Type B1 I-Boat | HistoryNet MENU
Despite its impressive range, the Type B1 I-boat's size, slow dive time, relatively shallow operating depth and sluggish underwater performance made it vulnerable to improving Allied anti-submarine technology.

Type B1 I-Boat

By Jon Guttman
8/29/2016 • Military History, MH Tools

Length: 356 feet 6 inches
Beam: 30 feet 6 inches
Height: 31 feet 6 inches
Draft: 16 feet 9 inches
Displacement: 2,584 tons surfaced, 3,654 tons submerged
Maximum speed: 23.5 knots surfaced, 8 knots submerged
Range: 14,000 nautical miles surfaced, 96 miles submerged
Power: Two 12,400 hp diesel engines, two 2,000 hp electric motors
Standard armament: One 5.5 mm deck gun aft, Type 96 dual 25 mm guns on conning tower platform, six bow torpedo tubes, 17 torpedoes
Aircraft carried: One Yokosuka E14Y1 floatplane
Crew: 94

No naval power produced as wide a variety of submarine types—ranging from suicidal Kaiten (“Return to Heaven”) manned torpedoes to 400-foot giants capable of launching floatplane bombers—nor put them to as many uses as did Japan in World War II. For all that, due to Japan’s insistence on using its force as a surface fleet adjunct with commerce raiding secondary, few submarine fleets accomplished so little for such a heavy expenditure of ingenuity, resources and lives.

Arguably the most successful (relatively speaking) of Japan’s many submarine variants was the Type B1 I-boat, one in a succession of long-range cruiser subs meant to support the fleet in its “decisive battle” against the Allies. Besides packing a 5.5-inch deck gun, dual 25 mm anti-aircraft guns and 17 of the 21-inch kerosene/oxygen-propelled Type 95—the fastest torpedo in use by any navy—the B1 boasted a streamlined hangar that housed a catapult-launched Yokosuka E14Y1 floatplane. In 1944 I-36 was modified to carry six Kaiten, I-37 four. Despite its impressive range, the I-boat’s size, slow dive time, relatively shallow operating depth and sluggish underwater performance made it vulnerable to rapidly improving Allied anti-submarine radar, sonar and weaponry.

The 20 Type B1s completed between the fall of 1940 and spring of 1943 saw widespread service, including attacks on the U.S. West Coast. B1s accounted for 50 merchant and auxiliary vessels (11 of them by I-21). In 1942 I-26 crippled the U.S. aircraft carrier Saratoga and sank the light cruiser Juneau. On September 15 of that year I-19 sank the carrier Wasp and destroyer O’Brien and damaged the battleship North Carolina with a single torpedo spread. The I-boats were also used—make that misused—as transports, notably on three special cargo runs to Europe. Only I-36 survived the war. MH

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