On the morning of July 24, 1943, Lieutenant Commander L.R. Daspit and the submarine Tinosa launched what may have been the most frustrating attack of the United States' World War II submarine campaign against Japan. Alerted by cryptanalysts in Hawaii that the 19,000-ton Tonan Maru No.3 was cruising on an easterly course from Palau to Truk, Daspit set a course to intercept the enemy vessel. She and her sister ship, Tonan Maru No.2, were originally built as whale factory ships but had been converted to oil tankers for wartime use. They were two of the largest vessels in Japan's precious merchant marine fleet.
As he manuevered his submarine into a favorable attack position, Daspit calculated Tonan Maru No.3's speed to be 13 knots. Curiously, the heavily loaded tanker had no surface or air escort and was not zigzagging as an anti-submarine measure. After taking up a position from which her torpedo tracks would be nearly perpendicular to the target's course, Tinosa launched a spread of four torpedoes. Only two small geysers of water erupted alongside the vessel, however. To Daspit's dismay, the tanker did not explode or begin to list, but rather turned away and put on speed. Tonan Maru No.3's abrupt course change left the submarine in a poor firing position, but Daspit fired the remaining two torpedoes from his forward tubes by instinct. Both weapons struck the ship aft at obtuse angles and exploded, causing the ship to stop and begin to settle slightly by the stern. Although dead in the water, the well-compartmented tanker was in no immediate danger of sinking. Although fire from Tonan Maru No.3's deck guns forced Tinosa to remain submerged, the Japanese could do nothing to prevent the next salvo of torpedoes.
Repositioning to correct the poor firing angle, Daspit placed Tinosa in a textbook attack position, approximately 875 yards off the tanker's beam, and launched one torpedo. The soundman reported a straight and normal run. At impact, the skipper saw only a disappointing splash alongside the vessel. The torpedo was a dud.
Torpedoes In The Water
Undaunted, the skipper ordered that every remaining torpedo be inspected before he continued. Each weapon was found to be in perfect working condition. Another torpedo was fired with great precision, yet the submariners were rewarded with only a deafening silence.
After seven more torpedoes were launched at the stationary target without success, Daspit wisely decided to save his 16th and final torpedo and take it back to Pearl Harbor for a complete overhaul. By methodically eliminating all possible factors except the ordnance, Daspit refocused attention on the Mark XIV torpedo, and he even returned with the perfect specimen to illustrate what had been the bane of the submariner's existence for the past year and a half.
"The only reliable feature of the torpedo was its unreliability."
For 18 months, several flaws had combined to render the Mark XIV torpedo, upon which submariners' lives and success depended, virtually impotent. From the onset of Mark XIV production, inherent defects had existed within the design of the torpedo and the Mark VI magnetic influence exploder mechanism. Each flaw that was discovered and corrected exposed another malfunction. As Theodore Roscoe, author of the official naval history of submarine operations, put it, "The only reliable feature of the torpedo was its unreliability."
After the initial Japanese naval onslaught in late 1941, the U.S. Southwest Pacific Command was established. Rear Admiral Charles Lockwood assumed command of all former Asiatic fleet submarines and divided the flotilla between the Australian harbors of Brisbane and Perth/Fremantle. Unlike a number of flag officers who held a wide variety of posts during their careers, Lockwood considered himself a true submariner. He proved to be an extremely pragmatic commander and a widely respected leader, which served him and his country well during the dark months after Pearl Harbor.
As yet unaware of their torpedoes' faults, submarine skippers reported an alarming number of prematures, duds and inexplicable misses during the first full year of the war. Frustrated captains watched helplessly as their torpedo wakes passed under sterns or just aft of targets. In response to repeated requests by field commanders, the Bureau of Ordnance conducted check firings to evaluate the depth control of the Mark XIV. By February 1942, the bureau reported a variance of four feet in depth control during the initial 880 yards of a run. Since four feet of depth would make little difference when engaging a capital ship, and most attacks took place at the 1000-yard range, the bureau concluded that the torpedoes were not at fault; rather, it must have been the crews' inexperience and errors that were causing failures. The bureau further argued that even if a torpedo did slip under a shallow-draft target, the magnetic detonator would activate the warhead. Faced with such apparently sound arguments, the submariners could only redouble their fruitless efforts. After five months of desperate action, little tonnage to show for their sacrifice and continued pleas from his skippers for reliable torpedoes, Lockwood decided to conduct his own tests.
Lockwood and his amateur scientists bought 500 feet of net from a local fisherman and moored it in deep water just outside Frenchman's Bay near Albany, Australia. A Mark XIV was obtained from an incoming submarine, Skipjack, whose crew was more than willing to part company with it. Lockwood's men modified the Mark XIV by replacing the warhead with an exercise head. This replacement head contained a calcium chloride solution that made its weight exactly the same as the warhead. The modified torpedo was loaded into a submarine, and Lockwood ordered a series of test firings.
