Hit Counter
Since 10-08-04

The dispute within the submarine force over the effectiveness of the Mark 6 exploder consumed BuOrd during the Second World War which resulted in a post war distrust by many in the operating force toward innovations by BuOrd. The controversy is still a matter of interest and SRC has received a request for clarification from Robert Marples. The clarification is presented in terms of the exploder and how it works and in terms of the historical facts involved in the controversy.

Operation of the Mk 6 Exploder

The exploder is really two exploders in one. A contact portion consists of an inertial ball and electrical contact. The influence portion consists of a voltage sensing coil, a voltage triggering thyratron, a capacitor, a solenoid and an inertia ring. Since the influence exploder is intended to detonate the charge immediately below a ship's keel the running depth of the torpedo is important and therefore that mechanism represents the third portion of the exploder. The portions are described below in inverse order.

Depth Control

A hydrostatic sensor acts off of sea pressure and a linkage to the horizontal planes keeps the torpedo on desired depth. This is it in its simplest form, however, the problem gets complicated fast. Just as lift is created on the upper surface of an airplane wing by creating a partial vacuum, so the torpedo moving through water creates different water pressures along the side of the torpedo. The faster the torpedo, the greater the pressure difference. If the calibrated depth mismatches the real depth because of the difference between hydrostatic pressure and hydrodynamic pressure the torpedo is going to run off depth.

Influence Exploder

This exploder operates on the influence that a ship's hull has on the Earth's magnetic field. A ship's hull made of ferrous metal should cause a ripple in the Earth's field and when this is picked up by the exploder the sensing coil induces a voltage. The voltage triggers a thyratron which discharges a capacitor into a solenoid. The solenoid operates a lever which displaces an interia ring which triggers the mechanical exploder. Once again, all this sounds in order, but a complication comes into account with variations in the Earth's magnetic field.

Contact Exploder

The contact exploder portion is much simpler than the influence portion. It is simply a ball on a spring which is centered within a hollowed frame. At the forward end of the mechanism is an electrical contact. When the ball is displaced forward by an inertial shock it pushes against the contact and this causes an electrical impulse to the exploder trigger. The problem is the sensitivity of the ball to shock. If the shock is too much it destroys the mechanism before the ball can do its job.

Historical Development

Development of the Mk 6 Exploder took place during the nineteen thirties at Newport, Rhode Island. The engineers were proud of their exploder after having tested it only once. It worked and the facility set about to manufacture hundreds of the exploders, then to put them away in a locked vault. When Pearl Harbor was bombed Newport brought the exploders out of moth balls, dusted them off and sent them out to submarines.

Submarines operating in the southwest Pacific shot their Mark 14-3A's equipped with Mk 6 exploders and reported about a two thirds failure rate. The skippers ranted and raved that they were getting pounded by the Japanese and had only duds to use in both offense and defense. The movie "Operation Pacific" showed John Wayne as a submarine captain who had his torpedomen dropping Mk 14s from the roof of the Pearl Harbor SuBase torpedo shop in an attempt to find a cause of the failures. And that wasn't far from the truth. In Fremantle they ran their own tests and found that the Mk14 was running eleven feet deeper than calibrated - not conducive to a successful influence exploder. On Oahu they shot the fish point blank at a rock. It failed to explode.

Back in Newport they were producing the exploder as fast as they could, believing that notices of failures were really failures of a good firing solution.

The following is taken Theodore Roscoe's, United States Submarine Operations in World War Two:

The armed torpedo would explode only when it passed through the magnetic field of an iron or steel vessel, or struck the hull of a vessel - that is, if the exploder mechanism were working properly. The contact feature of the Mk 6 exploder was a relatively simple affair. If the torpedo struck an object with sufficient force, the impact caused an inertia element to be dislodged. This contrivance released the firing pin which set off the works. The magnetic influence feature employed a more complicated device. Subtle electronics were involved. When the torpedo passed through a ship's magnetic field, the electromotive force generated in the exploder's induction coils underwent a change. Amplified by vacuum tubes, this change of EMF caused release of the firing pin. . .

The always hard-to-analyze human element contributed to the problem. Torpedoes, after all, are adjusted, aimed and fired by human endeavor. Faulty judgment, errors in calculation, mishandling, poor maintenance, all these could account for torpedo misbehavior. . . Massive military organizations, with their interdependent ramifications and internal complications, resist change. They may move at a glacial pace where the speed of an avalanche is imperative. And there were several top-level officers of the ordnance organization who ascribed all torpedo faults to errors of the operating personnel, and chose to define "dud" as a skipper's alibi to explain his miss."

