1. The Task Force will launch a surprise attack at the outset of war upon the U.S. Pacific Fleet supposed to be in Hawaiian waters, and destroy it.
2. The Task Force will reach the designated stand-by point for the operation in advance.
3. The date of starting the operation is tentatively set forth as December 8 [Japan time; December 7 Hawaii time], 1941.11
* * *
Being prepared for combat did not mean, however, that aerial bombing performance had been brought to concert pitch, at least in Yamamoto’s estimation. One day late in October, having learned of Yamamoto’s dissatisfaction from Combined Fleet air staff officer Comdr. Sasaki Akira, Fuchida went personally to see the C-in-C on board Nagato. There he reviewed the air training to date in Kagoshima Bay and proposed to Yamamoto that demonstration maneuvers be conducted in Saeki Bay utilizing all six carriers dedicated to the Pearl Harbor mission. Yamamoto agreed, pointing out that he wanted to see the aircraft drop their bombs or torpedoes not at safe distances from but within close proximity to their targets. Accordingly, four days of maneuvers were laid on for 4–7 November. In four groups—dive-bombers, torpedo bombers, horizontal bombers, and fighters—the aircraft displayed their newly acquired skills in mock attacks on Saeki, in northern Kyushu, which served as Pearl Harbor. Yamamoto pronounced himself satisfied.12
But there was one problem affecting the torpedo bombers that remained unresolved. Though they were now pressing their attacks close in to their targets, the torpedo-bomber pilots were achieving a disturbingly low percentage of hits. Part of the reason, it was decided, was the tendency of the torpedoes, when dropped, to plunge too deep in the water before ascending to their programmed attack depth, 13 to 20 feet. The water depth at Pearl Harbor was only 30 feet, except in the channels, where it was 45 feet. The successful British aerial torpedo attacks at Taranto had generally been made in depths of 84 to 90 feet, with a few runs made at 66 to 72 feet. The Japanese studied the site and took photographs, but Genda said after the war that Taranto had had no influence on the Japanese tactics for Pearl Harbor.13 Unless some modifications were made to the Japanese torpedoes it was certain that they would dive uselessly into the muck at shallow Pearl. This had been one of the two chief concerns about the raid expressed by Onishi in April.
The principal expedient devised by Japanese engineers to prevent that happening was the attachment of extended wooden fins to four metal horizontal and vertical tail fins of the Type 91 aerial torpedo (“Thunderfish”) then in use. The four other tail fins were left as they were. When the torpedo was dropped by an aircraft, the function of the wooden fin extensions was to “catch” the surface of the water and thus retard the torpedo’s plunge, before breaking away. Similar, though smaller, breakaway wooden fins were affixed to two antiroll stabilizer fins that protruded from the aft upper body of the weapon. After an initial, relatively shallow dive, the modified torpedo assumed a specifically programmed depth, usually 13 to 20 feet, the latter depth designed to place its 452-pound warhead below a warship’s armor belt. Once launched, the torpedo became an independent, self-propelled submarine, with guidance system, engine, propellers, rudders, and hydroplanes, which steered itself to immolation against the pilot’s target. The minimum run underwater required to arm the contact pistol (detonator) was about 650 feet, which was well suited to Pearl Harbor’s narrow waters. In tests of three such Modification 2 torpedoes, one hit bottom at 12 meters (39.3 feet), but the other two made successful runs to target. On that basis Fuchida estimated that, out of the 40 drops planned for, 27 Thunderfish would hit home. (In the actual attack, there were 36 successful drops, of which 25 scored hits; 11 others missed, malfunctioned, or bore into the muck; four met unknown fates.)
Still one more refinement was required, and that was finding what should be the pilot’s correct altitude, airspeed, and trim for making the drop. Sometime during the period 11–13 November the answers were found and the success rate in hits climbed to 82 percent.14 It was a tactic worked out just in time, since to make an aerial torpedo attack at Pearl Harbor on 8 December (7 December in Hawaii), carriers would have to depart by 26 November (25 November in Hawaii). The importance of all these operational advances may be measured by the fact that by far the most significant damage received by the Pacific Fleet’s capital ships on 7 December was inflicted by torpedo action.