Set to run at 10 feet, the torpedo was launched from a distance of approximately 900 yards. When divers inspected the net, they discovered the torpedo had cut the net 25 feet below the surface of the water. The next day, two additional torpedoes cut the net at eight and 11 feet deeper than set. Since he believed this extra depth had also kept the magnetic detonator from working, Lockwood ordered all of his skippers to adjust their torpedo depth settings accordingly. Most captains, not taking any chances, set their torpedoes for zero depth. Lockwood and his staff realized, however, that the malfunctioning torpedo needed to be corrected, not merely jury-rigged.
The Bureau Of Ordnance Response
Later in July, the Bureau of Ordnance responded to Lockwood's tests by announcing they were flawed and thus not conclusive. The Stateside bureau claimed improper trim conditions had been created when the field testers used an exercise head that was shorter than the warhead. Undaunted, Lockwood's team lengthened their exercise head to warhead length and immediately produced the same incriminating evidence.
In response, Commander James King was brought out of retirement and made chief of the bureau's Research and Development section to address the depth-control problem. King had earlier been responsible for adding the extra TNT to the Mark XIV warhead and for designing the torpedo's turbine engine, the best in the world. He immediately began to conduct tests similar to Lockwood's, launching torpedoes into nets from submarines, not barges, as had been the common practice. Not surprisingly, King achieved the same results as Lockwood. On August 1, 1942, he advised the fleet that the Mark XIV ran approximately 10 to 12 feet deeper than set.
The initial culprit was the depth-control mechanism. This intricate device sets the tension of the depth spring to correspond with the water pressure at the desired running depth. The two controlling elements within the depth mechanism are the hydrostatic valve, or diaphragm, and the pendulum. Ideally, when the torpedo reached the prescribed depth, the force exerted on the diaphragm by the water would equal the force exerted on the diaphragm by the spring. The setting was adjusted and indicated on a graduated dial called the depth index wheel.
On older torpedo models and early Mark versions, the hydrostatic valve was located in the middle section of the weapon, just behind the warhead. To increase range and speed, this space eventually became filled with additional parts and fuel. As a result, the valve was moved farther aft. This revised layout was originally perceived as a benefit because the depth control mechanism would be closer to the rudders it controlled. Its final location was the tapered section of the torpedo near the tail. No one realized that by placing the valve at a slight angle to the weapon's longitudinal axis, it would cause a corresponding change in how the valve reacted in determining depth control. This variance was minimal under what were considered to be normal testing conditions -- shallow depths, weak currents and calm seas.
Further complicating the problem, it was later found that the depth-recording instrument used by the bureau to check the reliability of all hydrostatic valves was miscalibrated. Years later, technicians discovered that the recording instrument and the misplaced valves erred in the same direction and amount. The bureau had been cursed with pure bad luck. Two completely different devices, each responsible for checking the other, deviated identically for vastly different reasons. This unfortunate coincidence explains the bureau's initial testing results and its rejection of Lockwood's evidence. It was a very peculiar and costly twist of fate.
Adding insult to injury, earlier improvements by Commander King, although well-intentioned and initially successful, added to the depth-control riddle. When the additional 115 pounds of TNT were squeezed into the Mark XIV warhead, the exercise heads were not correspondingly altered to reflect the change. The extra explosive had been packed into the warhead by increasing density, so although the water-filled exercise head continued to occupy the same space as the warhead, it no longer had the same weight. Thus, the Bureau of Ordnance was using one version of the Mark XIV for testing and issuing quite a different Mark XIV.
The problem of designing identical torpedo heads was solved by using Lockwood's calcium chloride solution, which correctly matched the warhead in size and density. The hydrostatic valve problem was alleviated when a new, calibrated depth-control valve was designed and installed on all Mark XIV torpedoes. Once these improvements brought the Mark XIV up to the correct depth, however, the Mark VI magnetic detonator presented additional problems. The "Silent Service" was no closer to having a reliable torpedo than it had been eight months earlier.
The Magnetic Detonator
During World War I, the Germans had developed a mine with a magnetic detonator. With continued improvement, it became a very effective weapon in World War II. The key to the secret detonator was a compass needle that moved when acted upon by the hull of a steel or iron vessel. When the magnetic needle swung, it activated an electrical contact that exploded the mine. Between the wars, every major navy attempted to duplicate the magnetic exploder in its standard submarine torpedoes. Conventional theory held that if a torpedo could be exploded under a ship, as opposed to alongside, the damage would be much greater. Ideally, one or two torpedoes detonated directly under a vessel would be enough to break the ship in half.
