Captain C.B. Momsen suggested loading inspected torpedoes, including Tinosa’s 16th, into a submarine, then firing them against the vertical cliffs off the island of Kahoolawe. The first torpedo that failed to detonate would be recovered and carefully dissected for clues. Lockwood agreed and assigned the submarine Muskellunge to the task.

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Maneuvering as close to a 90-degree track as possible, the submarine fired three torpedoes against the rock cliffs. The first two exploded, but the third threw up the familiar geyser of compressed air and water. Divers carefully retrieved the activated yet unexploded torpedo. The valuable dud was then hauled back to Pearl Harbor for examination.

The technicians removed the contact mechanism and discovered that the device had correctly released the firing pin, but the pin had not struck the fulminate caps with sufficient force to set them off. Curiously, the stud guides that directed the firing pin into the primer caps were severely bent and deformed. With the weak link apparent, experiments began to focus on the malfunction.

Lockwood’s men replaced the TNT in several warheads with cinder concrete and attached the normal contact mechanism. Test torpedoes were then dropped 90 feet along a wire suspended from a crane into an empty drydock where they landed squarely on steel plates. A direct, 90-degree hit produced a dud seven out of 10 times–a 70 percent failure rate almost two years into the war. By adjusting the target plates to a 45-degree angle, the failure rate was cut in half. At a still greater angle, the exploders worked without fail. Lockwood immediately directed his boats at sea to launch their torpedoes from large, obtuse angles. They were ordered to improvise, to use anything but the textbook 90-degree track.

The internal failures of the contact mechanism can best be understood through the forces at work in a live torpedo. When a 3,000-pound torpedo traveling at 46 knots struck the hull of a ship, incredible forces were unleashed. The initial force of deceleration equaled approximately 500 times the force of gravity. Transferred to the firing pin, this force appeared as friction between the pin and the guides along which it traveled for accuracy. These stud guides were exposed to nearly 190 pounds of pressure from the contact and resulting deceleration. The firing spring was unable to overcome this tremendous friction and pressure with enough force to drive the firing pin successfully into the primer caps. When a torpedo struck a glancing, angled blow, the force of impact was lessened enough to allow the spring to push the pin into the caps, causing detonation.

The solution turned out to be relatively simple. The Pearl Harbor workshops designed and mass-produced modified firing pins from the propeller blades of Japanese aircraft downed in the December 7, 1941, attack. The new pins were made as light as possible in order to reduce the friction on the stud guides. Testing this handiwork, Lockwood ordered the submarine Halibut, armed with modified exploders, to repeat the Kahoolawe tests. Each torpedo was again set to run as close to 90-degrees as possible to fully test the new pins. Six out of seven torpedoes exploded. Although one still failed, it was a significant improvement from a 70 percent failure rate.

During the 1930s, the Bureau of Ordnance had conducted similar tests designed to ensure a reliable contact mechanism in time of war. The Newport Torpedo Station flung torpedoes against steel plates over sand and discovered then that the firing pins failed to strike the caps with sufficient force. Their solution was to increase the strength of the firing spring. The tighter spring seemed to solve the problem, but it did so at the speed of 1930s torpedoes. Torpedo speeds had increased to 46 knots by World War II, and this increase created greater impact forces. The increased speed essentially negated the strengthened spring. If Tinosa’s torpedoes had been set for slower speeds or obtuse angles, Tonan Maru No. 3 would not have escaped. It took almost two years of wartime trials and tribulations, but American submariners were finally equipped with reliable and effective torpedoes.

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