilots who carried Fido were told only that it was a special anti-ubmarine weapon and were forbidden from discussing its capabilities or even acknowledging its existence.
This security was so effective that Japanese intelligence never fully understood what was killing their submarines.
Survivor accounts and recovered debris suggested some kind of homing torpedo, but Japanese scientists dismissed this as impossible given the technical challenges involved.
The acoustic torpedo project had begun at Harvard University’s underwater sound laboratory in January 1941, nearly a year before Pearl Harbor.
Dr.
John Hayes Hammond Jr.
, a pioneer in radio control systems, led the research team that would eventually create the weapon.
Hammond understood that submarines produced distinctive sounds, particularly the cavitation noise created when propellers spun rapidly through water, creating tiny bubbles that collapsed with characteristic acoustic signatures.
If a torpedo could detect these sounds and steer toward them, it would solve the fundamental problem of anti-ubmarine warfare.
The target was invisible beneath the surface, making accurate aim nearly impossible.
The technical challenges were formidable.
The torpedo needed hydrophones sensitive enough to detect submarine propellers from hundreds of yards away, but not so sensitive that they picked up the torpedo’s own motor noise.
It required an analog computer capable of processing acoustic signals and converting them into steering commands, all while surviving the shock of water impact from a low-flying aircraft.
The guidance system had to distinguish between submarine sounds and dozens of other noises in the ocean.
whale songs, shrimp colonies, surface ships, waves breaking, even the torpedo’s own propulsion.
Most critically, the entire system needed to be reliable enough for combat use, simple enough for safe mass production, and cheap enough to be manufactured in quantities of thousands.
By March 1942, the research team had developed a working prototype.
The weapon measured 68 in long and 19 in in diameter.
Small enough to fit in an aircraft’s bomb bay, but large enough to house the necessary components.
An electric motor powered by a seawater activated battery drove a single propeller at 12 knots.
The slow speed was deliberate.
Faster movement would generate propeller noise that would interfere with the acoustic receiver.
The passive hydrophone array consisted of four receiving elements positioned around the torpedo’s nose.
each tuned to frequencies between 20 and 30 kHz.
An analog computer compared the signal strength from each hydrophone, determining whether the target sound came from port or starboard.
Servo motors adjusted the rudder accordingly, keeping the torpedo pointed toward the loudest noise source.
The warhead contained 92 lb of torpex, a explosive mixture 60% more powerful than TNT.
This relatively small charge was sufficient because acoustic torpedoes almost always struck submarines directly rather than detonating nearby like depth charges.
The proximity fuse activated when the torpedo closed to within 15 ft of a large metal object, ensuring detonation at the optimal range for maximum hull damage.
The entire weapon weighed 680 lb, light enough for a single aircraft to carry while retaining enough battery capacity for 15 minutes of operation.
Initial testing at the Naval Torpedo Station in Newport, Rhode Island, revealed unexpected problems.
Early prototypes circled endlessly, unable to distinguish their own propeller noise from target sounds.
Others ran erratically, their guidance systems confused by multiple acoustic sources.
The most dangerous malfunction involved torpedoes that locked onto the launching aircraft’s engine noise and pursued it instead of diving after submarines.
This problem required extensive redesign of the acoustic receivers’s frequency response, tuning it to ignore higher frequency sounds characteristic of aircraft engines while remaining sensitive to the lower frequencies of submarine propellers.
By January 1943, these issues had been resolved sufficiently for operational deployment.
The first production batch of 100 Mark 24 torpedoes was delivered to the Atlantic fleet in February 1943.
The weapons combat debut came on May 14th, 1943 when Lieutenant Robert Williams, flying a TBF Avenger from escort carrier USS Bogue, attacked German submarine U456 southwest of Iceland.
Williams had surprised the Yuboat on the surface, forcing it to crash dive.
Rather than dropping conventional depth charges, Williams released a Mark 24 from 400 ft altitude.
The torpedo splashed into the water, activated its homing system, and destroyed U456 47 seconds later.
Post-action analysis confirmed the weapon worked exactly as designed.
The success rate of acoustic torpedoes in the Atlantic proved remarkable.
Of the first 50 Mark 24 weapons deployed in combat, 11 achieved confirmed submarine kills, and another six scored probable kills or severe damage.
This 22% kill rate compared favorably to the 9% success rate of conventional depth charges.
More importantly, acoustic torpedoes required far less precision in delivery.
