During World War I, the standard American submarine torpedo was the 18-inch Mark 7. This had a maximum range of 5,000 yards, and a top speed of 35 knots, with a 326-pound warhead. As this torpedo was used in the “O” class submarines, it remained in service in training boats through World War II.
By the time the United States entered World War I the inadequacy of the Mark 7 had been recognized. As capital ships grew larger, and became better protected against torpedo attack, the small warhead couldn’t cause enough damage to consistently put the target out of action. Twenty-one-inch torpedoes had already been adopted for use in surface ships, so it was logical enough to create a submarine version in the same size.
Size constraints didn’t allow a 21″ torpedo to be retrofitted to the “O” class boats—there was barely enough room for the 18-inch tubes—so they were installed in the new “R” and “S” class boats that were being built at the time the U.S. entered the war. (None of these boats were commissioned until after the war had ended, the first R-boat being commissioned in December 1918, and S-1, while completed in 1918, wasn’t commissioned until 1920.)
With the advent of the S-boats, the standard U.S. submarine torpedo became the 21-inch Mark-10. While having a diameter only three inches larger that the old Mark-7, this was enough to allow an increase in warhead size from 326 pounds to 497 pounds. The Mark-10 was slightly faster, 36 knots vs. 35, though the range was reduced to 3,500 yards. Considering the state of aiming ability at the time, the shorter range was unlikely to be much of a problem. Most commanders would want to get the range under 1,000 yards in any case.
During World War II, some of the R-boats, which were employed for training, used the older Mark-9 torpedo, which had been designed for battleship use. The range of the Mark-9 was twice that of the Mark-10, but it had only a 210-pound warhead and, at 27 knots, was also slower.
World War I experience had shown that the most effective way to destroy a ship was to “break her back”—that is, to set off the torpedo’s warhead under the centerline of the vessel, breaking the keel. Both the United States and Germany set to work on a method of doing this consistently, and both came up with essentially the same solution. The Japanese, who would be the United States Navy’s main antagonists in World War II, took a different route.
The Japanese, always ready to one-up their potential enemies, developed the 24-inch Type 93 “Long Lance” oxygen torpedo, with a 1,720-pound warhead, that has generally been recognized as probably the most effective anti-shipping torpedo ever fielded. This was a destroyer torpedo, but Japanese submarines were equipped with the 21-inch Type 95 oxygen torpedo, with an 893-pound warhead (increased to 1,213 pounds in the Model 2 version. Rather than attempting to work out a way to insure detonation under the keel, the Japanese simply opted for a warhead large enough to insure a kill.
A major advance in interwar torpedoes was the ability to manually set gyro angles. With the older, straight-running torpedoes, it had been necessary to aim the submarine at the target—or, really, at where the target would be by the time the torpedo reached it. By setting the gyro angle, the torpedo would turn onto the proper track after it was fired.
During the 1930s, the Navy worked on the development of an electro-mechanical Torpedo Data Computer (TDC). Concurrently, the Bureau of Ordnance was busy at the Torpedo Station at Newport, Rhode Island, on a project to produce the next generation of 21-inch torpedoes. Requirements for this project included greater speed, a larger warhead, and incorporation of a magnetic proximity exploder to allow detonation of the warhead beneath the target, instead of against its side.
The Navy’s TDC was far advanced over anything to be found in any other navy at that time. Where other navies had essentially taken the route of mechanizing the functions of an IsWas (the circular slide rule used by the Approach Officer to work out firing angles), the U.S. Navy drew their inspiration from the complex electro-mechanical computers that directed the aiming and firing of the main batteries in battleships. The result was a fairly large device, located in the conning tower, that could keep track of the boat’s position, course, and speed, compute and project the target’s track based on multiple observations—each one, it was to be hoped, reducing the error—and transmit the information to the torpedoes, updating their targeting information right up to the moment they were launched.
If the TDC operator did his job properly, the torpedoes ran as programmed, and the target didn’t change course after they were fired, the chances of a hit were about as close to 100% as it was possible to get.
The new TDC, which went into the fleet boats, where there was room to shoehorn it into the conning tower, was designed for use with the 21-inch Mark-14 torpedo. A considerable improvement over the old Mark-10, the Mark-14 was a wet-heater type, could travel at 46 knots, carried a 600-pound warhead (later increased to 660 pounds), and had a range of 4,500 yards. It could also be set to run at 31.5 knots, which extended the range to 9,000 yards. For the Mark-14, the explosive was changed from TNT to Torpex.
