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IJN I-21

Ocean-Going Diesel-Electric Attack Submarine

Imperial Japan | 1941

"The Japanese submarine I-21 took part in the naval attack on Pearl Harbor, Hawaii in December of 1941."

Authored By: Staff Writer | Last Edited: 05/30/2017 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site; No A.I. was used in the generation of this content.
As an island nation, much of Japan's immediate success heading into World War 2 (1939-1945) lay in her naval power projection. A massive naval buildup ensured that the Japanese military maintained the capability to conquer its desired holdings across Asia and the Pacific before arranging its surprise attack on Pearl Harbor, Hawaii and attempting to isolate the Australian mainland. The Japanese invasion of the mainland proved a very real threat to everyday Australians in early 1942.

Part of the Japanese naval prowess in Pacific waters was the Type B1-class submarine which included "I-21". The vessel was laid down on January 7th, 1939 under the direction the of Kawasaki Shipyard of Kobe, Japan and she was launched to sea on February 24th, 1940. Completed on July 15th, 1941, the boat was commissioned into active service and was to become the more successful Japanese submarine in Australian territory, credited with sinking some 44,000 tons of Allied goods during her storied career.

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As built, I-21 displaced at 2,625 tons when surfaced and 3,715 tons when submerged. She featured a running length of 356.5 feet with a beam of 30.5 feet and a draught of 16.9 feet. Propulsion was by way of a diesel-electric arrangement to include 2 x diesel engines outputting at 12,400 horsepower for surface travel and electric motors outputting at 2,000 horsepower for undersea travel. As with other submarines of the period, I-21 was required to surface to release her dangerous CO2 gasses, take on new oxygen stores and recharge her battery cells. When surfaced, I-21 could make headway at nearly 24 knots and reached 8 knots underwater. Range was out to 14,000 nautical miles, approximately 26,000 kilometers, which gave her good range. As such, strong ocean-going capabilities were a must for the design. The hull was engineered to reach depths down to 330 feet. Her full crew complement numbered 94 officers and enlisted.

As an attack-minded submarine, I-21 was granted use of 6 x 533mm (21") torpedo tubes in bow-facing launchers set three to a hull side. Seventeen torpedo reloads were carried. Surface warfare was handled by 1 x 140mm deck gun which proved useful against surface threats and offshore targets.

Unlike other submarines of the war, the I-21 managed a reconnaissance-minded existence as well, granted facilities for launching a single Yokosuka E14Y series floatplane aircraft for scouting work. This provided I-21 with "over-the-horizon" detection facilities which could spot potential new targets for the vessel to engage or to supply Japanese naval command with the current status of Allied movements and positions.

I-21 was soon assigned to Sixth Fleet's Submarine Division 3 (Submarine Squadron 1). She then made up a portion of the Japanese fleet charged with the surprise attack on Pearl Harbor, Hawaii on December 7th, 1941 in an attempt to limit U.S. naval capabilities in the region - particularly its carrier group. With the operation underway, I-21 supplied much-required patrolling north of Oahu on the day of the attack. Following its partial success (the American carrier fleet was not in harbor as planned), I-21 moved on an unknown aircraft carrier and its support force but mechanical issues and aggressive Allied air cover prevented her from attacking her prey. I-21 was then reassigned to trade routes along the American West Coast where it could harass incoming and outgoing merchants at will.

On December 23rd, 1941, I-21 gained the advantage on a passing oil tanker named SS Montebello, sinking her with, amazingly, no loss to the Montebello's crew who managed to escape via lifeboats. On June 8th, 1942, I-21 was called to attack Newcastle dockyards of New South Wales with her deck weaponry. Though little damage was achieved, it showcased the vulnerability of key Australian shoreline positions now within reach of Japanese aggression. On January 17th, 1943, I-21 sunk the SS Kalingo which killed two - though some thirty-two managed to escape. Along her patrol routes during January, I-21 managed to engage several other vessels with good results, some damaged while others capitulated in full. On February 8th, 1943, I-21 engaged SS Iron Knight and sunk her with thirty-six of her crew lost to the sea. The Starr King then followed on February 11th, 1943 when she was sunk at the hands of the I-21 near Port Macquarie.

I-21 continued in its wartime patrols throughout most of 1943 until about November when it was believed that the vessel was located, targeted and sunk by American naval warplanes (from USS Chenango) on November 29th, 1943 off of Tarawa. While officially unconfirmed, this is believed to have been I-21 for the vessel and her crew were never heard from again.

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Power & Performance
Those special qualities that separate one sea-going vessel design from another. Performance specifications presented assume optimal operating conditions for IJN I-21.
2 x Diesel engines developing 12,400 horsepower with electric motors developing 2,000 horsepower; 2 x shafts.
24.0 kts
27.6 mph
Surface Speed
8.0 kts
9.2 mph
Submerged Speed
14,034 nm
16,150 miles | 25,991 km
The bow-to-stern, port-to-starboard physical qualities of IJN I-21.
356.6 ft
108.69 meters
O/A Length
30.5 ft
9.30 meters
16.1 ft
4.91 meters
Displacement (Submerged)
Available supported armament and special-mission equipment featured in the design of IJN I-21.
6 x 533mm (21") torpedo tubes (17 x reloads)
1 x 140mm deck gun
Air Arm
Available supported fixed-wing / rotary-wing aircraft featured in the design of IJN I-21.
1 x Yokosuka E14Y floatplane
Ships-in-Class (20)
Notable series variants as part of the IJN I-21 family line as relating to the Type B1 group.
I-15; I-17; I-19; I-21; I-23; I-25; I-26; I-27; I-28; I-29; I-30; I-31; I-32; I-33; I-34; I-35; I-36; I-37; I-38; I-39
Global operator(s) of the IJN I-21. Nations are displayed by flag, each linked to their respective national naval warfare listing.
National flag of modern Japan

[ Imperial Japan ]
1 / 1
Image of the IJN I-21
Image courtesy of the Public Domain.

Mission Roles
Some designs are single-minded in their approach while others offer a more versatile solution to seaborne requirements.
Some designs stand the test of time while others are doomed to never advance beyond the drawing board; let history be their judge.
Going Further...
IJN I-21 Ocean-Going Diesel-Electric Attack Submarine appears in the following collections:
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