By 1925, the Bureau of Ordnance had completed a basic magnetic detonator. Unlike its distant German cousin, the American model was not activated by a compass. Instead, the bureau used induction coils that generated an electromotive force, which changed when the torpedo passed through or under a target's magnetic field. Vacuum tubes magnified the change within the coils to release the firing pin. The design was extremely complex for its day, but that complexity compromised the detonator's reliability -- as did the secrecy imposed by the bureau.
The bureau felt that the Mark VI magnetic detonator constituted a secret weapon by the late 1930s. The detonator was cloistered from all but a select few until the spring of 1941, when war seemed imminent. The bureau feared that knowledge of its existence would influence the design and construction of a potential enemy's fleet, primarily Japan's. In April 1941, Mark XIV torpedoes with Mark VI detonators were finally issued to the fleet, although security restrictions continued. Only commanding officers and torpedo officers were allowed access to the secret weapon and its manual. Common sense, however, dictated that enlisted torpedomen should also be allowed access, because they were expected to maintain and service the ordnance. Yet when war struck seven short months later, few if any men in the Pacific theater understood the inner workings of the detonator, and since only a few knew what a Mark VI would do under perfect working conditions, even fewer could recognize a malfunction. As with the Mark XIV torpedo's depth mechanism, it would take the rigors and sacrifices of combat to expose the detonator's fatal defects.
During the opening months of the Battle of the Atlantic, the Germans discovered that their updated magnetic torpedo detonators were malfunctioning in waters near the Arctic Circle. They correctly theorized that the Earth is a large magnet whose magnetism varies by location. They understood that different magnetic fields would surround a ship depending on its longitude and latitude. By mid-1941, the Germans had deactivated their magnetic exploders and were relying solely on contact detonators. The British soon followed suit. In a struggle as paramount as the Battle of the Atlantic, neither side could afford unreliable or ineffectual ordnance. American submariners, on the other hand, were just beginning a similar naval conflict in which they would not have reliable torpedoes for 18 months.
More Finger Pointing
By August 1942, the faulty depth mechanism had been isolated and corrected, and the Mark XIV was striking more targets. Curiously, however, skippers began to report a large percentage of duds and prematures. Frustrated captains and crews now suspected the mysterious Mark VI detonator.
The submariners attempted to make in-field adjustments, in the process trying to accumulate sufficient evidence to warrant deactivation. Admiral Lockwood, now in charge of all Central Pacific submarines from Pearl Harbor, would have ordered immediate deactivation had it not been for the possibilities and flexibilities the Mark VI theoretically offered. When dealing with shallow-draft escort vessels, the under-the-keel shot was a must, and it was accepted that such a detonation against any size vessel was most effective. Early in 1943, however, the Bureau of Ships released a study contradicting that assumption. The study, based on Atlantic convoy sinkings, concluded that broadside hits that created instability were the most effective attacks against merchantmen, which lacked the armor belt and compartmentation of warships.
Since Japan's lifeline was her merchant marine fleet and because the Bureau of Ordnance would only suggest slight technical adjustments to the Mark VI, Admiral Lockwood determined that the magnetic feature was more a liability than an asset. On July 24, 1943, he ordered his submarines to deactivate the Mark VI magnetic influence detonators and fire for contact hits only.
As later tests illustrated, the failure of the Mark VI design was twofold. In broad terms, the magnetic theory advanced by the Germans months earlier was correct. Depending on location, the magnetic field around a ship varies, and there were definate variances between the waters around New England where the Mark VI was tested and the southern Pacific. Additionally, internal construction flaws increased the chances of unreliable performance. Brush riggings, located on the generator that supplied power to operate the magnetic exploder, were discovered to be inadequate, and leaky base-plate castings allowed water into the exploder cavity. Having endured two major malfunctions in their primary weapon over a year and a half of disheartening combat, U.S. submariners eagerly abandoned the Mark VI detonator in favor of the contact mechanism. Fate, however, was to test their mettle one more time.
The contact device's name alone suggested reliability and consistency. Although less advanced than the magnetic feature, however, the contact exploder was still a complex device with numerous parts capable of perplexing malfunctions. In fact, a malignant flaw in the contact mechanism had been hidden while other malfunctions were slowly and painstakingly resolved.
When Tinosa arrived in Pearl Harbor, her 16th torpedo was given a complete inspection. After an all too familiar examination, the torpedo was declared to be in perfect working order. Commander Daspit had received the same report from his chief torpedo man prior to launching more than 10 torpedoes on 90-degree tracks at a stationary target, yet each torpedo had failed to detonate. Was the 16th torpedo an exception? Admiral Lockwood sought to answer this question with the type of common sense test that identified the depth control problem.
Next: Trouble With The Tin Fish
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