The Mark-14 could be fitted with the new, and very secret, Mark-6 exploder. This included a magnetic influence component that was designed to detect the changes in the earth’s magnetic field that occurred as it passed under ship’s hull. Just in case, there was also a standard contact exploder incorporated. The Mark-6 exploder was considered so secret that it wasn’t issued to the fleet, but was held in reserve, to be issued only after commencement of hostilities. The Mark-5 exploder, a pure contact type, was issued to the peacetime fleet in its place.
Strangely, for such important technology, there were only a few live fire tests with the new torpedoes and exploders, and these were almost all conducted with exercise heads, where the explosive was replaced by water ballast that could be blown at the end of the run, bringing the torpedo—which was, of course, a very expensive piece of equipment—back to the surface to be picked up and re-used. The real consequences of this particular economy wouldn’t be recognized until later.
When the war started, the submarine force was immediately sent into action, with the order to wage “unrestricted submarine warfare” against Japan. As it turned out, it would be 18 months before this really happened, and most of the problem during that time was torpedo related.
Up to a point, the American experience paralleled that of the Kriegsmarine (German Navy), which experienced its own torpedo problems during the Norway campaign.
Initially, there was a depth keeping problem. A torpedo set to run at fifteen feet would actually run as much as ten feet deeper. This problem was compounded by the blunt statement from the Bureau of Ordnance that there was nothing wrong with the depth keeping mechanism, and the commanders were obviously just missing their targets.
Eventually, it was no longer possible to ignore the commanders’ complaints. Tests were run, using the relatively simple expedient of firing torpedoes at a fish net, and it was confirmed that the Mark-14 torpedo was running ten feet deeper than set. After this, BuOrd finally did their own tests, at last conceding that there was a depth problem. The commanders were ordered to adjust the depth settings to compensate for the error, and new torpedoes were modified to fix the problem.
This having been accomplished, it was presumed that the success rate would now soar.
Commanders were complaining that, even with the corrected depth settings, and perfect shots, the magnetic exploders were either detonating prematurely, which only served to warn the target and alert the escorts, or they were passing under the target and not exploding at all.
This was the second time where American torpedo problems ran along the same lines as German torpedo problems (though the German depth keeping problem came from a leaky seal on a balance tank, and not from an engineer rather stupidly basing the settings on practice torpedoes with warheads that weighed 200 pounds less than the production version). In Norway, the u-boats had experienced the same problems with prematures and failures, using their own magnetic exploders.
The two experiences diverged at this point. The Germans recognized the problem, ordered the magnetic exploders deactivated, and went back to blowing up targets. The Americans, on the other hand, insisted that the exploder worked, and that the problem had to be in the people using it.
The Mark-6 magnetic feature was, it turned out, based on a pair of false premises. First, that the earth’s magnetic field was essentially the same everywhere and, second, that a steel-hulled ship is going to disturb that field.
In fact, the earth’s magnetic field varies considerably. An exploder that worked unfailingly off Newport could fail miserably in the Pacific. And it’s a relatively simple process to degause (demagnetize) a ship’s hull—something that was done routinely to warships and others going into combat areas once magnetic mines were introduced.
The American problem was compounded by RAdm Robert English, at Pearl Harbor, and RAdm Ralph Christie, in Australia. Christie had worked on the Mark-6 exploder at Newport, and was convinced that it worked. He presumed that any problems came from poor maintenance or other user error. And it wasn’t until English died in a California plane crash, and Lockwood took over at Pearl, that anyone would really listen to the commanders. Lockwood allowed the magnetic exploders to be deactivated on Pearl Harbor boats, though Christie persisted in mandating their use for a while longer.
But there was a third part of the problem. Because the captains had been under orders to use the magnetic exploder, and had been setting their torpedoes to run the required five feet under their targets, few of them had had the opportunity to realize that that contact exploder was also defective.
Now, time after time, a perfect shot would send a torpedo squarely into the side of a target, only to have it fail to explode. It might punch a hole in the side of a freighter, but most likely not something that couldn’t be repaired at sea. And with a warship, made of thicker steel, it might do nothing more than cause a small dent.
Curiously, bad shots, made at extreme angles, where the torpedo hit the target at an oblique angle instead of square, very often resulted in the warhead detonating and the target going down.