The following is taken from, John Alden's The Fleet Submarine in the U.S. Navy.

"The defects existed mainly in the Mark VI magnetic exploder which was developed under extreme secrecy just prior to the war. It was so secret that the operating submarine force was kept largely unaware of its existence. The device was tested by the experts at Newport under carefully controlled conditions instead of firings from operating submarines, and as a result it was not discovered that the exploder would only work if it passed close to the bottom of the target. When installed on torpedoes in the war zone, the exploders failed to detonate, but the operating forces were forbidden to disassemble them to look for the problem or even to conduct firing tests. The torpedo specialists refused to believe reports that something was seriously wrong with the weapons until the problem grew into a major scandal. It was ultimately discovered that the magnetic exploder, the torpedo depth control mechanisms and the contact exploder were all defective."

The Problems

The depth control mechanism worked just fine on the Mk 13 which ran at thirty three knots. The technicians simply stuck the same mechanism on the Mk 14 when it came along. No one considered the fact that the Mk 14 would run at 46 knots at high speed. Certainly, no one thought that the difference between hydrostatic pressure and hydrodynamic pressure increases exponentially as the speed of the torpedo increases. The difference between calibration depth and real depth was twice as great at 46 knots as at 33 knots. The boys at Newport thought that heavier war heads might be the cause and suggested a three foot difference. As it turned out the submariners in the field were right. The actual depth difference was somewhere between ten and eleven feet. This meant the fish were running much deeper than what was being set on the spindles.

The magnetic exploder didn't work because the experts in Newport were at sixty degrees latitude and the boats in the southwest Pacific were operating around the equator. The difference in the horizontal component of the magnetic perturbation at the equator is effectively less than half that at Newport.

When the deeper running fish passed under the keel of a ferrous metal ship near the equator the signal simply wasn't strong enough to actuate the thyratron.

The contact exploder was crushed on impact before it could function when the strike angle was near perpendicular. The casing was flimsy and the parts not substantial enough to withstand the punch. The fast Mk 14 torpedo had a higher inertial impact that caused the firing pin to miss the exploder gap. In a typical MOT, aft, fwd spread the MOT shot was least likely to explode.

Any of these problems could have been solved were they to have been presented in singles. Submarine torpedomen scratched their heads over problems that went far beyond their ability to make a poorly designed exploder work.

The Macro Problems

Newport had its eye on economics during the years of exploder development. It used recoverable torpedoes and minimized testing.

Newport kept the exploder super secret. This kept any potential problems hidden from view until it was too late.

No facility other than Newport had any authority in torpedo development.

Simultaneous problems tend to cover each other so that an outside source for the problem is sought, in this case, the commanding officers of operating submarines.


The concept of influence exploders was abandoned in favor of acoustic guidance. The Mk 14 continued using a greatly modified contact exploder through the sixties.


The following observations reflect the opinions of writers on the subject:

Edwin Gray, (The Devil's Device), ". . .the unhappy saga of the Mark 14 torpedo and its Mark VI exploder is perhaps a perfect example of the mayhem that can be created when experts bury their heads in the sand and steadfastly refuse to face the facts."

Robert Gannon, (Hellions of the Deep), "America entered the war with torpedoes far inferior to those of the enemy, and the fault lay squarely with the United States Bureau of Ordnance. It was ineffectual in research and development, inept in testing. It was inadequate in manufacturing, and feeble in its supervision of Newport. It was wanting in collegiality with the rest of the Navy, and it failed to trust those fighting under the Pacific surface."

Clay Blair Jr. (Silent Victory), "That. . .desk-bound staffers refused to listen to suggestions and criticisms from those they had sent into combat with this weapon seems, in retrospect, incomprehsensivly stubborn and stupid . . .The torpedo scandal of the U.S. submarine force in World War II was one the worst in the history of any kind of warfare."

Submarine Research Center

Dear webmaster:

Looking over this bulletin on the MK 14 Mod 3a torpedo and its MK 6 exploder mechanism, I am somewhat disappointed in what was published.
"Operation of the Mk 6 Exploder"

You wrote: "The exploder is really two exploders in one." Not really! This should read as follows:

"Properly, the exploder is the entire MK 6 assembly. It has a magnetic-influence feature and a contact feature. This leads to awkward verbiage, so we refer to the magnetic-influence exploder and the contact exploder. Both are parts of the MK 6, which weighs approximately 92#, and some elements of the exploder function in both modes. The exploder also contains important safety features. This complex arrangement was presumably designed so that an exploder, MK 5, without the magnetic-influence portion, but otherwise identical to the MK6 exploder could be produced and issued to the fleet in peacetime. Security was the overall motivation for this convoluted approach."
source: US NAVY TORPEDOES by Frederick J. Milford - Part two.