A pilot only needed to drop the weapon reasonably close to where a submarine had dived.
Fido would handle the search and attack autonomously.
The Pacific Fleet received its first allocation of Mark 24 torpedoes in August 1943.
Initially, only squadron commanders and specially selected pilots were briefed on the weapon’s capabilities.
Standard briefings told pilots simply that the Mark 24 was a new anti-ubmarine weapon that should be dropped near the swirl where submarines had dived.
No mention was made of acoustic homing or autonomous tracking.
This operational security prevented word from spreading even within American forces, ensuring that Japanese intelligence would learn nothing from captured pilots or monitored radio communications.
The first Pacific deployment of acoustic torpedoes occurred on August 29th, 1943 when TBF Avengers from escort carrier USS Lisk Bay conducted anti-ubmarine patrols near the Gilbert Islands.
Japanese submarine I19, which had achieved fame by sinking the carrier USS Wasp in September 1942, was detected on the surface by radar.
The submarine crash dived before the Avengers arrived, but Lieutenant Commander James Daniels dropped a Mark 24 at the swirl.
The torpedo found I19 at a depth of 80 m and detonated against her pressure hull.
The submarine sank with all 94 hands, becoming the first Japanese submarine destroyed by acoustic torpedo in the Pacific theater.
The loss of I19 sent shock waves through Japanese submarine command.
The boat had been one of Japan’s most successful submarines, credited with sinking carrier Wasp, battleship North Carolina with torpedo damage, and destroyer O’Brien.
Her commander, Lieutenant Commander Kinashi Takayichi, was an experienced officer with multiple successful patrols.
The circumstances of her loss puzzled Japanese analysts.
American aircraft had attacked after I-19 had safely dived to depth where conventional depth charges should have been ineffective.
Yet, the submarine had been destroyed by what appeared to be a single torpedo hit.
How had the Americans achieved such precision against an invisible target? Over the following months, additional submarine losses followed similar patterns.
Boats would be surprised on the surface, crash dive successfully, then be destroyed minutes later by single explosions at depth.
Survivors who reached the surface, rare in submarine actions, reported hearing torpedo screws circling their boats before impact.
Some described the sound as growing closer, then fading, then returning, as if the weapon was searching for them.
Japanese naval intelligence compiled these reports, but struggled to develop a coherent explanation.
The concept of an acoustic homing torpedo seemed too advanced for wartime production, requiring electronics and guidance systems beyond what intelligence assessments suggested American industry could manufacture in quantity.
This underestimation of American technological capability represented a consistent failure of Japanese strategic planning throughout the Pacific War.
Japanese planners believed that American manufacturing while capable of mass production lacked the sophistication for complex weapons requiring precision engineering.
The Zero fighter superiority early in the war had reinforced this assumption.
But by 1943, American industry was producing not just vast quantities of weapons, but increasingly sophisticated systems.
Proximityfused anti-aircraft shells that exploded near targets without contact.
Radar systems small enough to fit in fighter aircraft and acoustic torpedoes that hunted submarines autonomously.
Japan’s technological development, hampered by limited industrial capacity and resource shortages, could not match this innovation.
The statistical impact of acoustic torpedoes on Japanese submarine operations became apparent in the casualty figures.
In 1942, before acoustic torpedoes were deployed, Japanese submarines lost to aircraft attack numbered 14 out of 19 total losses.
A 74% aircraft attribution rate.
However, most of these losses occurred when submarines were caught on the surface and destroyed by conventional bombing or strafing.
Successfully crash dived submarines had an 85% survival rate against air attack in 1942.
By 1944, these numbers had reversed catastrophically.
Of 47 Japanese submarines lost that year, 31 were sunk by aircraft, representing 66% of total losses.
More significantly, 19 of those 31 air attack losses involved submarines that had successfully crash dived before the attack.
The survival rate for submarines that completed emergency dives dropped to 42% in 1944, a collapse directly attributable to acoustic torpedo deployment.
The compression of survival time created by acoustic torpedoes paralleled the experience of German hubot facing sentiment radar in the Atlantic.
Japanese submariners discovered that successfully diving no longer guaranteed safety.
The 45 seconds between aircraft sighting and crash dive completion represented merely the first phase of survival.
Even after reaching depth, submarines remained vulnerable to weapons that could hunt them through the darkness.
This psychological shift proved as devastating as the physical casualties.