Lockwood ordered more tests. Swede Momsen suggested firing live torpedoes at the cliffs on Kahoolawe, which rose vertically from the sea. Three torpedoes were fired from U.S.S. Muskallunge, with the third failing to explode. Navy diver John Kelly found the dud and swam down to attach a line. The torpedo was hauled aboard the rescue/salvage vessel Widgeon and returned to Pearl Harbor, where it was taken apart.
It was found that the contact mechanism, built to essentially the same standards as that in the slower Mark-10 torpedo, had failed under the greater impact of the much faster Mark-14. Instead of striking the primer, the firing pin had bent and jammed in the guides, which had also distorted. More tests were made, this time by dropping dummy warheads fitted with live exploders onto steel plates from a height of 90 feet, confirming the diagnosis.
Once understood, the problem was fairly easy to fix. New firing pins were machined from a light, high-strength aluminum alloy—the metal reportedly came from the propellers of Japanese fighters shot down during the Pearl Harbor raid—and the guides were strengthened, so that they would hold up long enough for the firing pin to strike the primer and detonate the warhead. This “PHM” (Pearl Harbor Modification) was fitted to all the torpedoes in the inventory, and the changes incorporated into new production.
After that, the Mark-14 torpedo suddenly became a model of reliability, and sinkings finally did soar.
American torpedoes were wet heater, or “steam” types. Alcohol was burned in a combustion chamber, using compressed air as an oxydizer—the largest part of a torpedo was the air flask—which produced a high-pressure “steam” exhaust that could be used to power a small turbine. This was geared to a pair of counter-rotating screws. The Japanese used a similar design, but with pure oxygen instead of compressed air, which reduced exhaust—mostly the incombustible nitrogen from the compressed air—and took up less room, allowing for the larger warheads in Japanese designs.
The exhaust was one of the greatest perceived problems with the Mark-14 and its predecessors, since it left a trail of bubbles marking the course of the torpedo as it ran in on the target. Not only did this potentially warn the target, but it could also give the escorts a starting point for their hunt.
The solution was obvious—build a torpedo that didn’t leave a bubble trail. As far as the Bureau of Ordnance was concerned, obvious and accomplished were usually two different things. This was no exception. Newport experimented with electric torpedoes, but the basic design of the Mark-18 electric torpedo came from an entirely different source. The British supplied a captured German G7e torpedo, and this was copied, with the modifications needed to fit correctly in American torpedo tubes and interface with the TDC. When the Torpedo Station couldn’t build them fast enough, Westinghouse was contracted to build torpedoes, which they did, in a highly efficient manner.
With a maximum speed of 30 knots, the Mark-18 was slower than the Mark-14, but also much harder to spot in the water. It had a 575-pound warhead, and a maximum range of 4,000 yards.
As for the exhaust problem, studies after the war suggested that the bubble track was almost never spotted by Japanese lookouts. At least, not until the torpedo was too close to avoid. And as the Mark-14 ran much faster, it was actually more likely to get a hit.
One of the more interesting wartime developments was the Mark-27 “Cutie” homing torpedo. This had a number of problems, but could be effective under the right circumstances. It was very slow, which limited its utility against anything doing more than 8.5 knots. It was also non-discriminating. The rules dictated firing no shallower that 150 feet, since the torpedo simply homed in on the loudest noise in the area. Too shallow, and that might just be the submarine that fired it.
The Mark-27 was a submarine version of the air-dropped Mark-24 “Fido” anti-submarine torpedo—referred to as a “mine” for security reasons—adapted for use against surface ships. One former Navy Aviation Ordnanceman, who was involved in dropping the originals from a B-26 in the Pacific, pointed out one minor problem that sometimes cropped up. If two were dropped together, they would often forget to look for the submarine and chase each other around in a circle instead. After this was noticed, the torpedo release mechanism was modified to allow only single releases.
Submarines would fire only one torpedo at a time, so this problem didn’t arise. These were 19-inch models, and carried a very small warhead (92 pounds for the Mark-24, 95 pounds for the Mark-27). In the original anti-submarine role this was enough, since it only needed to punch a small hole in the pressure hull and water pressure would take care of the rest. In the anti-escort role, the “Cutie” would home in on the escort’s screws, and the small warhead was likely to be enough to blow off a screw and put the escort out of action. It wasn’t necessary to kill an escort so long as you could get it stop trying to kill you.