Attack on Pearl Harbor (25 page)

Group Attacks on Battleship Row and Hits

Of the ten salvoes, six salvoes scored hits for 60%, two less than the 80% expected. Usually, as a rule of thumb, combat results are a third less than training results, which appear to be a good approximation in this case. Four of the hitting salvoes also put bombs into outboard ships, boosting the overall hit percentage to 10 hits out of the 50 bombs carried by the level bombers.

This was a phenomenally good performance for level bombers. The pattern mean point of impact (MPI) CEPs were likely on the order of 100 to 150 feet, compared to US heavy bombers which typically had a MPI CEP on the order of 2,000 feet. Japanese level bombers at the start of the war were very effective—they devastated Clark Field in a fatal blow to US air power in the Philippines, burned Cavite Naval Shipyard to the ground, and sank the aircraft transport
Langley
while she was underway during the Java campaign. In contrast, USAAC aircraft employing the celebrated Norden bombsight during the Battle of Midway dropped 291 bombs from altitudes between 4,000 to 25,000 feet for zero hits.
²⁸

The Japanese level bombers’ efforts were sabotaged by faulty weapons. Six of the ten bombs failed, seven if a bomb that passed entirely through
Vestal
before exploding is included. A dud rate of 5% is the norm, but 60% is outrageously high. In spite of having two independent fuzes, two of the bombs completely failed to explode, and four others had low order detonations that delivered only a small fraction of their destructive potential. The modifications that removed weight to convert the shells into bombs weakened them so that some shattered on impact.

The two bombs that penetrated the 4-inch Class “B” armor covering the turret roofs on
Tennessee
and
West Virginia
hit a joint between two armor plates and squeezed between the overlapping edges, a process which shattered the weakened bomb casing and cracked open the explosive cavity, scattering the explosive. The resulting low order detonations did only minor damage, as the turrets were empty of powder. Both turrets were repaired and their turret top armor replaced and thickened.
²⁹

Overall, it was a totally unsatisfactory performance by the Japanese weapons establishment.

The simulation was re-run to examine the potential of the attack under various conditions. The hit rate per salvo was changed to 60%. A 60% dud rate was used on bombs that hit engineering spaces. A 50% dud rate was used for magazine hits, assuming that some low order detonations might be enough to ignite powder that was broken open by the bomb’s impact. Since the magazines were shielded by gun turrets empty of powder, the probability of hitting a magazine was cut in half.

(12) AP Bombing Under Actual and Expected Conditions

The shaded bars show the original expected performance of the attack. The black bars show the results under the attack conditions. As would be expected, the expectations for blown up and crippled ships are reduced significantly. Over the 1,000 trials, 650 battleships were destroyed by magazine explosions.

(13) Distribution of Number of Inboard Battleships Sunk or Crippled

The second chart shows the numbers of battleships crippled or sunk under three sets of conditions. The “expected conditions” used the original hit rate assumptions as shown earlier. “Duds and Empty Turrets” uses the historical 60% hit rate and includes the historical dud rate, shielding of the magazines by the empty turrets, and distributes the attacks evenly over the targets. “Actual Attack Distribution” adds the actual attack distribution to the “Duds and Empty Turrets” conditions. Each bar shows the number of times out of 1,000 runs that zero, one, two, or three battleships were crippled or sunk.

The leftmost results (“Expected Conditions”) were shown earlier and are included for comparison. In about 85% of the trials either two or three battleships were crippled or sunk.

In the center set of bars (“Duds and Empty Turrets”) the most likely result is that one battleship would be crippled or destroyed, with only a 27% chance that two or three would be crippled or destroyed, down from 85%. The chance that no battleships would be crippled or sunk has risen from nearly zero to 30%, a significant risk that the entire level bombing effort could have come to naught.

The last set of bars (“Actual Attack Distribution”) shows the additional effect of the maldistribution of attacks as they occurred historically, where five groups targeted
Tennessee
, three groups targeted
Arizona
, and one group apiece attacked
California
and
Maryland
. The probability that none of the inboard battleships would be crippled or sunk increases to 35%. The expected value of the attack (the weighted average number of inboard battleships destroyed) drops from 1.02 battleships to .845 battleships, indicating that the actual attack distribution reduced the potential performance of the level bombers’ effort by about 18%.

The “surprise” plan called for the first wave D3A Val dive-bombers to wait until after the level bombers delivered their attacks. Instead they hit Ford Island early, generating clouds of smoke that interfered with the level bombers’ sight picture. Some of the level bombers aborted several passes and ended up dropping on whatever battleship was visible at the time. Thus, the 18% reduction in attack potential can partially be attributed to Fuchida’s communications error with the flare guns.