* * *
Why, then, did Admiral Kimmel not have his battleships protected against such attacks by torpedo baffles, or nets, extended out from and down ships’ sides that faced the water, in order to deflect torpedoes from ships’ hulls? There were reasons. The nets would interfere with ship movements inside the harbor, and their time-consuming removal would substantially delay an emergency sortie out the channel. Such nets had not been provided the fleet and Pearl Harbor had no facilities for manufacturing them. The most important reason was that Kimmel and his staff believed that any torpedoes launched in Pearl’s shallow water would dive steeply and bury themselves in the muck before hitting their targets. They had been encouraged in that view by CNO Stark, writing to Kimmel on the preceding 15 February:
Consideration has been given to the installation of A/T baffles within Pearl Harbor for protection against torpedo plane attacks. It is considered that the relatively shallow depth of water limits the need for anti-torpedo nets in Pearl Harbor. In addition the congestion and the necessity for maneuvering room limit the practicability of the present type of baffles.…
A minimum depth of water of seventy-five feet may be assumed necessary to successfully drop torpedoes from planes. One hundred and fifty feet of water is desired. The maximum height planes at present experimentally drop torpedoes is 250 feet. Launching speeds are between 120 and 150 knots. Desirable height for dropping is sixty feet or less. About two hundred yards of torpedo run is necessary before the exploding device is armed, but this may be altered.15
In the JCC hearings, Kimmel was asked to read aloud an additional passage from Stark’s letter:
Kimmel (reading): As a matter of interest the successful attacks at Taranto were made at very low launching heights at reported ranges by the individual aviators of 400 to 1,300 yards from the battleships, but the depths of water in which the torpedoes were launched were between 14 and 15 fathoms.
Chairman: That is 90 feet?
Kimmel: Yes, sir.16
Stark’s letter concluded with a comment on A/T nets (baffles), which he described as very expensive and extremely heavy. The anchors and moorings took up about 250 yards of space perpendicular to the line of the net and required a long time to lay. He acknowledged the need for deployment of a light and efficient net that could quickly be laid out and removed, and expressed his hope that such a net would become available “in the near future.”17 But no such net was made available before 7 December, indicating, we may assume, Stark’s continuing view that torpedo bombers should be of no concern at Pearl.
Arriving by Clipper, Stark’s letter was read by Kimmel on 8 March; after which, as the CINCPAC routing sheet shows, it was read by his staff, one of whom, Gunnery Officer Kitts, made this notation: “From considerations listed in letter, it appears Pearl Harbor does not need nets.”18 When a companion letter from Stark arrived for Bloch, it, too, was passed through Kimmel’s staff. Aviation Officer Davis commented on the routing slip: “Remember Taranto! I don’t think Pearl Harbor need worry, however—too shallow.”19 On 12 and 20 March, respectively, Kimmel and Bloch replied to Stark, concurring with his judgments, Bloch expressing a particular concern: “Most of the available berths are located close aboard the main ship channels, which are crossed by cable and pipe lines as well as ferry routes. The installation of baffles for the fleet moorings would have to be so extensive that most of the entire channel area would be restricted.”20
On 13 June, however, a modification of his earlier judgments was sent by Stark to commandants of the various districts, including Bloch’s Fourteenth, with a copy to Kimmel. In it Stark reported recent developments permitting torpedoes to be laun
ched in water considerably less than seventy-five feet deep, and to make excellent runs. “Hence,” he wrote, “it can not be assumed that any capital ship or other valuable vessel is safe when at anchor … if surrounded by water at a sufficient distance to permit … a sufficient run to arm the torpedo.” Then, as was often his wont, he diluted that caution with a qualifier: “It may be assumed that depth of water will be one of the factors considered by any attacking force, and an attack launched in relatively deep water (10 fathoms [60 feet] or more) is much more likely.”21
In his fourth endorsement (critical of Kimmel) to the findings of the NCI in 1944, Secretary of the Navy James V. Forrestal stated:
The records of the Navy Department indicate that in April 1941 there was circulated in the Department an Intelligence Report which described the demonstration of an aerial torpedo in England. It appears from this report that the torpedo described was equipped with special wings, and that it required no greater depth of water for its successful launching than the depth at which it made its normal run. It further appears from the records of the Navy Department that the British report aircraft torpedo attacks during the year 1940 in which torpedoes were successfully launched in 42 feet of water.22
The last of Forrestal’s three sentences makes reference to a British periodical summary entitled “Naval Aircraft,” No. CB 3053 (1), which summarized operations for the period ending 20 September 1940. Included were descriptions of British aerial torpedo attacks at sea on the German battle cruiser Scharnhorst, the Vichy French battle cruisers Strasbourg and Dunkerque, at Oran, and the battleship Richelieu at Dakar. In the attack on Richelieu in July 1940, the report stated that, “the charted depth was only 7 fathoms [42 feet].” This report, received in the Navy Department on 21 June 1941, was not sent on to Kimmel.23 Forrestal’s first two sentences probably refer to one of three U.S. Navy intelligence reports, the first dated 15 May 1941, the second 26 June 1941, and the third 5 September 1941. All three describe the operational British 18-inch, 42-knot Mark XII aerial torpedo fitted with glider wings. In an official enumeration of intelligence reports on aerial torpedoes received at the Navy Department, none is listed as having been dated at or received in April 1941.24 The May document identifies the new torpedo as the Mark I TORA, and gives this general description:
The TORA consists of a normal aircraft torpedo with air rudders removed and fitted with wings and tailplane. A gyroscopic unit applies aileron in response to deviation in roll and yaw so that after release the tora glides in an approx. straight path, at an angle of about 1 in 5 and at about a constant speed unit [until?] it strikes the water. It is thus possible to drop a torpedo from heights above those in use for release of ordinary torpedoes and at a considerable range from the target.
