Lost: How This American Submarine Took Down Imperial Japan's Aircraft Carrier Shokaku

By USN photo courtesy of http://ussubvetsofwwii.org. Photo i.d. courtesy of Chuck Norris ETC(SS), Retired. - http://www.navsource.org/archives/08/08244.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=28120
July 20, 2020 Topic: History Region: Asia Blog Brand: The Reboot Tags: World War IIAircraft CarriersMilitaryNaval WarfareWar

Lost: How This American Submarine Took Down Imperial Japan's Aircraft Carrier Shokaku

Submarines are deadly and stealthy killers.

Key Point: The carrier was struck by the American submarine USS Cavalla. By that point in the war, every loss was irreplaceable for Tokyo.

On the morning of June 13, 1944, the brilliant new aircraft carrier Taiho weighed anchor and slowly moved out of Tawi-Tawi anchorage in the Sulu archipelago in the southwestern Philippines. The vessel was serving as Vice Admiral Jisaburo Ozawa’s flagship. Departing with Taiho were the sister carriers Zuikaku and Shokaku and an assortment of cruisers and destroyers. The force headed northeast with other elements of Japan’s First Mobile Fleet. Two days later, on June 15, the Pacific island of Saipan in the Marianas was invaded by U.S. forces.

The loss of Saipan and adjoining islands in the Marianas chain would be a critical blow to the Japanese Empire, putting the home islands in range of heavy American bombers for the first time. To counter the American invasion, Ozawa devised Operation A-GO. The battle plan involved luring the American fleet into a position favorable for a Japanese attack. In addition to defending Saipan, Ozawa hoped to win a major victory over the U.S. Navy.

Ozawa’s Battle Plan in the Philippine Sea

Two serious issues hampered Ozawa’s ability carry out such a large scale fleet operation: his overall naval strength and the availability of fuel oil. A shadow of its former self, the Imperial Japanese Navy suffered heavy losses in battles at Midway, Guadalcanal, and elsewhere. However, using a nucleus of aircraft carriers and battleships, the admiral still felt he had enough forces available to make his plan work.  The shortage of fuel oil had been brought about by American submarine attacks on Japanese merchant shipping. As a result of the deficiency, Japanese ships were forced to use unprocessed Borneo oil. The low-grade fuel created dangerous fumes and damaged ship engines. In spite of these obstacles, Ozawa pressed forward with his plan.

The Japanese assembled nine aircraft carriers, five battleships, 10 heavy cruisers, two light cruisers, and a host of destroyers of the First Mobile Fleet to fight the Americans. Blocking Ozawa’s path to the embattled Marianas was the powerful U.S. Seventh Fleet under the command of Admiral Raymond A. Spruance. The American force included 15 aircraft carriers, seven modern battleships, and 956 carrier planes. The latter amounted to a roughly two to one advantage over the Japanese.

Wary of a potential Japanese move toward Saipan, Spruance was on guard for a big battle. He was concerned about the possibility that an enemy force might try to move around his ships and strike directly at the invasion fleet off Saipan. Spruance positioned his aircraft carriers to counter that threat.

With land bases in close proximity, Admiral Ozawa knew that the Americans would have to rely solely on carrier-based planes in the Saipan area. Accordingly, his battle plan relied on more than 500 land-based planes, operating from airfields on Guam, Yap, and Rota. Paying special attention to the American aircraft carriers, these land-based planes were to subject the enemy to a series of withering attacks prior to the arrival of the First Mobile Fleet. Ozawa would then use the greater range of his carrier planes to strike before the Americans could reply.

Forming up in the Philippines, the Japanese fleet refueled and moved east. Ozawa and his carriers departed the island group through the San Bernardino Strait, while the main force of battleships emerged from Surigao Strait farther south. Watchful American submarines guarding the Philippines spotted both forces and reported that two large groups of Japanese ships were heading east toward Saipan. The ensuing fight came to be known as the Battle of the Philippine Sea.

The Gato-Class Cavalla

Two days earlier and thousands of miles east at Pearl Harbor, Admiral Charles A. Lockwood studied a map of the Philippines Sea. The American command was on alert to a possible Japanese move to counter the Saipan invasion. As the leader of the American submarine force, Lockwood knew that some of his boats could play an important role in any battle.

