The four torpedoes did not destroy Shinano because they penetrated some magical weak point in a ship that should have been invulnerable.
They doomed her because the water did not remain where the torpedoes put it.
That distinction explains almost everything.
Shinano had inherited an enormous hull from the Yamato-class battleship program. She carried extraordinary armor, extensive underwater protection, a heavily protected flight deck, and enough displacement to make nearly every other aircraft carrier in the world look small. Yet when USS Archerfish struck her on the starboard side early on November 29, 1944, the carrier was not a fully completed warship with a trained crew operating a tested damage-control system.
She was an unfinished ship being hurried from Yokosuka to Kure.
Some watertight doors had not been installed. Openings cut through bulkheads for cables and pipes had not all been sealed. Many hatches remained open because workers still needed access to unfinished spaces. Important pumping equipment was incomplete. Large numbers of sailors had little experience working together, while civilian yard workers and technicians were still aboard. Then, after the torpedoes hit, Captain Toshio Abe initially kept the giant carrier moving at substantial speed, allowing seawater to continue forcing its way through damaged areas and into spaces that should have remained dry.
The result was not an instant structural failure.
Shinano survived the explosions.
She survived the first hour.
She continued steaming for dozens of miles.
What she could not survive was progressive flooding.
Water crossed boundaries that were supposed to stop it. The growing list reduced the effectiveness of damage-control efforts. Counter-flooding consumed additional reserve buoyancy. Machinery spaces became unusable. Electrical and pumping capability deteriorated. Eventually the ship that had seemed too large to sink became too flooded to save.
That was the real weakness Archerfish found.
Not inadequate size.
Not insufficient armor.
Incomplete watertight integrity.
The biggest carrier in the world went to sea before she was truly ready to be a ship.
Shinano had originally been designed to fight as something completely different
Shinano did not begin as an aircraft carrier.
She was laid down at Yokosuka as the third member of the Yamato class, the same family that produced Yamato and Musashi, the largest and most heavily armed battleships ever completed.
The original concept reflected Japanese naval thinking from the years before the Pacific War. Japan expected that it might eventually have to fight the numerically larger United States Navy. Because Japan could not easily match American industrial production ship for ship, designers attempted to gain qualitative superiority.
The Yamato class represented the extreme form of that approach.
Instead of building battleships comparable to American designs, Japan built vessels intended to overpower them individually. Yamato and Musashi carried 18.1-inch guns, enormous armor protection, and displacement far beyond most contemporary battleships.
Shinano was supposed to become another member of that force.
Construction began before Japan entered the Pacific War, but work slowed after the war began as Japanese shipbuilding priorities changed. Then came the Battle of Midway in June 1942.
Japan lost four fleet carriers in a single operation.
Akagi, Kaga, Soryu, and Hiryu disappeared.
The loss did more than reduce Japanese carrier strength. It demonstrated that aircraft carriers had become central to naval warfare and exposed serious weaknesses in Japanese carrier protection, firefighting, aviation-fuel systems, and damage control.
The incomplete Shinano suddenly represented something more useful than another battleship.
Japan already had Yamato and Musashi.
It desperately needed carrier capability.
The decision was therefore made to convert Shinano.
The transformation was enormous.
Below the main deck, much of the ship retained the fundamental structure of a Yamato-class hull. Above it, designers had to create a carrier with a flight deck, hangars, elevators, aviation workshops, fuel systems, aircraft-ordnance storage, and the other facilities necessary for air operations.
The conversion became almost equivalent to designing a new ship around an existing armored foundation. A contemporary postwar analysis by U.S. Naval Institute described Shinano as an unusual compromise whose massive battleship structure had to be combined with an aircraft carrier superstructure.
The result was unlike the large American fleet carriers being built at the same time.
Shinano was enormous, but she was not intended simply to carry the largest possible combat air group.
Her role evolved toward that of a heavily protected support carrier. She could carry her own aircraft, but she was also expected to transport replacement aircraft, fuel, weapons, and supplies that could support other carriers.
The ship's size therefore did not translate directly into an equally enormous operational air group.
Postwar Japanese carrier data published by the U.S. Naval Institute gives Shinano a standard displacement around 62,000 tons, a trial displacement of 68,059 tons, and a maximum speed of about 27.8 knots. U.S. Navy historical summaries often describe her as approximately 72,000 tons, reflecting her much heavier displacement under other loading conditions. Whatever measurement is used, she was far larger than any completed carrier then in service.
Her dimensions were equally extraordinary.
The flight deck extended more than 800 feet.
Armor accounted for an enormous portion of her weight.
One U.S. Naval Institute analysis estimated roughly 17,700 tons of armor, close to a third of her standard displacement.
This helps explain why Captain Abe and others could believe the ship would absorb serious damage.
Shinano did not look fragile.
She had been built around a hull conceived to survive punishment in battleship combat.
Yet armor and survivability are not the same thing.
A ship can possess thick armor while remaining vulnerable to flooding if the internal boundaries behind that armor are compromised.
That difference would become fatal.
Midway influenced the carrier built above the Yamato hull
Japanese designers attempted to incorporate lessons learned from earlier carrier losses.
One of the worst dangers aboard an aircraft carrier was aviation gasoline. Fuel vapors could spread through enclosed compartments and transform relatively localized damage into catastrophic fires and explosions.
The loss of Japanese carriers at Midway had shown how dangerous bombs, armed aircraft, aviation fuel, and enclosed hangar spaces could become when combined.
Shinano therefore received considerable protection against these threats.
Her flight deck was armored.
Her aviation ordnance spaces were heavily protected.
Ventilation ducts received armor in an attempt to reduce the spread of blast, flame, and dangerous fumes.
Some aviation-fuel spaces received additional protection inspired by lessons from the loss of Taiho, another Japanese carrier sunk by an American submarine in June 1944 after a torpedo hit led to aviation-fuel vapor problems and a devastating internal explosion.
The designers were trying to build a carrier that could absorb damage better than earlier Japanese carriers.
This is one of the great ironies of Shinano.
Many of the protective concepts were sensible.
The ship was not doomed because Japanese engineers had simply ignored wartime experience.
In several areas, they had learned from it.
The problem was that a theoretically well-protected ship still needed to be completed properly.
An armored ventilation duct cannot compensate for an unsealed penetration through a watertight bulkhead.
An armored flight deck cannot stop seawater moving through an unfinished opening deep inside the hull.
A torpedo bulge cannot preserve buoyancy if flooding spreads far beyond the compartments the protective system was designed to sacrifice.
Shinano contained impressive engineering features, but on November 28 those features existed inside an unfinished ship.
That was the contradiction Archerfish unknowingly attacked.
By late 1944, Japan no longer had the luxury of finishing Shinano slowly
The strategic situation had changed catastrophically for Japan.
The Imperial Japanese Navy had lost much of the carrier force that had attacked Pearl Harbor.
The Solomon Islands campaign had consumed ships, aircraft, and experienced aircrew.
American submarines were strangling Japanese merchant shipping.
The Philippine Sea in June 1944 had devastated Japanese carrier aviation.
Leyte Gulf in October further reduced the effective strength of the fleet.
Meanwhile, American forces had captured the Mariana Islands.
This placed Japan within range of the new B-29 Superfortress operating from Pacific bases.
Shinano was being completed at Yokosuka, one of Japan's most important naval construction facilities, and secrecy surrounding the ship had been extreme.
The secrecy did not last.