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Little historical attention has been given to the mining operations conducted by U.S. submarines during the Pacific war. In comparison with our submarines’ outstanding torpedo successes, their mine-planting forays appear as a minor sideshow. Indeed, the official Joint Anny-Navy Assessment Committee (JANAC) tally of Japanese ships sunk during the war attributes only five ships totaling 18,553 tons to mines laid by U.S. submarines, but it does not identify the boats credited with those sinkings. Consequently, the count of ships and tonnage sunk by individual submarines has never included the victims of the mines planted by those same boats.

Unfortunately, it is extremely difficult to pinpoint a ship’s sinking to a specific minefield, let alone to the submarine or other agent that may have laid the mines. Casualties usually occurred hours, days, or even months after a minefield was laid, when the enemy could have had ample time to sweep the mines or cordon off the dangerous grounds. In several locations both submarines and aircraft planted mines in close vicinity, while the positions reported for Japanese losses as well as those recorded for the Allied mine-fields themselves are often of questionable precision. The figures almost never correlate exactly with each other, and are usually several miles apart.

In addition to the possibility of being detected and swept, mines had their own internal weaknesses such as exploding prematurely, breaking their tethers and drifting out of position, or failing with age. In spite of such problems, mines were known to be very effective offensive weapons against enemy shipping when planted clandestinely in strategic locations such as harbors or channels. They were also widely used defensively to protect against enemy approaches to beaches or harbors. Accordingly, mines were extensively used by all combatants throughout the Pacific theater, often in the same general areas, where they were likely to become a threat to friend and foe alike.

The problems of identifying a mine victim are illustrated by the only instance when a U.S. submarine actually observed a victim exploding a freshly laid mine. LCDR Roy Benson in TRIGGER (SS 23 7) was in the process of planting a field of 19 magnetic mines on 20 December 1942 off the cape Inubo Saki when a freighter conveniently ran into one, blew up, jack knifed and sank. Two days later in the same area he torpedoed another victim which he last saw going down by the bow, and on 26 December he sighted yet another ship heading into the mined area, followed later by a distant explosion.

JANAC was never able to identify the ship seen to sink in the minefield, but Benson was credited with an Unknown Maru. His torpedo attack was later assessed as sinking the TEIFUKU MARU. Postwar Japanese records are somewhat confusing and contradictory, but the most likely conclusion seems to be that the ship seen to sink in the minefield (the Unknown Maru) was the MITSUKI MARU; the torpedo victim, which was damaged but not sunk, was the YOSHU MARU; no ship was sunk or damaged by the mine explosion heard on the 26’h; and the TEIFUKU MARU actually hit a mine on 29 December, was run aground, and became a total loss. The records for most of the other ships credited to mines suffer from similar confusion.

Minelaying was seldom regarded as a primary mission for U.S. submarines. Although many other navies included submarines specially fitted for laying mines, only the single USS ARGONAUT (SM 1) was designed primarily as a minelayer. By 1941 ARGONAUT, then the Navy’s largest submarine, was old, slow, unwiedly, under-armed, and overdue for a thorough modernization. Operating as an ordinary submarine, she was on station off Midway Island when the Japanese attacked Pearl Harbor. On her return, she was ordered to Mare Island for her much-needed updating. The original minelaying installation, featuring internal stowage and transfer facilities for60 Mk XI mines laid from two 40-inch diameter stem tubes, was retained. Although the authorities in Washington had deemed it worth refurbishing, forces afloat had other ideas. On her return to Pearl Harbor the mine gear was immediately stripped out to provide space for carrying Marines to the Makin Island raid. Being then reclassified as a submarine transport, ARGONAUT was ordered to Brisbane to conduct special missions such as evacuating refugees from the Philippines. While en route she was directed to attack a convoy, only to be sunk by Japanese destroyers with the loss of I 05 lives.

Abandoning the concept of dedicated submarine minelayers, the Navy shifted to developing mines that could be ejected through the torpedo tubes of all fleet submarines starting with SARGO (SS 188). The main drawback was that only a small load of mines could be carried. In the early months of the war, when Allied surface and air forces had been driven back from the Far East, distances to enemy targets were so great that submarines were the only effective means of laying mines surreptitiously in Japanese waters. Aircraft mining in the South and Southwest Pacific theaters did not start until March 1943. Herb Mandel, who was then on FINBACK (SS 230) during her shakedown early in 1942, recalls going out on GRUNION (SS 216) to observe a practice mine plant. This training must have been discontinued shortly thereafter, as his own boat never did such an exercise, nor did GRUNION ever lay a live minefield. However, as skipper of PERMIT (SS 178) at the end of the war, Mandel laid a dummy mine plant for the Bureau of Ordnance in Provincetown Harbor, so obviously even the oldest fleet boats had been refitted to handle mines.