Submarine crews that once felt relatively safe once submerged now understood they remained targets until the hunting aircraft exhausted its ordinance and departed.
Lieutenant Commander Toshio Kusaka, commanding submarine I 176, experienced this new reality on May 16th, 1944 while operating east of the Admiral T Islands.
His boat had been patrolling submerged during daylight hours when the sound operator detected high-speed propellers approaching from the south.
Kusaka ordered the submarine deeper and reduced speed to minimize noise signature.
The propeller sounds passed overhead without incident.
American destroyers hunting fruitlessly for submarines that remained undetected.
At 1700 hours, with daylight fading, Kuska brought I76 to periscope depth to observe the surface situation.
Finding the area clear, he surfaced to recharge batteries and ventilate the boat.
Within 12 minutes, lookouts spotted an aircraft approaching from the northwest.
The submarine’s radar detector remained silent, suggesting the aircraft had not yet acquired them.
Kusaka ordered an immediate crash dive.
I 176 slipped beneath the surface 42 seconds before the TBF Avenger passed overhead.
The dive appeared successful.
The submarine reached 70 m depth and leveled off, running on silent routine to avoid detection.
Then the sound operator reported torpedo screws in the water bearing steady and range closing.
Kusaka immediately understood what was happening.
His intelligence briefings had mentioned American aircraft dropping torpedoes that might pursue submarines, though the reports had been vague and contradictory.
He ordered flank speed and a sharp turn to starboard, hoping to outrun or evade the weapon.
For 90 seconds, the maneuver seemed to work.
The torpedo screws faded in bearing and range.
Then they returned closer than before.
The weapon was circling, searching.
Kusaka tried another turn, this time to port while driving the submarine to 100 m depth.
Again, temporary success, followed by the inexurable return of propeller noise.
The chase continued for nearly 6 minutes, an eternity for men trapped in a steel tube, knowing that death pursued them through the depths.
Each evasive maneuver provided brief restbite before the relentless screws returned.
The sound operator’s voice became increasingly strained as he reported bearings and ranges, counting down the meters until impact.
At 60 m range, Kusaka ordered all stop, hoping silence would save them as it had in exercises against conventional torpedoes.
The Mark 24 lost acoustic contact when I 176’s propellers stopped.
The torpedo began a wider search spiral, listening for any sound that might betray the submarine’s position.
For 40 seconds, the sonar operator heard nothing.
Then, faintly, growing gradually louder, the propeller screws returned.
The torpedo had found them again.
Kusaka had no options remaining.
The submarine could not outrun the weapon.
Silence did not protect them.
Evasive maneuvers only delayed the inevitable.
The acoustic torpedo struck I 176’s pressure hull at frame 63 just forward of the engine room.
The explosion breached the hull across a 6-foot section, flooding the engine room and maneuvering compartment instantly.
The submarine’s stern section flooded within 90 seconds, dragging the boat down by the stern at a 40° angle.
Emergency bulkheads held temporarily, giving the forward section crew time to dawn emergency breathing apparatus and attempt escape through the forward torpedo room hatch.
14 men made it out of the submarine, following the bubbles toward the surface from a depth of 112 m.
The rapid ascent from such depth should have killed them all from decompression sickness.
Remarkably, seven survived the ascent and reached the surface alive, though only three remained conscious.
American destroyer USS Jenkins recovered these three survivors, including Lieutenant Commander Kusaka, who would spend the remainder of the war as a prisoner.
His testimony provided Allied intelligence with detailed confirmation of Japanese submarine vulnerability to acoustic torpedoes.
The tactical evolution of acoustic torpedo deployment reflected increasing American confidence in the weapon’s reliability.
Initial doctrine called for dropping a single Mark 24 when a submarine was detected, conserving the limited supply of these specialized weapons.
By mid 1944, with production exceeding 200 torpedoes monthly, tactics shifted to multiple weapon attacks.
Aircraft would drop two or even three acoustic torpedoes in sequence, saturating the area where a submarine had dived.
This tactic proved devastatingly effective.
Even submarines that successfully evaded the first torpedo found themselves pursued by a second or third weapon, each searching it independently.
The attack on submarine I26 on November 6th, 1944 demonstrated this evolved tactic.
Two TBF Avengers from escort carrier USS Santi detected the submarine on radar northeast of Lee Gulf.
I26 crash dived successfully, reaching safe depth before the aircraft arrived.