The most dramatic difference is in comparing the chances for a clean sweep (destroying or crippling all three inboard battleships) under expected conditions versus the actual conditions. The chances drop from over 40% to less than 2%. The chances of getting two or three battleships drops from 85% of the trials to under 20%. Under historical conditions, the most likely result is that one battleship would be crippled or destroyed. So, the faulty bombs reduced the level bombers’ damage potential by one to two battleships.

The level bombers destroyed one battleship, meeting Japanese hopes.

The hit on the
Arizona
has often been represented as a “one in a million” or “lucky” hit. The simulation shows that the battleships were more vulnerable to the high level bombers than previously believed, and the loss of at least one battleship to AP bombs would be expected.

Overall, the performance of the level bombers was very good under trying circumstances. Compared to the performance of high-level bombers throughout the war, their effort was outstanding.

Assessment: Dive-Bombers of the Second Wave

Seventy-eight D3A Val dive-bombers arrived over the harbor to hit ships.
³⁰
With the 250kg GP bomb expected to be ineffective against battleships, and no carriers present, under the original prioritization scheme the dive-bombers’ assignment was to sink cruisers. Either just before or just after launch their assignment was changed. They were told to finish off ships damaged in the first wave attacks.

The second wave dive bombers had a difficult task trying to identify targets with the anchorage filled with smoke and a cloud layer at 3,500 feet, between their pitch-over attitude and their targets. They had to deliver their attacks through heavy AA fire from an awakened defense.

A Tactical Error? The Attack on
Nevada

The second wave arrived to see
Nevada
underway and heading towards the entrance channel. The Japanese had hoped for an opportunity to sink a ship and block the channel.

Fuchida claimed that he considered taking command of the dive-bombers, but noticed Egusa setting up to attack
Nevada
. Fuchida had his pilot bank to give him a better look, and related that he thought, “Ah, good! Now just sink that ship right there.”
³¹
This has to rank as one of the more foolish thoughts to come out of the battle.

There are several issues regarding this decision, and the conduct of the second wave dive-bombers in general:

Target Assignment

According to the planned prioritization of targets, the second-wave dive-bombers were to attack carriers first, followed by cruisers, then followed by battleships. Veterans recount that their targets were carriers. The 250kg GP bombs were capable of sinking carriers, possibly with as few as four hits.

If the torpedo bombers capsized any of the carriers, the dive-bombers were directed to hit the overturned hulls and reduce the ships to scrap. This was a bad decision. A sunken carrier would take more than six months to raise and recondition. If the war lasted longer than that, any bomb damage inflicted on a capsized hull would not add to the repair time. Such hits would require additional repair facility man-hours, but would not necessary extend the duration of the repair work. If Genda did not think of the idea, he surely approved.

The idea that dive-bombers were to attack sunken carriers was a bad planning decision that would not have contributed to achieving the mission of the attack. It is another reflection of the disparity between Yamamoto’s and Genda’s goals.

According to some testimonies, the second wave was on deck and preparing to launch when the aircrews received the word that the reconnaissance aircraft reported no carriers in Pearl Harbor. According to the prioritization scheme they should then have considered cruisers as their primary target; however, at least some of the aircrews were told just before launching to “hit the targets left over from the first wave,” meaning battleships. Forty percent did go after battleships, an inappropriate weapons-target match that did not contribute to accomplishing the attack’s objectives.

Targeting Decision Making and Bombing Accuracy

The second-wave dive-bombers’ target selection was miserable. GP bomb hits and significant DNMs (damaging near miss)
³²
tell a part of the story of what the dive-bombers attacked:

Only one hit and one DNM were achieved on the dive-bombers’ highest priority targets. None of these cruisers were sunk, and the other six were untouched. Seven hits and one DNM were achieved on battleships, 10% of the 78 bombs.
³³
Including all ships that were attacked (no matter how inappropriate), 15 bombs hit something of value, raising the hit percentage to 19%; the percentage improves to 24% if four DNMs are included.

Destroyers
Cassin
and
Downes
were in the same dry dock as
Pennsylvania
; the bombs that hit these destroyers were aimed at the battleship. This would reduce the “hit what was aimed at” percentage to 15%, with three hits in the category of “collateral damage.”