The first submarine minefield was laid out of Fremantle by W. J. Millican in THRESHER (SS 200) in the approaches to Bangkok on 16 October 1942. It was followed four days later by another in the same area planted by Donald McGregor in GAR (SS 206). Both submarines carried maximum loads of32 Mk 12 mines, which took the space of 16 torpedoes. Although their designed maximum load was 40 mines, in practice U.S. subs carried at least eight torpedoes for use in an emergency before the mine plant or to attack targets thereafter. All minelaying missions but one were carried out by boats of the Tamboror later classes, probably because the earlier types had fewer torpedo tubes. The only exception was the one by STINGRAY (SS 186)-a Salmon-class boat with only four tubes forward-in April 1943.

Most of the early mine loads were probably carried in the forward torpedo room. Later in the war typical loads were gradually reduced to only eleven mines, then increased again to 23 in 1945. According to E. C. Hawk’s report of the plant laid by POMPON (SS 267) in December 1943, his 11 mines were fired alternately from tubes 9 and 10 in the after torpedo room. When HARDHEAD (SS 365) laid a field of 23 mines, her commander, F. A. Greenup, fired 10 from tubes 3 and 4 forward and 13 from tubes 9 and 10 aft.

The Mk 12 was a non-tethered ground mine housed in a stream-lined case and actuated by a Mk 3 magnetic exploder, a complex device that had to be set according to the polarity and strength of the earth’s magnetic field in the location where it was to operate. It could also be adjusted to be sensitive to a particular size of target passing overhead and to detonate only after a selected number of targets had been counted. These features were intended to make the mines harder to find and sweep, and probably had to be preset in the shop before going on patrol. The ship count was set for the first target to be detected in all but six fields where the mines were set at various combinations between one and nine counts.

Although the Mk 12 mine’s explosive charge would remain active indefinitely, the exploder was powered by a battery, possibly activated by sea water, with an expected life of90 days. In order to function as designed, it had to be planted in depths ranging from seven to twenty fathoms with the submarine running either fully surfaced, with decks awash, or at periscope depth, depending on the circumstances. In the Pacific war U.S. submarines initially placed these mines spaced between 280 and 1500 yards apart, while in later fields the spacing was between 500 yards and one mile. Two of the reports I have seen note that the mines had to be laid in a carefully plotted sinusoidal curve, apparently to make sweeping more difficult. A delay mechanism could give the boat 45 minutes to clear the area, but in most cases no delay at all was set. The first five patrols using Mk 12 mines experienced 11 failures, including premature explosions in each case. In August 1944 these mines were refitted with the improved Mk 3 Mod 2 exploder, making them twice as sensitive.

Other characteristics of the Mk 12 mines and their exploders are apparently still classified, which leads to some questions about their performance. According to Captain Franklin G. West, Jr., Training and Readiness Officer of the Mine Warfare Command in 1990, the Mk 12 mine was inoperative after the 90-day battery had expired. However the life of a battery is not that exactly predictable, so allowance has to be made that mines might be viable somewhat longer. Also, it apparently did not have a sterilization mechanism. As will be seen later in a detailed analysis of claimed casualties due to mines, losses were credited to submarine mines much later than three months after the fields were planted, either due to lack of knowledge of the exploder’s real characteristics or to some other unspecified mechanism by which it might have been set off.

In October 1942 the WHALE (SS 239) under J. B. Azer sailed from Pearl Harbor on her first war patrol with a load of 24 Mk l 0-1 mines to be laid in Empire waters in Kii Suido. The objective was to plant them close inshore in order to force enemy traffic into deeper water where it would be more vulnerable to torpedo attacks. These were tethered mines touched off by contact with chemical horns and planted in fairly deep water with the mines themselves held at a selected depth below the surface. The WHALE’s were laid in 15 to 42 fathoms of water with the explosive casings held two fathoms below the surface, but later plantings were made in water as deep as 63 fathoms. These mines consisted of two major sections-the floating sphere and its anchor-and their connecting cable, without any outer casing. Like their Mk 12 counterparts, they too were susceptible to failures: in the WHALE’s case, one proved to be a floater. Only three later missions, all from Pearl Harbor, used these mines. B. F. McMahon in DRUM (SS 228) took 24 of them to Bungo Suido in December 1942, and in April 1943 W. N . Wylie in SCORPION (SS 278) carried the only load in which both Mk 12 and Mk 10-1 mines were laid together. In the final mission Creed Burlingame in the SIL VERSIDES (SS 236) planted 24 of them in Steffan Strait as part of a coordinated operation with aircraft, the strait being the only entrance to Kavieng, New Ireland, that aircraft mines could not block. Following that exercise, the boat continued on to Fremantle, Australia.