To avoid damage to the torpedo and disturbance of its water path it is necessary to jettison the wings and tailplane just prior to entering the water. To effect this a paravane is towed below the tora and on striking the water operates a quick release mechanism.25
There follows a fifteen-page highly technical description of TORA components, attachments, rigging sequence, and so on. Nowhere, however, is mention made of the water depth required, the depth to which the TORA dived upon reaching water, or the length of run needed to arm the warhead. An official Navy Department document prepared in 1945 or 1946 specifies: “There is no evidence that the report or enclosure was sent to C. in C. U.S. [Kimmel].”26
The second intelligence report, dated 26 June, describes the same aerial torpedo but under two new names, Paratorp and Torraplane. By this date two squadrons of Beaufort bombers had been equipped with the weapon. Here it is stated that the torpedo “enters the water at 9 degrees incidence” [9 degrees to the horizontal] and will “reach the proper depth without driving too deeply or broaching to, when dropped from an airplane.” Again, the report reads, the torpedo “would enter the water smoothly without ricochet or danger of striking the bottom.”27 The oblique angle of entry “is controlled by means of wooden air rudders fixed to the horizontal rudders of the torpedo, which are in turn actuated by a servo motor within the torpedo itself.” The additional information contained in the foregoing sentence is drawn from the third intelligence report, originating with the U.S. naval attaché in Ottawa, Canada, which was based on conversations held with RCAF flying officers in aerial torpedo squadrons, and received in the Navy Department on 8 September. Again, no precise numbers are given for the initial depth reached after entry, but normally the depth setting, rather than placed for impact against the hull, is deeper than the draft of the target vessel both as a means of circumventing a torpedo net and for the purpose of placing the torpedo’s electromagnetic Duplex pistol type of detonator below the ship’s hull, where the change in magnetic field generated by the hull activates the detonator.28
The last two of the three intelligence reports cited above were sent to Admiral Kimmel, according to Navy Department records.29 If they were read at Pearl it would seem that the fact that the British had succeeded in dampening the initial dive of an aerial torpedo would have been picked up by somebody and would have set alarm bells ringing in Kimmel’s staff offices, for the Japanese arguably could do the same. But later staff testimony makes no mention of the British breakthrough. That of fleet gunnery officer Kitts on 21 August 1944 may stand for the rest:
My views were that the particular kind of aircraft attack, namely, by torpedoes, was possible in Pearl Harbor. In that, we had considered counter-measures, such as nets and balloon barrages. The feeling in general in my own mind was that the feasibility of a successful torpedo attack in Pearl Harbor—to my mind, that was minimized by the receipt of information copies of one or two letters addressed by the Chief of Naval Operations to the District Commandant [Bloch], indicating to me that the water at Pearl Harbor was so shallow that the success of a torpedo attack on ships in Pearl Harbor was dubious. Nets were considered. The difficulty of procuring these nets, on which there was a low priority, and the need of moving the fleet on short notice from berths for a sortie, the difficulties of cluttering up the harbor with these nets—it was my understanding that all these factors placed the nets not only in a low priority, but gave us a feeling that the risk of a successful torpedo attack was slight.30
Kimmel and Bloch, too, make no mention at the time or later of any familiarity they had with aerial torpedo launching advances such as those recounted above. Indeed, in October 1945, while preparing his testimony to be given in the JCC hearings, Kimmel wrote to CNO King asking to see, as though for the first time, the intelligence report on British aerial torpedo developments to which Secretary Forrestal made reference in his fourth endorsement to the NCI findings.31 Subsequently, two of the reports discussed above (15 May 1941 and 5 September 1941) were sent by messenger to Kimmel, who had a temporary office in the Navy Department.32 The discontinuity of information here—Navy Department records that indicate certain torpedo reports were sent to Kimmel, despite the seeming ignorance of the existence of those same reports on the part of Kimmel and his staff—leaves the student of these events in an uncertain mind. The question of what Kimmel and his staff knew about aerial torpedo advances in 1941 and when they knew it must be answered at this date by acknowledging that we simply cannot say.
