Both Japanese and American carriers shared the many of the same vulnerabilities that were intrinsic to aircraft carriers. One useful analogy for early war carriers is that they were often like boxers with a strong hook but glass jaws; whoever hit first was likely to knock the other one out (either sinking or ending air operations). As the war progressed though, Japanese CVs proved to be far more vulnerable to damage than their American counterparts. There was no single factor behind this development, but rather a constellation of several factors that worked against the Japanese.

Factor One: Design: Aircraft carriers posed a particular problem for naval architects in the interwar period. These ships needed to be long enough for flight operations, carry a useful load of aircraft and their stores, and be fast enough to launch their aircraft. Combining all three of these factors was difficult and necessitated design choices that would have unforeseen ramifications in wartime. in hindsight, most of the Japanese solutions to these problems proved quite disastrous.

In the case of length, all large ships need a strength deck to protect them from the motions of the ocean. This was not much of a design problem for conventional ships, but carriers were a different matter. Naval architects usually have two choices for a carrier strength deck, the hangar or the flight deck. The Americans opted for a strength deck on a single hangar deck after experiments while the British preferred a flight deck arrangement. The Japanese instead had two hangars for their fleet CVs with the strength deck atop the lower hangar. This decision meant that Japanese hangars were rather narrow and enclosed (which was also a desirable trait for operating in Japanese home waters which often had inclement weather), unlike American CVs which were open. This design decision ensured that when bomb hits penetrated the unarmored flight deck and detonated inside this enclosed space, the explosions would rip through the ship and maximize damage. This was famously seen in Midway where after hits by dive bombers, Akagi, Soryu, and Kaga were immediately knocked out by bombs that exploded in the upper and lower hangar decks in the span of a few minutes. The magnified explosions damaged not only the hangars, but also a variety of subsystems like steampipes and generators.

One of the stock images of Japanese CV losses is exploding fuel and fires. The reason behind this archetype is that when Japanese avgas tanks were integral components of the ships' hull. Unlike British or American designs, which tended to use more cumbersome but safer means of storage, this integral design meant that avgas fuel lines would rupture if the ship were to experience a sudden shock. The Japanese had surrounded these areas with inert CO2, but the gas often leaked out and was not a solution to the issue of fire. In the aftermath of Midway, the Japanese recognized this problem, but could not find an adequate way to rectify it. EDIT The two Shokakus Zuikaku had concrete surrounding the voids around the avgas storage in the aftermath of the Philippine Sea, but this did not solve the problem. This avgas design was part of the reason for the loss of Taiho as Albacore's single torpedo hit her in the right spot to rupture her avgas. The avgas location of avgas tanks also played a significant role in the loss of Shokaku and Hiyo, with the latter ship's survivors confusing what was likely an avgas explosion with a submarine torpedo.

Factor Two: Damage Control: The loss of Taiho exemplified another problem of the IJN: damage control. The torpedo hit on Taiho led to her elevator well filling up with avgas. While this was an unenviable situation for the ship to be in, her DC officer made a number of poor choices that maximized the damage. Japanese DC suffered from both problems of personnel and equipment, and this played a role in the loss of Taiho and other CVs.

In the former case, the IJN tended to make DC the responsibility of specialists inside the engineering divisions. In the carriers, the IJN divided damage control between the engineering officer and the deck officer who were respectively responsible for DC bellow decks and flight decks. This meant only a very narrow clique of officers had knowledge or insight into how to contain damage once it had broken out. On top of this, DC training often interfered with their other duties, so that it tended to receive lower priority. The bifurcated command arrangement also ensured DC efforts could be quite uncoordinated. Japanese DC officers thus had to order around personnel with little experience or knowledge of the ships' DC equipment. Furthermore, a timely hit could knock out the Japanese DC personnel, which is what happened on Kaga when the immediate death of her two senior engineer officers meant her DC efforts were without a head. In contrast, the USN made DC a responsibility that filtered through the ranks. While Japanese DC certainly got better throughout the war, it never reached the proficiency of their American counterparts. Most of the IJN's DC was learned on the job, so that experienced ships like the two Shokakus could deal with damage better, but newer ships found the learning curve quite steep.

