I-1 Submarine & Admiral Yamamoto’s Demise (Solomon Islands — 90 fsw)

Introduction

On Thursday, April 18 1943, during Operation Vengeance, United States P-38 aircraft launched from the Solomon Islands shot down the aircraft carrying the Commander-in-Chief of the combined fleet of the Imperial Japanese Navy, Admiral Isoroku Yamamoto. Yamamoto was the architect of the Japan war strategy and was absolutely critical to their heretofore success in the Pacific Theater.

Yamamoto gave the order order to attack Pearl Harbor from the bridge of the IJN Nagato battleship — hence the aptly named Operation Vengeance.

It is widely recognized that Yamamoto was able to effectively implement a strategy involving both the Japanese Army & Navy. After his death, that cooperation largely ended and it greatly reduced the effectiveness of Japan’s war strategy.

The question is: how did we know where Yamamoto would be?

I-1 Submarine

The I-1 was a J1 type submarine and was one of four submarines in her class. She was originally built as “Submarine Cruiser No. 74” but was renamed 1-1 shortly after launching. She was 320 feet long (which is about the same as a Balao-class submarine) and had a beam of 30 feet with a complement of 68 officers and enlisted.

She was launched on 15 Oct 1924 and commissioned in 1926 and served in a lot of battles including supporting the attack on Pearl Harbor and then a few other patrols before participating in the Guadalcanal campaign where she was sunk (details below).

For armament, she had 2 x 140mm 40 cal deck guns and 6 x 533mm torpedo tubes with 20 x Type 95 torpedos on board. It should be noted that the Type 95 torpedos were very good and very accurate. At the time, it was the fastest torpedo in use by any navy in World War II and it had the largest warhead of any submarine torpedo.

The I-1 Submarine & Yamamoto

There is some conflicting information about the importance and the role of the I-1 Submarine in finding and shooting down Yamamoto’s plane.

One version is that we directly found Yamamoto’s travel plans in the I-1 Submarine (which I think skips a few steps and isn’t exactly the case). Based on my research of the history, the information that they retrieved from the I-1 eventually enabled us to decipher some of the Japanese communications which then allowed us to know where he would be.

Either way, the I-1 Submarine was a critical component of Operation Vengeance to kill Yamamoto.

Below is a more detailed summary of the events:

On January 29, 1943 there was a battle between the New Zealand corvettes HMNZS Kiwi and HMNZS Moa (which we also dived on this trip) and the Japanese submarine I-1. It was a fierce battle with the Kiwi actually ramming the I-1 multiple times during the battle and they were close enough to exchange small arms fire between the ships.

During the battle, the officer of the I-1 tried to run the submarine aground which it did late at night when it hit a submerged reef about 300 yards from the beach. The ship was badly damaged, the rear part of the hull was flooded, and the Abandon Ship order was given right before the submarine sank below the surface. The Japanese managed to take ashore some of the codebooks, secret messages, etc but had left behind a case with past and future codes. Those documents were seized by the crews of the Kiwi and the Moa.

The Japanese realized that they had left behind a lot of valuable information and tried to destroy the submarine. They returned on a barge with depth charges and explosive charges which were strapped to the wreck in an attempt to detonate the torpedos. The explosion was too weak to destroy the I-1 but did cause a lot of damage. The Japanese then tried to destroy the I-1 by air attack on February 10th to the tune of eight dive-bombers and 42 Zeke fighters. Clearly there was information they were afraid of getting into Allied hands.

On Feb 11th, US Army personnel examined the wreck hoping to find more information and on the 12th a submarine salvage vessel from the US went to work and conducted diving operations. The divers salvaged documents from the conning tower which included code books and call signs. The code books were shuttled to CINCPAC for analysis. The Japanese had also dispatched the I-2 Submarine in an attempt to stop our salvage work and destroy the I-1 but it was depth-charged and chased off.

At one point, the I-2 Submarine returned and fired torpedos which salvage divers saw buzz by about 20 feet deep!

The salvage divers continued their work for Feb 20 – 23 and found numerous books, maps, sketches, code books, etc. which were then sent on for analysis and decryption which continued throughout April. Specifically, we recovered about 200,000 pages of the JN-25 code books, charts, and manuals.

On April 13, 1943 we intercepted encrypted information regarding Yamamoto’s upcoming tour of the Solomon Islands. Those messages were encrypted with the JN-25 cipher and were decrypted by FRUPAC (Fleet Radio Unit, Pacific — also called “Station Hypo”) and FRUMEL (Fleet Radio Unit, Melbourne) and contained information about which airplanes and fighter escorts would be involved in Yamamoto’s travel plans. After the loss of the I-1 Submarine, the Japanese had made some changes to the JN-25 cipher but we were still able to decrypt it after a LOT of effort.

