Station HYPO

How did a battle at sea between two New Zealand corvettes and a Japanese submarine directly contribute to the shoot down of Admiral Yamamoto 79 days later?
74 years ago today on January 29, 1943 two New Zealand corvettes – HMNZS Kiwi and HMNZS Moa located and rammed the I-1, a Japanese submarine operating in the southwestern Pacific.  Gunfire was exchanged and eventually the I-1 hit a reef and was disabled near Kamimbo Bay on Guadalcanal. Seized from the submarine were approximately 200,000 JN-25 codebooks which proved very valuable in that they gave the Allies advance warning of raids and other details of Japanese naval force movements.

Background:

The I1 was a Japanese J1 type large cruiser Submarine that was built by Kawasaki. After 18 months of construction on the March 10, 1926 the submarine was handed over to the Imperial Japanese Navy and was placed under the command of Captain Lieutenant Commander Eiichi Sakamoto and began her service.

Throughout the World War II the I1 was part of many important battles and played a significant role in history.

During the attack on Pearl Harbor, the I1 stayed outside in the channel of Kauai ready to intercept and sink any escaping ships. She continued on and was part of two more attacks in December 1941.  She was first part of the bombardment of Kahului, Maui and later that month shelled the Harbor of Hilo, Hawaii. On March 3, 1942, the I1 intercepted the Dutch steamer Siantar, which was on its way to Australia and sunk it.

On August 1, 1942 her role changed. The I1 gave up her 140mm gun to make more room, which was later used for a 14m barge.  From now on her main role was as a cargo ship to evacuate troops, transport them between Islands and resupply them with food.

On January 24, 1943 the I1 left the Island Rabaul with supplies for the Japanese troops on Guadalcanal. During the night of January 29, 1943, while the I1 was submerged in front of Guadalcanal the two New Zealand corvettes HMNZS Kiwi and her sister ship the HMNZS Moa picked up her phosphorescent.  The Kiwi lunched six depth charges and followed with six more, forcing the submarine to surface.  The I1 ascended and tried to boat off while exchanging gun fire with both ships.  The Kiwi rammed the I1 damaging itself and the I1.  The Moa took over the fight and chased the submarine further towards shore while firing its 4” gun.  Two hours after the initial attack, the I1 unable to escape stranded in shallow waters on a reef.  Due to the JN-25 code books the I1 carried the Japanese Soldiers on board tried to blow up the submarine while abandoning it.  Fortunately, the Japanese failed total destruction!

Immediately after the sinking, the Allies started to explore the wrecked I1 and recovered about 200.000 pages of the JN-25 code books, charts and manuals. The code books included updated Japanese Navy codes which were immediately send to Station Hypo at Pearl Harbor where the code breakers used them to exploit the Imperial Japanese Navy communication messages. This eventually led to exploitation of Admiral Yamamoto’s flight itinerary and shoot down of his aircraft on April 18, 1943.

Source: cryptologicfoundation.org