The losses at Midway highlighted the inadequacy of Japanese firefighting equipment. The IJN had relied upon fire curtains and CO2 extinguishers to contain fires, but they discovered that blast damages rendered these items useless. The Japanese immediately after Midway began experiments with foam sprays and fire-retardant paint and retrofitted these systems into their surviving carriers and new construction. While these measures helped the Japanese CVs in the later carrier battles, they did not eliminate the various problems.

Factor Three: Expedient Designs: One problem that faced the Japanese war effort was that despite their optimistic forecasts, Japanese shipyards underperformed and could not deliver ships in a timely manner or in the quantities needed. This meant that the Japanese had to rely increasingly upon carrier conversions to make good their losses.This meant that often a carrier design became grafted onto a hull not really meant for the rigors of carrier operations. While this sometimes led to useful ships like the Hiyo-class (which were originally ocean liners), these ships were often quite unarmored and vulnerable.

Added to this problem is that Japanese shipyards tended to cut corners by mid-war in order to replace the escalating losses. This meant that various protection measures were often half-finished and could not work as designed. Shinano's loss was in no small measure due to these time-saving expedients as her watertight compartments proved to be anything but.

Another factor on the losses of large IJN ships was that by the late war, the Japanese began to impress these ships as high-speed cargo vessels carrying troops equipment from the Home Islands and returning with strategic materials from SE Asia. The storage of this excess equipment hindered the ships' operation and added a further explosive risk. This was clear in the catastrophic loss of Unryu in 1944 where the storage of Okha suicide bombs meant for Luzon greatly added to the conflagration that consumed her.

Factor Four: The Americans: Obviously, IJN ships did not damage themselves, but when considering damage, the Japanese underestimated developments in American weapon design. The advent of torpex and other innovations made American munitions far more deadly pound for pound than interwar amatol or TNT. Added to this, it was readily apparent that by 1943 the American had won the production battle against Japan. Not only were the Americans launching more carriers, they were also fielding a new generation of aircraft that were more sophisticated than their 1941 counterparts. The increasing masses of American aircraft by 1943 also ensured that Japanese ships were subject to increasing air attack. The poor state of IJN AA defenses meant that the US pilots had a much better chance of hitting their opponents. Once the USN had ironed out the bugs in its torpedoes, USN submarines became an increasing danger by late 1943. The compromised Japanese codes also allowed submarines to stalk zones in which high priority targets like carriers operated.

One important thing to realize is that the Americans were not immune to these problems. For example, the l1942 loss of Lexington in a number of ways mirrored Taiho's loss. Likewise, in 1945 Franklin was gutted by fire from two bombs, much like Akagi. American DC was much better organized and prepared than IJN's, but in USN personnel did not fight under the same conditions as their Japanese counterparts as the war dragged on. The increased effectiveness of USN CAP and AA meant that it was much more difficult for the Japanese to achieve hits against their foe. The Japanese solution to this problem, the Kamikaze, often attacked American ships at oblique angles that lessened the damage wrought by their bombs. In short, the American DC never faced the type of challenges that the Japanese faced.

Sources

Ahlberg, Lars, Hans Lengerer, and Leszek Erenfeicht. Taihō. Vol. 1 Gdańsk: AJ-Press, 2004.

Parshall, Jonathan B., and Anthony P. Tully. Shattered Sword: The Untold Story of the Battle of Midway. Washington, D.C.: Potomac Books, 2005.

Peattie, Mark R. Sunburst: The Rise of Japanese Naval Air Power, 1909-1941. Annapolis: US Naval Institute Press, 2001.

Tully, Antony. "Star-Crossed Sortie: The Last Voyage Of Unryu and DesDiv 52." Nihon Kaigun. http://www.combinedfleet.com/Unryu.htm

Tully, Antony, Jon Parshall and Richard Wolff. "The Sinking of Shokaku -- An Analysis." Nihon Kaigun. http://www.combinedfleet.com/shoksink.htm