The first Avenger dropped a Mark 24 at the swirl, then circled while his wingman positioned for a second attack.
The first torpedo acquired I26’s acoustic signature and began pursuit.
The submarine’s commander, Lieutenant Commander Kusumi Toshiaki, ordered evasive maneuvers and managed to temporarily evade the first torpedo through a sharp turn combined with reduced speed.
Before Kusumi could congratulate himself on the successful evasion, the second Avenger dropped another mark 24400 yd from the submarine’s new position.
This torpedo immediately detected I26’s propellers and began its own pursuit.
The submarine now faced two weapons simultaneously approaching from different bearings.
Kusumi’s options evaporated.
He could not evade both torpedoes.
The first weapon, which had temporarily lost contact, reacquired the submarine’s acoustic signature when Kusumi increased speed to evade the second torpedo.
Both weapons converged on I26 from opposite directions.
The first torpedo struck the submarine’s port side amid ships.
The second weapon, following just 20 seconds behind, detonated against the starboard side aft.
The combined explosions broke I26’s pressure hull in multiple locations and snapped the keel completely.
The submarine sank stern first, flooding so rapidly that no crew members had time to dawn emergency breathing equipment.
All 78 men aboard died, including Kusumi, who had survived three previous war patrols and was considered one of the submarine forc’s most skilled commanders.
Japanese attempts to counter acoustic torpedoes proved largely ineffective due to incomplete understanding of the threat.
Without captured examples or detailed intelligence on how the weapons functioned, Japanese engineers could only speculate about countermeasures.
Some submarines deployed noise makers, devices that created loud sounds intended to decoy acoustic homing systems.
These proved marginally effective at best.
The Mark 24’s guidance system could distinguish between the artificial sound of noise makers and the specific acoustic signature of submarine propellers.
Other countermeasures focused on reducing acoustic signature through improved propeller design and sound dampening materials applied to submarine holes.
These modifications showed promise in testing, but came too late to affect the war’s outcome.
By the time Japanese shipyards began incorporating acoustic quieting into new submarine construction in early 1945, Japan’s industrial capacity had degraded to the point where submarine production had nearly ceased.
Of 12 new submarines laid down in 1945, only three were completed before Japan’s surrender.
The most effective Japanese response to acoustic torpedoes was not technological but tactical.
Avoid surfacing in areas with known American air activity.
This defensive posture, while reducing losses to air attack, also reduced submarine effectiveness to near zero.
Submarines that remained submerged continuously could not patrol effectively, could not recharge batteries adequately, and could not maintain speeds necessary to intercept convoys.
The Imperial Navy’s submarine force, which had begun the war with aggressive hunting tactics, ended it cowering beneath the waves, achieving almost no operational success while still suffering steady attrition.
The psychological impact on Japanese submarine crews exceeded the physical casualties.
Sailors who had trained for years, learning to think of the depths as sanctuary, now understood that safety no longer existed.
The ocean that had once protected them had become a hunting ground where invisible weapons stalked them through the darkness.
Morale plummeted across the submarine force.
Volunteers for submarine duty, once plentiful among the Imperial Navy’s most skilled sailors, became scarce.
Men sought transfers to surface ships despite those vessels own vulnerability to air attack and submarine torpedoes.
Petty Officer Firstclass Hiroshi Nakamura, who survived the sinking of submarine I373 on August 11th, 1944, provided a haunting account of the psychological transformation in his postwar memoirs.
Nakamura had been serving aboard I 373 when TBF Avengers from USS Manila Bay surprised the submarine on the surface west of Saipan.
The crash dive succeeded in getting the boat underwater before the aircraft could attack.
But moments after leveling off at 70 m, the sound operator reported torpedo screws approaching.
Nakamura described the next 4 minutes as the longest of his life.
The crew listened in absolute silence as the propeller noise grew steadily louder.
The sound operator provided bearing and range estimates, his voice becoming increasingly tense as the weapon closed.
At 50 m range, the submarine’s commander ordered flank speed and a radical turn.
The maneuver delayed impact by perhaps 20 seconds.
The torpedo adjusted its course and continued pursuit.
At 10 m estimated range, men began embracing each other, saying farewell, preparing for death.
The propeller noise became audible throughout the submarine without need for electronic amplification, a high-pitched whine that grew into a shriek.
Then inexplicably, the sound passed down the submarine’s starboard side and continued past without detonating.
