"What did the USN know about Yamato and when?"

by Joseph Czarnecki
Updated 21 August 2002

Oft-repeated questions about the famous Japanese Yamato class battleships include:  “What did the U.S. know about these ships and when did they know it?”

According to some distinguished authors, nothing concrete was known about their characteristics until after the Occupation of Japan and the subsequent capture of their construction records.  That this is not the case can be shown by a review of the available information in a few authoritative texts.  The following essay is a presentation in chronological order of the status of the USN’s on-going intelligence on these battleships as gleaned from those works.

As revealed in John Prados’s “Combined Fleet Decoded,” the U.S. had made the following assessment prior to the start of the Pacific War:

In 1936, Captain Fred F. Rogers, a Japanese-language officer (which denotes an American naval intelligence officer who could speak and write Japanese, having been so taught in Japan) and the U.S. naval attaché in Tokyo, reported that “Japan had designs for warships of 45,000 to 55,000 tons,” that she “would not hesitate to build ships of at least 35,000 tons” and “would not revert to the Washington restriction on tonnage or its limit for guns.”  Rogers quoted secret Diet testimony from Navy Minister Admiral Nagano Osumi to the effect that:

“...as a result of the coming no-treaty period we shall enjoy freedom of action in construction of warships in respect to category, quality and characteristics.  With this freedom we may construct those ships particularly adapted for our national requirements, thereby gaining an advantage which obviates the necessity for numerical equality.”
Captain Rogers’ successor, Captain Harold M. Bemis, reported in January 1938 that “Japan has under construction two 16-inch battleships of considerably greater tonnage than 35,000 tons and is planning to lay down a third and possibly a fourth.”

Bemis’s fellow members of the international “Attachés’ Club” apparently agreed that the new ships would exceed Treaty limits.  The British, German and French attachés all felt these would displace 45,000 tons.  The Italian attaché was more specific at 46,000 tons with a possible tonnage increase to 50,000 tons.  The Soviet attaché thought the ships would barely exceed 35,000 tons.

There was apparently some agreement among the attachés that the armament would be twelve 16-inch (41 cm) guns despite the interest shown in 18-inch guns for the No. 13 class in the 8:8 Program.  The Soviet attaché had erroneously concluded that the Japanese could not lathe such large weapons without importing the necessary equipment, but rightly pegged the location of the development facility.  The Italian attaché thought testing of such a weapon had failed with a burst powder chamber.

Following the start of the Pacific War, a document captured on Tulagi in August 1942 apparently provided the U.S. with their first crude schematic of the design, and for the first time confirmed a barrel-count of nine guns.

In October 1942, ONI (Office of Naval Intelligence) estimated the ships at 35,000 tons and armed with nine 16-inch guns.

In February 1943, the of interrogation of a survivor of the submarine I-1 sunk off Guadalcanal, Kuboaki Takeo, confirmed the nine-barrel count.

By the spring of 1943, CINCPAC (Commander In Chief Pacific) Intelligence listed the ships as being armed with nine 17.7-inch (45 cm) guns (the source of this information is not discussed by Prados).

In July 1943, interrogation of a captured Japanese aviator betrayed the conversion of the third ship in this class, Shinano, to a carrier and that her intended size was 50,000 tons.  ONI did not believe the ship would be so large and hedged its estimate by speculating the Shinano might displace 30,000 tons after being stripped down from a battleship to a carrier.

An October 1943 message intercept referred to “special type 40 cm” ammunition and contributed to the USN thinking that these ships carried 16-inch guns.  This was as intended by the Japanese, who from the inception of the super-battleship program had always referred to the 46 cm (18.1-inch) guns on the Yamato class as the “special type 40 cm” as a disinformation ploy.

On 4 February 1944, two B-24 reconnaissance aircraft overflew Truk, taking photographs.  These were processed by 25 February 1944 and one of them showed a Yamato class ship visible in one corner.  ONI photo interpreters at the Photographic Reconnaissance Interpretation Section Intelligence Center (PRISIC) at Pearl Harbor correctly estimated that this ship was armed with 18-inch guns.  Ship-design experts called in to consult estimated the ship at a minimum of 60,000 tons and noted that this huge size was necessary in order to mount nine 18-inch guns.  However, these experts then proceeded to thoroughly play Devil’s Advocate against their own position, claiming such a design was “impractical.”