The admiral laid an imaginary square over the Philippine Sea and believed that any Japanese surface force intending to challenge the American invasion fleet at Saipan would have to pass through the cube. He directed four submarines, Albacore, Finback, Bang, and Stingray, to patrol 30-mile radii at each corner of the square. He later shifted the shape 100 miles south based on intelligence received. One additional submarine, Cavalla, was not among the initial patrol group but would play a key role in the days ahead. She had already been at sea for weeks.

On her first war patrol, Cavalla departed Midway on June 4, 1944. After pulling away from the submarine tender Holland in the late afternoon, she slowly moved through the channel that led to the open sea. Two planes acting as her temporary escort flew above. After clearing the channel, she made for open sea on a westerly course.

Built by the Electric Boat Company in Groton, Connecticut, Cavalla was put into commission on February 29, 1944, by  Lt. Cmdr. Herman J. Kossler. One of 73 Gato-class submarines built during the war, she measured 311 feet in length and had a standard displacement of 1,526 tons. Her maximum surface speed of just over 20 knots fell to under nine knots when submerged. Main armament consisted of 10 torpedo tubes, six forward and four aft, with the ability to carry a total of 24 underwater missiles. A deck gun and a small assortment of light antiaircraft guns were available for use on the surface.

Patrol in the Pacific

After about two months of workups, test dives, and crew training, Cavalla made the long voyage to Pearl Harbor via the Panama Canal to join the war effort. She stayed at Pearl Harbor less than a month before venturing to Midway. For the boat’s first war patrol, Kossler was to prowl along the eastern Philippines.

On June 8, Kossler’s boat entered the 500-mile circle that surrounded Japanese-held Marcus Island. Lookouts kept a careful watch for enemy planes as Cavalla spent the majority of the daylight hours riding on the surface. One day later the submarine had an unusual encounter. “A slight impact was felt aft, and shortly afterwards a whale broached astern in what appeared to be a pool of blood,” Kossler explained. “No apparent change in the propeller beat nor any additional vibration has been noted, so it is not believed any damage was done to the propellers.” Cavalla continued her slow and largely uneventful voyage west.

During the early hours of June 14 the weather began to change. “Seas increasing, barometer dropping steadily,” Kossler recorded. “Took several [waves] over bridge and down conning tower hatch with no serious damage.” The commanding officer drafted a weather message for Pearl Harbor, but the radio operator was unable to send it because of the deteriorating conditions. Kossler decided to ride out the storm below the waves and submerged for much of the day, only surfacing mid-afternoon when the weather seemed to be slowly improving.

At about 7 pm Cavalla received a message about a surface contact reported by the submarine Flying Fish. Kossler immediately set a course for the location. The sea conditions had now improved substantially. “Storm completely past us,” he noted just before midnight. “Increased speed to sixteen knots to make up for lost time.”

In the early evening hours of June 15, Cavalla entered her assigned patrol area. Kossler began to patrol the probable route of the enemy reported by Flying Fish. She was not the only submarine on the hunt for the reported contact. The next morning the patrolling submarine sighted Pipefish. Kossler exchanged communications with the fellow American submarine, and it was decided that the two boats would make a coordinated search with each patrolling on one side of the target’s reported track. Having made no contact by 8 pm, Kossler ended the search and Cavalla continued within her assigned patrol area.

Several hours later, just before midnight, Cavalla sighted a convoy tentatively identified as two tankers and three escorts. It was unclear to Kossler if it was the same group that she and Pipefish had searched for earlier in the day. He spent the early morning of June 17 stalking the convoy but had to abandon his torpedo attack when a fast-moving Japanese destroyer forced him to go deep.

By the time Kossler was able to bring his boat to the surface, the convoy was gone. He unsuccessfully gave chase until receiving orders at about 5:30 pm to move to a different patrol area. Unknown to Cavalla’s commanding officer, the convoy that he had been chasing was one of two tanker trains servicing Admiral Ozawa’s battle force.

Sighting the Japanese Fleet

From the sighting reports transmitted by the submarines, Admiral Spruance knew for certain that the Japanese fleet was on the move. The reports provided the general positions of the Japanese ships, but his search planes were unable to pinpoint the precise location of the enemy carriers. The American reconnaissance planes did see Japanese float planes, indicating the enemy fleet could not be far off. Ozawa spent June 17 at sea moving east, trying to keep out of range of the American carrier planes until conditions were favorable to strike.