As might be expected with weapons such as these, submariners did not like handling mines. In addition to their inherent hazards, they required taking one’s boat into dangerously shallow waters near enemy ports, displaced more versatile and familiar torpedoes, and almost never produced visible or creditable results. Several missions were to replenish older minefields, in which cases accurate navigation was crucial. Examples of the risks are numerous. When J. B. Azer took WHALE inside Japanese minefields in order to lay his mines in a shipping lane, he detected a Japanese mine in the process. After planting his load in three sub-fields, he was forced down by destroyers, but had the satisfaction of seeing some ships previously damaged in a torpedo attack head straight toward the mines and later heard four heavy explosions. Unfortunately, these may have been premature, as no victims have been identified in post-war Japanese records.

Roy Benson in TRIGGER (SS 237) recorded having to pass up favorable torpedo targets to avoid alerting the Japanese, start his mine plant while surfaced in bright moonlight, and break off temporarily when ships appeared. These difficulties were offset by his unique experience of actually watching his victim blow up and sink. While patrolling in the Gulf of Siam on 13 June 1945, BERGALL (SS 320), under J. M. Hyde set off an Allied mine and was lucky to escape with reduction gears so badly damaged that she had to return to the States for repair. Patrol reports are replete with similar examples of mine hazards. In April 1945, GUIT ARRO (SS 363) had to run for miles on the surface under a bright moon, dodging traffic all the way, to reach her assigned position in Berhala Strait. Her skipper, T. B. Dabney, has provided this account of his experience after leaving Fremantle and reaching the area to be mined.

“We ran on the surface, with all four main engines on the line, since it was a race against time. Arriving in the strait at about midnight, we had loaded our mines in the tubes, in preparation for accomplishing our mission. We were surprised to find two small ships with escorts exiting through the straits. Since we were in the narrow confines of the straits, in shallow water, and small boats all around us, we had to download our mines in the forward tubes and reload torpedoes, in case we were suddenly detected before we could commence our mission. The convoy passed within a thousand yards, apparently without detection. The small fishing boats, although close at hand, gave no indication of giving our presence away. We reloaded our mines and took position to lay our mines in a sinusoidal curve. We successfully completed our mine field operation … and started our return at top speed on the surface, just before daybreak. We had a hundred miles of open water to cover before arriving at the I 00 foot curve, suitable for diving. A Japanese plane spotted us and we had to dive. The bomb load fell around us but there was no damage.

In his patrol report, Dabney aptly referred to the area as Wader’s Paradise. The water there was only seven fathoms deep.

Thanks to RADM M. H. Rindskopf we have a first-hand account of the Mk 10-1 mine plant laid by DRUM (SS 228) on her fourth patrol. On 12 December 1942, en route to Bungo Suido “in the unfortunate condition of having two of her forward tubes loaded with mines,” skipper B. F. McMahon encountered the 13,360 ton carrier RYUHO with a deck load of planes. He fired the available four tubes and obtained one hit, but was driven deep before he could swing around for a stem shot, allowing the damaged carrier to escape. According to Rindsko of, who was a junior officer at the time, two mines were stowed in a tube but had to be fired one at a time.

“We carried mines only forward so with four in tubes (two each) that meant 20 in the room, two to a rack. That meant that we carried four torpedoes in tubes and no reloads forward with four and four aft. It is even possible that the torpedoes in the after room were Mk 15 destroyer type which had to be loaded through the tube because of the length. That was due to the shortage of the Mk 14 early in the War. We did not have to back down to launch as the mine was ejected by the same air impulse as torpedoes. I have some recollection that there were two aspects which might have been affected: first, the gyro spindle in the side of the tube was not required for the mine and might have gotten in the way during loading; second, is the lever at the top of the tube which triggered the torpedo starter. That wasn’t required for mines but whether it got in the way or how it might have been withdrawn is fuzzy indeed. The mines may have been the same diameter as the torpedoes or a bit small.. .. The Mk 10 had the anchor attached to the case and antenna without any streamlining or outer casing. Shoving them around the torpedo room and loading was no particular problem since we fired at a planned fairly rapid pace …. We did not stick around long enough to see whether any targets ran through the field …. I do recall that from the continual firing and venting inboard, the pressure in the boat went to something like 12 inches. Since we did not have a compensating depth gauge, the diving officer had to make adjustments in gauge depth to keep us at 62 feet.”

Notwithstanding the many problems, senior commanders recognized the mine’s strategic value, and mines also constituted an alternative weapon when torpedo shortages would have necessitated going on patrol without full racks. In all, 33 Commanding Officers in 32 submarines planted minefields between October 1942 and May 1945, laying 576 Mk I 2 magnetic bottom mines and 82 Mk l 0-1 of the tethered type. Of these, 13 Mk l 2s were failures, six of which exploded prematurely, and three Mk I 0-1 s were floaters. Ten patrols were made from Pearl Harbor and 23 from Freman tie, Australia. The only boat to lay two fields was TAUTOG (SS 199), first under J. H . Willingham on 2 November 1942 and then under W. B. Sieglaff on 7 March 1943. Apparently one mine plant per skipper was considered enough of a sacrifice.