Of one thing we may be certain: in later years, Kimmel understood thoroughly how decisive the breakthrough in torpedo delivery during 1941 had been. In an aide-mémoire dated 15 August 1945, he wrote: “The fact that the Japanese aerial torpedoes could be launched in less than 40′ of water and make successful runs was unquestionably a considerable factor in the Japanese decision to make the attack on the fleet at Pearl Harbor.”33
* * *
On 7 November Admiral Yamamoto ordered all ships assigned to the Hawaiian Operation to depart their various anchorages on or about the fifteenth and to proceed as single vessels or in small formations to rendezvous in isolated Tankan (Hitokappu Wan) Bay, Etorofu Island, in the northern Kuriles, arriving no later than the twenty-second. On the sa
me day, Yamamoto issued Combined Fleet Top Secret Operation Order No. 2, implementing Order No. 1 and establishing 8 December (7 December Hawaii time) as “Y” Day—the approximate date for the commencement of operations. An Imperial Naval Order issued by the Imperial General Headquarters on 2 December would confirm Yamamoto’s No. 2 and declare 8 December to be “X” Day—the exact date on which the raid on Hawaii would be made: “The hostile actions against the United States of America shall commence on 8 December.”34 The provisional date (“Y”) and the finally determined date (“X”) proved to be one and the same.
The fleet that assembled in Tankan Bay consisted of, for aerial attack: the Navy’s six first-line carriers, Akagi (flagship) and Kaga (1st CV Division), Soryu and Hiryu (2nd CV Div.), and Zuikaku and Shokaku (5th CV Div.); for guard and support: battleships Hiei and Kirishima (3rd BB Div.) and heavy cruisers Tone and Chikuma (8th CA Div.); for guard and escort: light cruiser Abukuma and destroyers Tanikaze, Urakaze, Isokaze, Hamakaze, Akigumo (17th DD Div.), Kasumi, Arare, Kagero, and Shiranuhi (18th DD Div.); for patrol: submarines I-19, I-21, and I-23 (2nd SS Div.); for bombardment of Midway Island on return voyage: destroyers Akebono and Ushio (7th DD Div.); and for supply: tankers and auxiliaries Kyokuto Maru, Kenyo Maru, Kokuyo Maru, and Shinkoku Maru (1st Supply Train), Toho Maru, Toei Maru, and Nihon Maru (2nd Supply Train).
These were not the only naval vessels assigned to the Hawaii Operation, however. Already at sea was the Advance Expeditionary Force composed almost entirely of fleet submarines, twenty in number. Most were assigned to blockade of Oahu and strangulation of the entrance and sortie channel at Pearl. Two were assigned to reconnaissance of key points. And five were each to launch a deck-borne forty-five-ton midget submarine in the southern approaches to Pearl. Seventy-four feet in length, carrying two crewmen and armed with two torpedoes, the midgets’ mission was to pass submerged beneath the torpedo net guarding the channel entrance and to launch torpedoes at capital ships from inside the harbor. Since the existence of the midgets that formed what was called the Special Attack Unit had successfully been kept secret, it was thought that the American defenses would miss their penetration. In one midget that was captured after beaching off Bellows Field, U.S. investigators would find silhouettes of Oahu from every angle of approach, exact silhouettes of all ships of the fleet, and charts of Pearl Harbor that were impressively accurate. Meanwhile, the larger boats outside the harbor would torpedo any American ships that tried to sortie during or after the air assault, and to the east, interdict any surface reinforcements dispatched from the West Coast.
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