The Mark 24 had missed, its proximity fuse failing to detect the submarine’s hull, or perhaps malfunctioning.
The crew barely had time to process their survival before the sound operator reported another torpedo approaching from a different bearing.
The second Avenger had dropped its weapon while the first torpedo was making its attack run.
I373 attempted another evasive maneuver, but the submarine’s batteries had depleted significantly during the high-speed evasion of the first torpedo.
Maximum speed had dropped to 4 knots, insufficient to outrun a weapon moving at 12 knots.
The second torpedo tracked straight to target without deviation, striking the pressure hull just forward of the control room.
The explosion killed 41 crew members instantly, including the commander.
Nakamura, stationed in the Aft torpedo room, survived the initial blast, but found himself trapped in the flooding submarine.
Seven men in the aft section managed to reach emergency breathing equipment and exit through the stern torpedo tube.
Three survived the ascent from 90 m depth and were recovered by American destroyer escorts.
Nakamura spent 6 months recovering from severe decompression sickness in a Navy hospital in Pearl Harbor before being transferred to a prisoner of war camp.
His description of hearing the torpedo screws approaching, knowing that death hunted him through the darkness, unable to see the threat or effectively fight it, captured the existential horror that acoustic torpedoes inflicted on submarine crews.
The technical specifications of Japanese fleet submarines theoretically gave them capabilities to evade acoustic torpedoes.
Type B submarines could dive to 300 m depth, well beyond the operating range of most depth charges.
Their maximum submerged speed of 6 1/2 knots, while slower than the Mark 24’s 12 knots, could be sustained for extended periods.
More importantly, Japanese submarines carried sophisticated sound equipment capable of detecting surface ships at ranges exceeding 10 km.
This equipment should have been able to detect approaching acoustic torpedoes in time for evasive action.
The reality proved far different from theory.
Japanese submarine sound equipment, while sophisticated, was optimized for detecting surface ships, not underwater torpedoes moving at relatively slow speeds.
The acoustic signature of a Mark 24 approaching at 12 knots was subtle, often masked by the submarine’s own propeller noise and the ambient sounds of the ocean.
By the time sound operators detected the distinctive high-pitched wine of an approaching acoustic torpedo, the weapon was typically within 200 m, providing less than 60 seconds for evasive action.
Moreover, Japanese submarine commanders lacked training and doctrine for evading acoustic homing torpedoes because Japanese intelligence had not confirmed such weapons existed until late 1943.
Standard evasion tactics for conventional torpedoes, sharp turns and depth changes proved marginally effective at best against weapons that could track and follow.
Some commanders developed intuitive countermeasures through trial and error, discovering that stopping all propulsion sometimes caused acoustic torpedoes to lose contact.
But this information spread slowly through the submarine force, hampered by the fact that submarines destroyed by acoustic torpedoes typically went down with all hands, leaving no survivors to report what had killed them.
The escort carrier task forces that deployed acoustic torpedoes represented a broader revolution in American anti-ubmarine warfare.
These small carriers converted from merchant ship hulls displaced approximately 10,000 tons and carried 28 aircraft typically a mix of TBF Avengers and FM Wildcat fighters.
Their primary mission was not fleet operations, but anti-ubmarine patrol, providing continuous air coverage over convoy routes and hunting Japanese submarines in their patrol areas.
USS Anzio, the escort carrier whose aircraft destroyed I41, exemplified this class.
Commissioned on August 27th, 1944, Anzio operated in the Philippine Sea and South China Sea throughout late 1944 and early 1945.
Her air group conducted hundreds of anti-ubmarine patrols, attacking 23 submarine contacts and achieving six confirmed kills.
The carrier’s success demonstrated how escort carriers multiplied American anti-ubmarine effectiveness by providing air coverage in areas that fleet carriers could not regularly patrol.
The coordination between escort carriers and their destroyer escorts created hunter killer groups specifically optimized for submarine destruction.
The carriers provided radar surveillance and air attack capability.
The destroyers offered sonar detection and depth charge attacks.
Together, they created multiple overlapping threat layers that Japanese submarines struggled to evade.
A submarine detected on radar by carrier aircraft would be forced to dive where it became vulnerable to acoustic torpedoes.
If it evaded the torpedoes and attempted to escape underwater, destroyers would track it with sonar and attack with depth charges.
The submarine had no safe option.
For more detailed historical breakdowns, check out the other post of this page