British and Dutch submarines, including three designed specifically as minelayers, also laid 30 minefields, at first from Ceylon and later while patrolling from Fremantle under U.S. operational control. Although these are beyond the scope of this article, J. L. Mccallum in BREAM (SS 243) had an unsettling experience on a special mission carrying British commandos with limpet mines to attack some anchored Japanese ships. On 14 March 1945 two of the frogmen were launched in a rubber raft but never returned. This demoralizing occurrence did not exempt the crew from planting a regular minefield on BREAM’s very next patrol.

What were the results of these heroic efforts? Unfortunately, they are both meager and uncertain. The largest number of submarine mine victims claimed in any official U.S. source appears in the report of the Strategic Bombing Survey (SBS), which was conducted immediately after the war. Although its main thrust was obviously aerial bombing, it also investigated offensive mining and concluded that 27 ships were sunk and 27 damaged by mines laid by U.S. submarines. As noted earlier, the official JANAC report of 1947 listed only five ships as sunk by U.S. submarine mines, of which four are also claimed by SBS.

Since 194 7 significant new data sources have come to light, and I have used these to check the SBS and JANAC assessments. (To save space, these sources are described in the Appendix). My analysis reduces their claims to at best nine sinkings and eight cases of damage that can probably or possibly be credited to U.S. submarine mines. (None are assessed as fully confirmed, because sources are incomplete, indefinite, or even contradictory.) On the other hand, from these additional sources I have identified three cases of possible or probable sinkings and six of damages not claimed by SBS or JANAC. Table I summarizes the 26 cases that I consider credible.

In determining whether a claimed mine casualty should be categorized as probable, possible, or neither, I have tried to take into account all available data including the relative positions of the casualty and the minefield, the age of the mines and likelihood that they could have been swept, or other mines known to be in nearby locations, possible air or torpedo attacks, and the general reliability of the data sources. My conclusions are necessarily subjective and other analysts may differ. New data and information on other sources will be appreciated.

For readers interested in a more detailed analysis of the date, Table II gives particulars of the minefields laid by U.S. submarines. Table III lists all 28 sinkings claimed by SBS and JANAC, with notations to the applicable Japanese sources. Similarly, the 27 SBS damage claims are listed in Table IV, and the nine other cases in Table V.


SBS or Strategic Bombing Survey –The Offensive Mine Laying Campaign Against Japan; originally published 1946, reprinted by Headquarters Naval Material Command, 1969. This survey was conducted immediately after the war and includes many sections and appendices other than the above. The data were derived from intelligence reports but clearly not including naval Ultra intercepts. I am indebted to Ted Hajduk of Detroit for original SBS records detailing the ships attributed to the different minefields.

J or JANAC-Japanese Naval and Merchant Shipping Losses During World War II by All Causes; Gov’t Printing Office, February 194 7. J ANAC counted only ships sunk but excluded merchant types, including small converted naval types with maro names, ofless than 500 gross tons. Its intelligence sources apparently including sanitized information from Ultra messages and Japanese records captured at the end of the war. JAN AC also attributed seven sinkings to British (including Dutch) submarine-laid mines. Only one of these appears in the SBS tally and is more likely to have been sunk by a U.S. submarine.

I OR UN-The Imperial Japanese Navv in World War II. Part IV, Monthly Losses of Combatant and Non-combatant Vessels; Military History Section, U.S. Army Far East Command, 1952. After the war General MacArthur had Japanese researchers compile an extensive list of all ships believed sunk or damaged during the war, which was issued as a monograph. The ships are listed by month with separate sections for warships and non-combatants. Tables and maps give the date, ship type and tonnage, location, cause, and extent of damage. Not all records are complete, and locations are often given as general areas rather than latitudes and longitudes. This publication contains the most extensive records of damaged ships.

W or WIJN-Jentschura, Jung, & Mickel: Warships of the Imperial Japanese Navy, 1869-1945; Naval Institute Press, 1982. This book is based on data originally compiled by Shizuo Fukui and Erich Groner in the 1950s and updated in several printings. It covers converted as well as regular warships in considerable detail, but includes little infonnation on damage short of sinking.

S-Translations from Japanese publications by William Somerville of Lincolnshire, England. The major sources are Senji Sempaku Shi (Wartime Ships History, 1991) and Senji Yuso Sendan Shi (Wartime Transportation Convoys History, 1987) both by Shinshichiro Komamiya. The fonner is an alphabetical listing of ships sunk; the latter lists convoys chronologically and includes much information about the ships involved. Both lists have gaps and occasionally conflict. I am indebted to Mr. Somerville for data from these and other Japanese sources .

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