In June 1944, the U.S. captured Admiral Yamamoto’s former yeoman, Noda Mitsuharu, who had now been serving with Admiral Nagumo on Saipan.  Edwin Layton of CINCPAC Intelligence interrogated him and the prisoner revealed the “special type 40 cm” deception by recounting current jokes about the designation in the IJN.  At this point, Layton abandoned his previous conviction that the Yamato class carried 16-inch guns and so informed a Seventh Fleet intelligence officer passing through Pearl Harbor on his way to the Southwest Pacific.

The intelligence available in February 1944 and June 1944 is discussed in a footnote on pages 534 and 535 of “Combined Fleet Decoded” which states:

“On February 25 the Truk photos were processed at PRISIC in Pearl Harbor: one picture showed the huge ship, obscured at the corner.  The Office of Naval Intelligence brought ship-design experts into the discussion, concluding that the Yamato class must displace at least 60,000 tons.  That also was the size experts thought necessary to mount 18-inch guns, but the same people argued that problems of stowage and propulsion, plus complications with docking and navigation, would render such a warship impractical.  A Seventh Fleet intelligence officer was given this word just before returning to SOWESTPAC, then passed through Pearl Harbor, where he talked to Eddie Layton.  Previously a staunch defender of the proposition the Yamatos had only 16-inch weapons, Layton was by this time interrogating prisoner Noda Mitsuharu, former yeoman to Admiral Yamamoto.  Noda repeated many jokes current in the Imperial Navy about the ‘special type’ 16-inch guns of the Yamato, converting Layton.  Still the ONI Weekly on August 30 carried a feature on Japanese naval guns discounting the possibility of the larger size for Yamato, and still listing her at 40,000 tons.  The first reasonable [sic] accurate artist’s conception of the appearance of the Yamato appeared in the ONI Weekly on September 20 based directly on the Truk photo.”
[Re: “sic” should read “reasonably.”]

This information is apparently alluded to in Morison’s Volume XII with a footnote on page 162, stating:

“Although Yamato had participated in the Battle of Midway, these battleships were ‘mystery ships’ to the world at large; but the CINCPOA and Seventh Fleet Intelligence officers, by piecing together scraps of information, had a good idea of their size early in 1944, which was confirmed by a talkative officer prisoner in September.”
In Dulin & Garzke’s “Battleships:  Axis and Neutral Battleships in World War II” we find two drawings of the class attributed to the USN during the course of the Pacific War.  The first is a crude and inaccurate drawing correct only in the ship’s gross features and attributed to “summer 1944.”  The second is much more accurate and given as dating from “March 1945.”

This book includes several war-time photographs of Yamato and her sister-ship Musashi, mostly taken while they were engaged during the Battle of the Sibuyan Sea on 23 October 1944.  Dulin & Garzke contains two very clear images of Yamato from this battle, a less distinct photo of Musashi amidst bomb and torpedo splashes and an unidentified overhead photo of one of the ships evading attack*.  The clarity of some of these photographs, as opposed to the earlier, indistinct ones, would have obviously aided intelligence officers in assessing the size and armament of these ships.

Yamato was also photographed under attack in the Inland Sea on 19 March 1945.  One frame taken during this attack appears in Dulin & Garzke.  Further photographs were taken during Yamato’s final engagement on 7 April 1945, including four that appear in Dulin & Garzke.

The intelligence assessment for these ships as of the time of Yamato’s “Final Sortie” may be judged by the following paragraph taken from Morison’s “History of U.S. Naval Operations in World War II,” Volume XIV:

“An eagerly anxious evening followed for TF 54.  Staff officers familiar with range tables took care to remind others that Yamato’s 18.1-inch guns should have a maximum range of 45,000 yards, as against 42,000 for the 16-inch gunned battleships in Deyo’s force and 37,000 for [the 14-inch gunned] Tennessee; and that her speed should enable her to make an ‘end run’ and thrust at the transports.”
This statement effectively shows that at some point between the October 1944 battles around Leyte Gulf and Yamato’s final sortie in April 1945, the USN had come to a more complete and accurate appreciation of the Yamato’s true size and armament and was actively using that assessment for battle planning.


John Prados, “Combined Fleet Decoded:  The Secret History of American Intelligence and the Japanese Navy in World War II

Samuel E. Morison, “The History of United States Naval Operations in World War II,” Volumes XII and XIV

Robert O. Dulin, Jr. and William H. Garzke, Jr., “Battleships:  Axis and Neutral Battleships in World War II

* Editor's Note:  This photograph is actually of Yamato.  USN photographs of these battleships taken during the war and a few acquired afterwards may be found at the U.S. Navy Historical Center.



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