Japan Stunned When America Fixed Mark 14 Torpedoes and Cut Supply Lines

September 15th, 1943. The American submarine USS Tang prowels the East China Sea under Commander Richard Okaine. Her torpedo rooms carry the first batch of Mark1 14 torpedoes with reliable contact exploders. For 21 months, American submarines had fired nearly worthless weapons at Japanese shipping.

 Now with three critical defects finally corrected, the underwater war was about to transform from embarrassing failure into devastating success. The stakes were Imperial Japan’s entire logistics network spanning 6,000 m from the home islands to the conquered territories of Southeast Asia. The torpedo crisis had begun the moment Pearl Harbor burned.

 In the Philippines, 23 state-of-the-art American submarines of the Asiatic Fleet failed to sink a single Japanese warship despite scoring direct hits. The torpedoes simply refused to explode. For the next 2 years, submarine commanders would fire 14,748 torpedoes with a success rate barely reaching 20%.

 Japanese merchant ships entered port with American torpedoes protruding from their hulls below the waterline, unexloded and harmless as steel logs. Lieutenant Commander Frederick B. Warder of the USS Seawolf had endured six war patrols watching his Mark1 14 torpedoes fail. Off Christmas Island, he approached a Japanese invasion force and fired at a light cruiser from perfect range.

 Only one torpedo struck, allowing the target to limp away for repairs. Throughout the attack, his crew heard explosions but saw no damage. When Warders switched back to the older Mark 10 torpedoes, his hit rate improved dramatically. The newer weapons, supposedly superior in every way, were actually inferior to torpedoes designed in 1915.

 The Mark14 torpedo carried a warhead of 680 lb of torpex explosive, enough to break the back of any merchant ship. But three separate design flaws had turned America’s primary submarine weapon into an elaborate mechanical failure. The depth control system contained a critical error that sent torpedoes running 10 to 11 ft deeper than their settings.

 Magnetic exploders triggered prematurely due to sensitivity to Earth’s magnetic field. Contact exploders featured firing pins too heavy to function properly, creating duds even on perfect hits. Bureau of Ordinance officials in Washington refused to acknowledge these problems existed. When submarine commanders filed reports of torpedo failures, Bour blamed poor approach techniques and inadequate maintenance.

 The peaceime testing had been woefully inadequate. At $10,000 per torpedo, the Navy deemed them too expensive to actually explode during trials. Test firings were considered successful if the exhaust bubbles passed under target ships. Not a single Mark14 had been detonated in testing before the war began. Captain Atsushi Oi of the Imperial Japanese Navy watched American submarine activity from his position on the staff of Japan’s shipping protection forces.

 Throughout 1942 and early 1943, he observed what appeared to be remarkably ineffective American submarine attacks. Japanese merchant ships reported being hit by American torpedoes that failed to explode. Some vessels reached port with unexloded American fish embedded in their hulls. The Americans seemed to possess excellent submarines operated by skilled crews, but their weapons were fundamentally defective.

 The first breakthrough came in August 1942 when Admiral Charles Lockwood forced Bour to conduct proper depth testing. Torpedoes fired through nets at various depths revealed they were running approximately 11 ft deeper than set. The problem stemmed from poor sensor placement on the torpedo’s nose where water flow created pressure differentials that fooled the depth control mechanism.

Moving the sensor to the torpedo’s midbody, where hydrodnamic effects were minimized, solved the depth problem. But fixing the depth issue only revealed the second major flaw. With torpedoes now running at correct depths, magnetic exploders began triggering more frequently. However, they were detonating prematurely before reaching optimal position beneath enemy holes.

Submarine commanders estimated that 10% of their torpedoes exploded prematurely, while board claimed the rate was only 2%. The magnetic influence exploder was designed to sense changes in Earth’s magnetic field caused by a ship’s steel hole. In practice, the system proved hypers sensitive and unreliable. Lieutenant Commander John Scott in the USS Tunny experienced the magnetic exploders failures firsthand on April 9th, 1943.

 From only 880 yards, he fired all 10 torpedo tubes at Japanese aircraft carriers Hyo, Juno, and Teao. He heard seven explosions, but achieved minimal damage. The magnetic exploders had triggered too early, wasting a perfect attack opportunity on three major fleet units. Admiral Lockwood faced enormous pressure from his submarine commanders to abandon the magnetic feature entirely.