Why aircraft attacked the destroyer
Shaw
is unknown. It is possible that the destroyer and floating drydock combination was misidentified as a cruiser or battleship. Recall that the Japanese dive-bombers were to attack at about a 55-degree dive angle beginning about a half-mile from the target. At that angle, visually the sides of the floating drydock would merge with the sides of the ship, particularly in the dramatic shadows of early morning light. With both painted grey, the pair would very much have the bulky look of a battleship when viewed through cloudy skies.

It is also possible that the bombs that hit
Shaw
were actually aimed at
Nevada
as she passed by the floating drydock in her sortie down the channel.

Studies of German dive-bombing accuracy by Ju-87 Stukas (an aircraft very much like the D3A Val) under combat conditions found a circular error probable (CEP) on the order of 30 meters, or about 90 feet. Battleships were on the order of 600 feet long by 90 feet in beam, while the Americans’ larger 10,000-ton cruisers were approximately 600 feet long with a 65-foot beam. A 50–60% hit rate ought to be expected if the Japanese dive-bombers’ accuracy were comparable to that achieved by the Germans. In practice sessions the Japanese had scored 50–60% hits on a target the size of a small battleship underway at 14 knots.
³⁴

The 1941 edition of the NWC Maneuver Rules specifies that dive-bombers against a stationary, battleship size target would obtain 16% hits, close to what the Japanese actually achieved. A 1951 OEG study changed that rate to 20%, using a CEP under “ideal combat conditions” of 175 feet, based on the wartime performance of US dive-bombers.

At Pearl Harbor, the dive-bombers’ accuracy was well below Japanese expectations, in fact, below the historical performance of dive-bombers in the Pacific over the entire war, and their target selection was poor. From the hits and near misses that were observed, it is likely that approximately 31 of the dive bombers assigned to ship targets went after the battleships
Pennsylvania
,
California
, and
Nevada
, rather than cruisers—40% of the dive-bombers wasted their weapons in inappropriate weapon-target matches.

The ultimate ignominy was that the dive-bombers were out-scored in hit percentage by the level bombers, who before the battle were expected to get a hit rate about a quarter of that expected of the dive-bombers.

Level of Effort Against
Nevada

Egusa’s decision to use dive-bombers to attack
Nevada
was an inappropriate weapon-target match. The 250kg GP bomb was not a battleship killer. It did not have the penetration to breach the citadel or reach the magazines or engineering spaces, and was not a direct challenge to the ship’s watertight integrity. The chances that these weapons would sink a battleship in the channel were exceptionally low.

If the basic decision to attack the
Nevada
was bad, it was made worse by the level of effort that the flight commanders allocated. Approximately 14 D3A Vals (possibly as many as 18) attacked
Nevada
. Under training conditions this might get eight hits. The dive-bomber commanders would have no way of knowing that
Nevada
had been previously hit by a torpedo, so they had to expect to sink the ship on their own. Were eight 250kg bomb hits sufficient to sink a battleship?

The answer is a resounding “no.” The Japanese knew that GP bombs had no chance of sinking the ship, and the Americans’ assessment was the same.
Striking Power of Air-Borne Weapons
, a US Naval Intelligence document, makes this assessment of GP bombs v. battleships: “Even though fires and extensive structural damage above the armored deck result from the employment of G.P. bombs, sinking is not to be expected except when a comparatively large number of such hits are received.”
³⁵
In the NWC Maneuver Rules of June 1940, 64 500-pound GP bomb hits would be needed to sink
Nevada
, 55 in combination with a single aerial torpedo hit. In this case the deterministic model of ship damage is appropriate, since the bombs did not have the penetration to get into magazines or engine rooms.

The Japanese expected that it would take 18 dive-bombers to sink a fragile carrier.
³⁶
How, then, could they expect to sink a tough battleship using only 14?

The Japanese dive-bombing attack on
Nevada
was a wasteful half-measure. Even if all the bombs had hit, 14 hits were well short of what they must have known was needed. The attempt to sink
Nevada
in the channel was a waste of ordnance that could have been better employed against targets more vulnerable to their effects.

A charge of negligence has to be directed against the authors of this hare-brained idea—Fuchida (who briefed the idea), Genda (who allowed the idea to be briefed), and doubly again against Fuchida who, as the on-the-spot strike commander, allowed it to proceed.
³⁷
Fuchida should either have cancelled the effort, or poured on sufficient dive-bombers to have at least a chance of sinking the ship. The dive-bomber crews or commanders ought not be blamed—they operated as briefed, although they should have known that the numbers committed were inadequate.