You wrote: "The contact exploder consists of an inertial ball and electric contact." This type of contact exploder is not the one that was issued the fleet at the start of WWII. It was issued as a last resort, makeshift substitute for the original MK 5 contact exploder that was mounted on the same baseplate. It was a totally unnecessary feature added by BUORD, in order to save face. After the modification of the MK 5 contact exploder by the Torpedo Shop personnel at Pearl Harbor, (by fabricating a firing-pin block assembly from aluminum, and strengthening and re-positioning the vertical brass guide posts), successful firings resulted. This was named the (PHM) Pearl Harbor Modification, accomplished by field personnel, and not by BUORD.

Btw, where did the electricity for operating this "contact switch" and the magnetic-influence feature come from? Answer: During the first part of the torpedo run, an 8mfd capacitor is charged by the output of a DC generator driven by the impeller shaft. The generator output passes through a voltage regulator tube, which keeps its voltage nearly constant regardless of generator speed. The magnetic-influence feature was disabled by removing the "pick-up" rod from the warhead (coil assembly) by removing a plug from the forward underside of the warhead. When the new brass ball contact switch was used, an electrical "squib" attached to the 65-grain fulminate-of-mercury detonator, started the chain reaction, by detonating the theTetryl booster and the main charge of TNT or TORPEX.

You wrote: "The problem is the sensitivity of ball to shock. If the shock is too much it destroys the mechanism before the ball can do its job." The MK 15 Mod 3 surface type torpedo used the same MK 6 exploder mechanism with the brass ball contact switch, and it was launched from the 02-level of a destroyer, and this "shock" problem was non-existent. Incidentally, there is no electrical current available to "destroy" the mechanism at launch, because the output of the generator is shorted out until the impeller has turned about 481 turns, and the detonator has been screwed in the cavity of the Tetryl booster.

I have ten torpedo related questions for you.........

What is the arming distance of the exploder mechanism on a MK 14 or MK 15 torpedo?

Why are the warheads made of phosphor bronze on MK 14 and MK 15 torpedoes and steel on the MK 13, MK 18 and MK 23 torpedoes?

What type of exploder mechanism is installed in a MK 23 torpedo?

Can a MK 15 (surface to surface) torpedo be used in a submarine?

What prevents a torpedo from rolling over after it is launched?

How does the anti-countermining feature on a MK 6 exploder mechanism work?

How does the anti-circular run feature work on a MK 6 exploder mechanism?

What is the maximum depth for opening a fleet submarine's torpedo tube for launching a torpedo?

What successful post-WWII modifications were made to the troublesome MK 6 exploder mechanism?

What is the difference between a run-down gyroscope and a constant-spin type?

On page two you wrote: "The movie "Operation Pacific" showed John Wayne as a submarine captain who had his torpedomen dropping MK 14s from the roof of the Pearl Harbor Sub Base torpedo shop......." I know that torpedomen can do almost anything, but they can't drop MK 14s off the roof of the Sub Base torpedo shop at Pearl Harbor.......... Run that movie over again and you'll notice that John Wayne used a "cherry-picker" to drop those plaster-filled warheads (on MK 14s) on a steel plate on a pier. Also, refer to: Robert Gannon's "Hellions of the Deep", on page 90, 3rd paragraph.

On page 3 you wrote: "Disposition: The concept of exploders was abandoned in favor of acoustic guidance. The MK 14 continued using a greatly modified contact exploder through the sixties."

Passive-acoustic guidance was first in the MK 24 "Fido" aerial ASW torpedo on May 14. 1943, destroying the U-266. Then the 19" diameter MK 27 "Cutie" was sent to the Pacific in the summer of 1944. The 21" diameter Mk 28 was sent to the Pacific Fleet in the summer of 1945.

The MK 27" passive-acoustic torpedo used a MK 142 nose-mounted contact exploder mechanism.

The whole point of my requesting information, in the first place, on the post WWII modification of the Mk 6 exploder mechanism, was to confirm the fact that I viewed a B&W 16-mm training film (at the Naval Torpedo School in early 1950) of a surfaced fleet boat launching a stern shot MK 14 at an AKA/APA in Long Island Sound, using a MK 6 Mod 10 magnetic-influence exploder mechanism, with devastating results.
Again, do you have any info on this Mk 6 Mod 10 exploder mechanism?
Robert F. Marble TMCS(SS) USN (Ret)
Port Charlotte, FL