 On June 24th, 1943, he officially authorized the activation of the Mark 6 magnetic exploder. Submarines would rely solely on contact detonation. This decision immediately improved hit rates, but soon revealed the third and final major defect in the Mark1 14 torpedo. Lieutenant Dan Daspit, commanding the USS Tonosa, discovered the contact exploder problem during one of the most frustrating attacks in submarine warfare history.

 On July 24th, 1943, he found the 19,250 ton tanker Tonin Maru, number three, dead in the water. From close range, he fired four torpedoes and achieved two hits that stopped the target completely. Approaching from optimal angle and range, Daspit then fired nine more torpedoes at the stationary target, observing each impact through his periscope. All nine were duds.

 The contact exploders firing pin was too heavy and crushed upon impact, preventing detonation. This attack proved that the better the shot, the more likely the torpedo would fail to explode. Direct hits at 90° angles created maximum deceleration forces that destroyed the firing mechanism. Glancing blows at oblique angles might allow the firing pin to function, but perfect shots guaranteed failure.

 American submarine commanders were being penalized for accurate gunnery. The solution came from an unexpected source. After the Pearl Harbor attack, workshops at subbase Pearl Harbor had recovered numerous Japanese aircraft shot down during the raid. Engineers examining the wreckage discovered that Japanese propeller blades were manufactured from lightweight aluminum alloy, ideal for torpedo firing pins.

 The irony was unmistakable. Japanese metal would fix American torpedoes that would then sink Japanese ships. By September 1943, new contact exploders with lightweight firing pins were reaching American submarines throughout the Pacific. The first reliable Mark14 torpedoes were finally deployed after 21 months of war. USS Haddock proved their effectiveness immediately, sinking two Japanese ships with four torpedo hits.

 No duds, no prematures, no misses due to incorrect depth. The American submarine force finally possessed dependable weapons. The transformation in submarine effectiveness was immediate and dramatic. In 1942, American submarines had conducted 350 war patrols and sunk approximately 180 Japanese merchant ships totaling 725,000 tons.

 In 1943, conducting the same number of patrols, they sank 335 ships totaling approximately 1.5 million tons. The difference was reliable torpedoes and improving intelligence. Major Y. Horry of the Imperial Japanese Army served as liaison officer with the Navy’s convoy escort forces. From his position at first convoy escort fleet headquarters on Formosa.

 He witnessed the dramatic increase in Japanese shipping losses beginning in late 1943. American submarine attacks, previously ineffective nuisances, suddenly became devastating strikes that sent entire convoys to the bottom. Horry observed that the Japanese Navy’s offensive-minded doctrine had left merchant shipping virtually unprotected.

The intelligence breakthrough that complemented improved torpedoes came from crypton analysis of Japanese naval codes. In November 1942, British codereakers at the Far East combined bureau broke the J40 code system used by Japanese merchant shipping. Known to Americans as the Maru code, this system provided real-time intelligence on convoy movements throughout the Japanese Empire.

 Daily noon position reports from Japanese merchant ships were intercepted and decoded, allowing American submarines to plot exact intercept courses. Commander Richard Okaine, former executive officer of the legendary USS Wahoo, exemplified the new effectiveness of American submarines. Taking command of the USS Tang in 1944, he sank 24 confirmed Japanese ships in 9 months of operations.

 His success rate averaged one ship destroyed for every 10 days on patrol. The combination of reliable torpedoes and precise intelligence had transformed submarine warfare from hit or miss hunting to systematic slaughter. Captain Atsushi Oi noted the dramatic change in American submarine effectiveness beginning in late 1943 where previously Japanese ships might survive torpedo hits due to American weapon failures.

 Now every submarine contact became potentially fatal. The Imperial Japanese Navy finally recognized the severity of the shipping crisis and established the combined escort fleet on November 15th, 1943, exactly when American torpedoes became reliable. This centralized organization assumed responsibility for all convoy protection throughout the empire.

Japan’s response revealed the scale of American success. For 2 years, while American submarines struggled with defective weapons, Japan had failed to develop adequate convoy protection. Now, as American submarines reached peak effectiveness, Japan attempted to implement convoy escorts with insufficient ships, inadequate training, and obsolete doctrine.

