The most numerous of all the World War II Imperial Japanese Navy (IJN) submarine classes was the B1-Type, nicknamed the "I-15" class after the first boat launched. Twenty were produced and the I-15 was commissioned on September 30, 1940 at the Kure Navy Yard, Japan. Commander Nobuo Ishikawa was assigned as her commanding officer and remained so throughout her short war time career.
The B1 was 356.5 ft (108.7m) long, her beam was 30ft 5 in (9.3m) wide and the boat's draft was 16 ft 8 in (5.1m). In comparison to the 1940-era Gato American submarine class, the B1 was 45ft longer and had a 3,000 nm greater range as well as faster cruising speeds. Because of the vastness of the Pacific, Japan built boats for extreme range and size, many of which were capable of cruises lasting more than 100 days. In comparison, the Gato class had a maximum of a 75-day patrol.
The B1 (or I-15) had two diesel engines producing 12,400 horsepower (9,200kw) for running on the surface and to charge her batteries for submerged running there was an electric motor producing 2,000 horsepower (1,500kw). This power allowed the boat to operate at roughly 23.5 knots (44km/h) along the surface and 8 knots (15km/h) when submerged. At 16 knots (30km/h) she could cruise 14,000 nm (26,000 km). She required 94 officers and enlisted men and had an additional crew of two pilots that were needed to operate her onboard Yokosuka E14Y "Glen" seaplane used in reconnaissance duties. Some of the class carried this single seaplane, located in a hangar just in front of the conning tower, and was launched by a catapult. One Hundred Twenty-Six of the IJN Type 0 Small Reconnaissance Seaplane (Japanese designation of the aircraft) were built. The E14Y holds the distinction of being the only Japanese aircraft to drop bombs on the United States mainland in all of World War II, this occurring on September 9th, 1942 off the coast of Oregon. Besides a forest fire from the dropped incendiary bombs, no one was hurt and no property was damaged.
The I-15 is assigned to the Fleet (Submarines) under Rear Admiral Sato Tsutomu's command with Vice Admiral Shimizu Mitsumi's Sixth Fleet's Advance Expeditionary Fleet. Under "Operation Z" the I-15 was sent on her first war patrol off the north coast of Oahu on December 7,1941. She received the code signal "Climb Mt. Niitaka", a code indicating war would begin on 12/8/1941 Japan time. Her mission was to sink any warships that came into her patrol area from Pearl Harbor after the initial attack had begun. No ships appeared in her vicinity and I-15 was ordered to return to Japan. On 12/10 she received a signal that an American fleet was sighted nearby, this fleet including the USS Lexington and her escort. Admiral Shimizu ordered six boats to sink the carrier including the I-15. The American fleet was not sighted and she was ordered to proceed to the west coast to attack American ships.
A plan to shell San Francisco was reviewed and ordered for December 25th, 1941. Second thoughts stopped the attack due to concerns of retaliation for possible civilian deaths. She was then assigned to Sub Ron 1 and was operating around Kwajalein supporting a planned flying boat attack on Pearl Harbor named "Operation K1". The I subs were spotted by a ship from Halsey's Task Force 8 from USS Enterprise. I-15 was not hit but a number of Japanese ships were damaged including a submarine depot ship. After the attack, I-15 was ordered to attack the carrier but was recalled to support the K1 operation instead, this on February 5th, 1942. Five submarines equipped with floatplanes were used in support of the attack. The planes were removed and fitted with extra fuel tanks for the long flight. The target at Pearl was the ship repair docks that were missed on the opening December 7th, 1941 attack .
On February 20th, 1942 the USS Lexington was spotted by a Kawanishi flying boat and, with the element of surprise lost, the attack on Rabaul was cancelled. I-15 was diverted to intercept the carrier and sink her but the carrier was not found. Again she returned to aid the operation to attack Pearl Harbor and proceeded to French Frigate Shoals. The Kawanishi flying boats arrived and were refueled. The mission was a failure as cloud cover over the harbor caused the flying boats to miss Pearl and the planes dropped the bombs on Honolulu instead. I-15 was sent to Yokosuka for a scheduled overhaul.
In May of 1942 Commander Nobuo Ishikawa took I-15 on her second war patrol. Proceeding to Adak Alaska with the Northern Force she patrolled the Aleutians for allied shipping. Operation "AL" was the deception preceding the attack on Midway Island. I-15 used her float plane to scout and she carried out periscope observations of Kodiak Island. Twenty ships under Admiral Boshiro landed troops on Attu Island without opposition. I-15 was relieved and was returned to Yokosuka.
