On November 1, 1944, an American B-29 reconnaissance flight photographed the huge carrier while construction continued. U.S. Navy historical material notes that the ship had remained unknown to American intelligence until this photographic discovery.
The photograph changed the risk calculation.
A vessel that had been hidden inside the Japanese naval construction program could now become a bombing target.
Japanese planners accelerated the schedule.
Shinano was commissioned on November 19, 1944, with Captain Toshio Abe in command.
Only nine days later, she left Yokosuka.
A ship can be commissioned before every final fitting detail is complete, but Shinano's unfinished condition was far more serious than cosmetic work.
She still needed substantial internal completion.
Her crew had not received enough time to become an experienced damage-control organization.
Many systems had not been fully tested.
Shipyard personnel remained aboard because work had to continue during or after the transfer.
The destination was Kure, where completion could continue under conditions thought to be safer.
The decision had a logical wartime purpose.
Leaving Yokosuka reduced the risk of losing Shinano to an air raid while she sat almost helpless in port.
Yet moving her introduced a different danger.
An unfinished ship that remained inside a protected naval base could tolerate open hatches, incomplete piping, unfinished bulkhead penetrations, and inexperienced crew organization.
A ship crossing submarine-patrolled waters could not.
The measures that accelerated Shinano's escape from one threat exposed her to another.
The carrier left Yokosuka carrying more than an unfinished crew
On November 28, Shinano departed Yokosuka bound for Kure.
Three destroyers accompanied her as escorts.
The carrier also carried wartime cargo, including 50 Ohka rocket-propelled suicide attack aircraft and six Shinyo explosive motorboats, according to U.S. Navy historical accounts.
The presence of those weapons demonstrates that the voyage was not a simple shipyard transfer.
Japan was trying to turn an unfinished carrier into a useful component of a rapidly deteriorating war effort.
Captain Abe commanded a ship that looked overwhelming from the outside.
A submarine commander viewing Shinano in darkness would see a gigantic silhouette surrounded by destroyers.
He could not see the unfinished internal spaces.
He could not see the unsealed openings.
He could not measure the experience level of the damage-control teams.
He could not know which pumps were missing.
The ship's greatest weaknesses were hidden inside the hull.
USS Archerfish would discover them without ever knowing they existed.
Archerfish was not originally waiting for Shinano
USS Archerfish, a Balao-class submarine under Commander Joseph F. Enright, had not been sent specifically to sink Japan's new carrier.
Her primary mission in the area involved lifeguard duty for B-29 crews that might have to ditch during air operations against Japan.
On November 28, Archerfish learned that no B-29 raids were scheduled for the day.
That freed Enright to hunt.
At dusk, lookouts spotted a large Japanese vessel leaving the Tokyo Bay region with escorts.
At first, the contact looked like a tanker.
Night identification at sea was difficult. Distance distorted scale, and the Americans had no reason to expect that a carrier larger than anything they knew Japan possessed would suddenly appear in front of them.
As Archerfish maintained contact, Enright began to understand that the target was something far more important.
The submarine remained on the surface during much of the chase because surface speed exceeded submerged speed. A Balao-class submarine could make more than 20 knots on the surface under favorable conditions, but only a fraction of that submerged.
Shinano was capable of nearly 28 knots in ideal conditions.
The American submarine therefore faced a serious positional problem.
Enright could not simply dive, chase the target underwater, and expect to catch it.
He needed to get ahead.
For more than six hours, Archerfish followed and maneuvered while Shinano and her escorts continued through the night.
The chase was not a simple line.
Enright had to anticipate Japanese course changes.
He needed to remain close enough to maintain contact without allowing the destroyers to force him away.
He also had to decide whether to attack on the surface or submerge before firing.
A night surface attack could offer speed and visibility advantages, but the escort destroyers made it dangerous.
Enright rejected the surface option.
Getting caught by destroyer gunfire at close range could destroy Archerfish before she fired.
He wanted a submerged torpedo attack.
That required reaching a position ahead of Shinano's path.
Fortune eventually helped him.
The Japanese carrier altered course in a way that improved Archerfish's geometry.
The target was coming toward the submarine's firing position.
After hours of pursuit, the enormous carrier finally began solving Enright's tactical problem for him.
Captain Abe had reasons to keep moving quickly
Shinano's movement through the night reflected another strategic calculation.
Abe understood the danger from submarines.
Speed provided protection.
A submarine attempting a submerged interception had limited mobility. A fast surface ship could reduce the amount of time it remained in a particular threat area and make firing solutions more difficult.
The destroyers also offered protection.
Yet the escort was not enough to prevent Archerfish from maintaining contact.
There were indications of an American submarine nearby, but Shinano continued toward Kure.
The Japanese captain had to balance competing dangers.
Slowing down made the ship easier to intercept.
Changing course too frequently could delay the voyage.
Turning toward possible contacts could disrupt the formation.
The unfinished carrier needed to reach Kure.
The longer she remained exposed, the longer American submarines had to find her.
This helps explain why speed remained important to Abe even after the torpedoes eventually struck.
His instinct was to keep the ship moving toward safety.
Under normal circumstances, that decision might make sense.
On Shinano, it helped progressive flooding.
Enright prepared the torpedoes for a giant target
By the early morning of November 29, Archerfish had achieved the position Enright wanted.
The range was about 1,400 yards.