 The statistics of destruction told the story of Japanese logistics collapse. In 1944, American submarines conducted 520 war patrols and sank 603 Japanese merchant ships totaling approximately 2.7 million tons. This exceeded the combined totals for the first 25 months of war. Japanese bulk commodity imports fell from 19.4 million tons in 1942 to 16.

4 million tons in 1943, then crashed as submarine attacks intensified. Oil transportation became the most critical vulnerability in Japan’s logistics network. The Dutch East Indies produced sufficient petroleum to meet Japanese military requirements, but transporting crude oil and refined products across the vast Pacific distances to Japan proved increasingly impossible.

 In 1943, Japan consumed oil at peak rates from captured Southeast Asian fields. By 1944, submarine attacks on tankers made oil shipments so dangerous that much of Japan’s battle fleet remained stationed near the oil fields, fueled by unrefined crude. 124 Japanese tankers fell victim to American submarines during the war.

 Each sinking represented not just lost tonnage, but irreplaceable specialized vessels essential for oil transport. The USS Flasher scored a devastating blow on December 22nd, 1944. sinking three large tankers in a single convoy attack off Indo-China. The Eroda Maru and sister ships Odawa Maru and Omirosen Maru represented major losses in Japan’s shrinking tanker fleet.

 Major Horry witnessed the practical effects of shipping losses from his positions with convoy escort forces. Ships waited weeks or months in Pacific harbors for escort protection that never materialized. On one occasion, 32 merchant ships remained idle in Palao Harbor for 95 days, awaiting a single escort vessel to accompany them to Japanese home ports.

The submarine threat had effectively paralyzed Japanese maritime logistics. The human cost of Japan’s shipping crisis extended beyond military personnel. Transport ships carrying troops and civilian workers became priority targets for American submarines. The Kamakura Maru, Japan’s largest passenger ship, sailed south in April 1943, carrying 1,000 civilian engineers and technicians assigned to increase rubber and petroleum production in Southeast Asia.

 USS Gudgeon’s torpedoes found their mark in the South China Sea. Only 28 civilians survived. Japan possessed no replacements for either the technical expertise or the largest military transport in their fleet. By February 1944, over 10% of Japan’s remaining merchant marine was sunk in a single month. Merchant ship losses at truck during American carrier raids represented approximately half the total monthly losses.

 Captain Oi noted that tanker losses during the truck and Palao raids were particularly devastating for combined fleet operations and oil transportation from the Netherlands East Indies to the Empire. The collapse accelerated through 1944 as American submarine numbers increased and tactics improved. Surface search radar allowed submarines to locate convoys at night and conduct high-speed surface attacks.

The submarine’s 16 knot surface speed exceeded most convoy speeds, enabling running attacks that maximize torpedo expenditure. Convoy HI71, sailing to the Philippines in August 1944, lost the attack transport to Matsumaru so quickly that she dragged 4,541 soldiers and merchant crewmen to their deaths.

 The final phase of logistic strangulation came in early 1945. Air and submarine attacks in the South China Sea and East China Sea forced abandonment of convoys between the Empire and Southwest area in February. Complete cessation of shipping on this vital route occurred in March 1945. The last tanker from Southeast Asia reached Japan in March 1945, ending 3 years of petroleum imports from the conquered territories.

 American submarines had achieved what strategic bombing and surface naval forces could not accomplish alone. They severed Japan’s maritime lifelines and strangled the logistics network that sustained the Empire’s war effort. By wars end, American submarines had sunk 5.3 million tons of Japanese shipping, representing 54.

6% of all Japanese vessel losses during World War II. The United States Strategic Bombing Survey concluded that submarine warfare against shipping was perhaps the most decisive single factor in the collapse of the Japanese economy and the logistic support of Japanese military and naval power. The transformation from defective torpedoes in early 1943 to systematic destruction of enemy logistics demonstrated how technical excellence combined with accurate intelligence could achieve strategic victory.

 Japan’s empire, built on control of sea lanes spanning half the Pacific Ocean, collapsed when those same sea lanes became hunting grounds for reliable American torpedoes.