In August of 1942 the Japanese were building an aircraft landing field on Guadalcanal. As a result, the American "Operation Watchtower" was hastily conceived for the Invasion of Guadalcanal in the British Solomon Islands. The American operation on Guadalcanal opened up a seven month campaign to take the island.
In September of 1942, during the Battle of the Eastern Solomons, Commander Ishikawa sighted an enemy task force. He recognized the USS Enterprise, the battleship USS North Carolina (BB-55), two cruisers and five destroyers in the group. Commander Ishikawa wanted to conduct a coordinated attack and tried to contact the I-17 patrolling nearby with sonar using Morse code, but the I-17 did not answer the calls. Two of the screening destroyers detected the I-17 and subsequently dropped depth charges. After the attack ended, the I-15 surfaced and maintained contact with the enemy force. Rear Admiral Yamazaki the ordered the I-15 and I-17 to attack the task force. Up to this time, the I-15 had not sunk any allied ships and the crew was eager to sink an American war ship. On October 24th, 1942, Commander Ishikawa sighted another major enemy task force heading South from Santa Cruz. The I-15 may have also been sighted by the Americans. While taking evasive action to avoid the submarine contact, the USS South Dakota and the USS Mahan (DD-364) collided causing heavy damage to one another. I-15 did not attack.
The I-15 was recharging her batteries on the surface when Lieutenant Commander John Tennent's USS Southard (DMS-10), a fast minesweeper, made contact and opened fire with her 4.5-inch main gun. The I-15 submerged and moved in to attack. Ishikawa fired two torpedoes at the minesweeper which both missed their target. DMS-10 acquired the I-15 on sonar and dropped six depth charges in patterns over the next few hours. Now damaged and low on air, the I-15 needed to surface regardless of the situation topside. The USS Southard opened fire from about a mile away with a salvo hitting the I-15's conning tower. The direct hit forced the I-15 to sinks by the bow with all hands on board. She was presumed lost in the Guadalcanal area by the INJ and on December 24th, 1942, her name was officially struck from the naval list. Commander Ishikawa was promoted Captain posthumously in a short career with no recorded enemy ships sunk.
Japan had the most diverse submarine fleet in World War II which is not a surprise considering her island nation status. She built the largest submarines of the day with some over 5,000 tons and over 400 feet in length. She produced midget subs and manned torpedoes as well. Many of the boats could carry aircraft and some even could field multiple bombers. The torpedoes carried on most were the Type 95 series that used kerosene instead of the alcohol used in Allied torpedoes. The IJN fuel mix gave the Type 95 three times the range of their Allied counterparts.
The IJN war plan for boats was to concentrate on warships and not so much merchant vessels as the Americans targeted. So instead of attacking slow unarmed merchant ships Japanese captains attempted to cross swords with well-armed fast surface warships. In comparison, Japan sunk 184 merchant ships during the war while Germany sank 2,840 cargo ships. The US accounted for 1,079 merchant vessels. One would have concluded the IJN should have deployed her submarine fleet off the West Coast of the United States, the Panama Canal and or near Hawaii and India for maximum effect. Instead her boats lurked waiting for the major battleship or aircraft carrier to cross into her patrol zone.
To add insult to injury to the strategy, many boats were also used by the Imperial Japanese Army to resupply island garrisons instead of being used for attack missions. Large submarines were easy to see and were slow to submerge, providing prime targets for enemy warships and dive bombers alike. As a result, many of these Japanese vessels were sunk in these inglorious roles. As the war progressed, losses mounted due to the improved anti-submarine warfare tactics and strategies developed by the Allies. Radar and underwater sonobuoys were consistently dropped by Allied aircraft resulting in the sinking of IJN boats with newly developed acoustic homing torpedoes. The IJN did not develop counter measures or new technological advances in an effort to help protect its submarine force. With mounting losses, moral inevitably declined in the IJN submarine service. It became apparent that boats on 40-day patrols reporting seeing no enemy ships was not due to the lack of allied targets, but the knowledge attacking them was sheer suicide at this point. The IJN submarine service was not prepared, however, to go the way of their Kamikaze brethren in the skies.