We discovered that Yamamoto’s plans on the 18th of April had him in and around Bougainville Island and there would be two G4M twin-engine “Betty” bombers carrying passengers and six A6M “Zero” escorts.

Nimitz was made aware of the information and had to decide what path to follow and consulted with Edwin Layton who was an expert on Japan and Nimitz’s principal intelligence officer. Layton spoke excellent Japanese, personally knew Yamamoto, and had served as the naval attache in Tokyo prior to the war. Layton told Nimitz “Aside from the Emperor, probably no man in Japan is so important to civilian morale. And if he’s shot down, it would demoralize the fighting Navy.” Also, if they shot down Yamamoto, Nimitz knew it would be crippling to Japan but it was also a case of “the devil you know vs. the devil you don’t.”

The general opinion was that nobody had Yamamoto’s stature, experience, charisma, and ability to lead the IJN. Nimitz mad the decision to go ahead with Operation Vengeance and tasked Vice Admiral “Bull” Halsey with implementing it who then tasked Rear Admiral Marc Mitscher. Given the distance the P-38s would have to fly for the mission and to avoid detection, they needed extra drop tanks and they dispatched a squadron of 18 airplanes.

They eventually gave credit to the pilot Captain Rex T. Barber for having shot down Yamamoto’s plane. His plane came back with 104 holes in it! Note that there is some debate about who actually shot down the plane with some accounts crediting Captain Thomas G. Lanphier, Jr.

The summary is basically that they recovered important information about the JN-25 cipher from the I-1 Submarine which then allowed us to decrypt the messages about Yamamoto’s detailed itinerary which then allowed us to intercept his transport and shoot it down.

The Wreck

Obviously, given all the attempts to sink the submarine, it is not intact.

In 1972, an Australian treasure hunter tried to salvage valuable metals from the bow section of the wreck. They planted explosives in the bow section which then also set off one or two live torpedos inside thus causing a lot more damage to the bow. The stern is still somewhat intact but the bow is split open and splayed apart.

The wreck is on a slope with the stern in about 90 feet of water and the bow in about 45 feet of water making it a recreational dive.

The main deck gun was salvaged in 1968 and was brought to Auckland, New Zealand and is on display in the Torpedo Bay Navy Museum.

Below are some photos of the wreck. Almost entirely all photos I took were for the photogrammetry model so I had to pilfer a few out to use here so they aren’t really good compositions but they will give you and idea of the state of the wreck. In one, you can see a very small hole that leads from one compartment to another near the stern. I think that was for somebody to fit through. Crazy small.

You can see how “splayed out” a few sections of the wreck are with the stern being relatively intact. There is quite a bit of coral and sponge growth and the wreck is home to a lot of small fish.

Photogrammetry Model

I did one dive on the I-1 and figured I would need about an hour to get the photos for a photogrammetry model. It is a challenging wreck to build a model since it is split open and splayed out on the rocks as it gets shallower.

One problem I ran into is that I set my camera exposure based upon the conditions where I started at 90-ish feet deep on the stern area. As I then progressed up the wreck, as the water got shallower, the light became greater and the photos became over-exposed. At one point, I realized this was happening so I corrected the exposure. You can see this in the model whereby it gets lighter as you go up the wreck and then around midships it changes back to “normal.”

I did some post processing to try to make this better which helped and then made another run at it for some additional exposure changes. Just for comparison, the three versions are below. I used the “Re-Texturing” Method I posted about a while ago. I don’t really plan on spending more time on the model since it accurately conveys the shape of the wreck and will be useful in the current form for other divers if not artistically the best model.

Below are screenshots of the final model with a link to the on-line 3D model.

Links to both a Desktop (8M faces) and a Mobile friendly (800k faces) models are below.

Desktop Model

Mobile Version

Thanks

Kevin was my safety / buddy diver on the dive. Most other divers didn’t want to spend an hour on this wreck but Kevin agreed to help and make sure I didn’t run into trouble. Thanks, Kevin!

Neil Yates, as usual, was an incredible resource for the history of the wreck.

References

Cryptologic Foundation : FRUPAC Intercepted Yamamoto’s Encoded Itinerary

Cryptologic Foundation : Shutdown of Admiral Yamamoto

Naval History and Heritage Command : Operation Vengeance — Admiral Yamamoto Shot Down 18 April 1943

Station Hypo : How Battle at sea between New Zealand and Japan directly contributed to Shoot down of Admiral Yamamoto

National Archives : The Sinking of the Japanese Submarine I-1 off of Guadalcanal and the Recover of its Secret Documents

Combined Fleet : IJN Submarine I-1 Tabular Record of Movement

Wikipedia : Japanese I-1 Submarine

Wrecks of Iron Bottom Sound – MS Word Document by Neil Yates

We are the Mighty : Meet the intelligence officer who urged Nimitz to kill Yamamoto