warhead was also modified to arm after only a short distance through the water, about 650 feet.
Three of the specially modified torpedoes were dropped for tests. Two ran successfully; the third hit bottom at 39.3 feet. “On that basis the Japanese estimated that, out of 40 drops planned for Pearl Harbor, 27 would hit home.” 45
The best of the practice runs occurred in early November, days before the end of training. The torpedo bombers achieved 82.5% hits. With this percentage, 19 hits could be expected from the 24 bombers allocated to attack the battleline. If the attack was distributed evenly over six battleships, that would give three hits per battleship, which according to Japanese thinking was sufficient to cripple them all.
The torpedo bombers were allocated months before the attack, before the shallow water problem was solved. As late as 4 November, during what has been referred to as “the dress rehearsal,” 46 only 40% of the torpedoes leveled off at the correct depth. At this rate only 10 torpedoes would hit the battleline, perhaps two per battleship, which was sufficient to damage, and perhaps to cripple, but not to sink. Using Fuchida’s original guesstimate, there would be 16 torpedo hits on the battleline, two or three per target if evenly distributed. And again, that was enough to cripple, but not to sink.
If 80% hits could be expected, then each B5N Kate with a torpedo would be worth 0.16 of a capital ship. Twenty-four torpedo bombers, cumulatively, would be worth 3.8 battleships sunk out of six potential targets.
The planners originally allocated only crippling power against the battleships, not killing power.
Carrier Moorings Attack Group
The other 16 torpedo-carrying bombers, two groups of eight, were assigned to attack the carrier anchorages on the northwest side of Ford Island. The Japanese formal estimate was that three of the carriers were operating out of Pearl Harbor, 47 although their intelligence agent’s reports would attest that rarely were all three in port together.
Three carriers were assigned to the Pacific Fleet, Enterprise , Lexington , and Saratoga . Saratoga was at San Diego. She had not been operating out of Pearl Harbor while in drydock at Bremerton. The intelligence officer’s reports would not have mentioned her presence since October.
The attack against the carrier moorings would involve a fairly easy approach along the length of the loch past Pearl City, with plenty of room to set up the attack and establish the proper release conditions. The approach would be at an angle so the targets would be foreshortened, reducing the target cross section by about 30%. The approach would pass several nests of destroyers, so surprise would be needed to avoid AA fire.
These 16 bombers represented 40% of the torpedo-armed bombers. The maximum number of carriers the Japanese could expect to find in port would be three, two more likely, fewer still considering the carriers spent a lot of time underway. At 40% hits, this group would be expected to score six hits, enough to sink two carriers; at 68%, ten hits, which could account for all three carriers or gut two; 82% would give thirteen hits, over four hits for each carrier assigned to the Pacific Fleet.
This allocation provided killing firepower against the carriers.
Weighting the Attack
The battleship anchorages might expose six battleships along battleship row. The Japanese assigned 24 aircraft to attack this group, four per potential target. The other group of 16 bombers would be attacking, at most, three carriers, more likely only two, for five to eight torpedo bombers per potential target. They maintained this overweight even after they received an intelligence report that none of the carriers were in port, in the hopes that the carriers might return.
Carriers did not have the built-in anti-torpedo protection of a battleship, and could sustain fewer hits before sinking. The Japanese felt that three to four torpedo hits were
Three of the specially modified torpedoes were dropped for tests. Two ran successfully; the third hit bottom at 39.3 feet. “On that basis the Japanese estimated that, out of 40 drops planned for Pearl Harbor, 27 would hit home.” 45
The best of the practice runs occurred in early November, days before the end of training. The torpedo bombers achieved 82.5% hits. With this percentage, 19 hits could be expected from the 24 bombers allocated to attack the battleline. If the attack was distributed evenly over six battleships, that would give three hits per battleship, which according to Japanese thinking was sufficient to cripple them all.
The torpedo bombers were allocated months before the attack, before the shallow water problem was solved. As late as 4 November, during what has been referred to as “the dress rehearsal,” 46 only 40% of the torpedoes leveled off at the correct depth. At this rate only 10 torpedoes would hit the battleline, perhaps two per battleship, which was sufficient to damage, and perhaps to cripple, but not to sink. Using Fuchida’s original guesstimate, there would be 16 torpedo hits on the battleline, two or three per target if evenly distributed. And again, that was enough to cripple, but not to sink.
If 80% hits could be expected, then each B5N Kate with a torpedo would be worth 0.16 of a capital ship. Twenty-four torpedo bombers, cumulatively, would be worth 3.8 battleships sunk out of six potential targets.
The planners originally allocated only crippling power against the battleships, not killing power.
Carrier Moorings Attack Group
The other 16 torpedo-carrying bombers, two groups of eight, were assigned to attack the carrier anchorages on the northwest side of Ford Island. The Japanese formal estimate was that three of the carriers were operating out of Pearl Harbor, 47 although their intelligence agent’s reports would attest that rarely were all three in port together.
Three carriers were assigned to the Pacific Fleet, Enterprise , Lexington , and Saratoga . Saratoga was at San Diego. She had not been operating out of Pearl Harbor while in drydock at Bremerton. The intelligence officer’s reports would not have mentioned her presence since October.
The attack against the carrier moorings would involve a fairly easy approach along the length of the loch past Pearl City, with plenty of room to set up the attack and establish the proper release conditions. The approach would be at an angle so the targets would be foreshortened, reducing the target cross section by about 30%. The approach would pass several nests of destroyers, so surprise would be needed to avoid AA fire.
These 16 bombers represented 40% of the torpedo-armed bombers. The maximum number of carriers the Japanese could expect to find in port would be three, two more likely, fewer still considering the carriers spent a lot of time underway. At 40% hits, this group would be expected to score six hits, enough to sink two carriers; at 68%, ten hits, which could account for all three carriers or gut two; 82% would give thirteen hits, over four hits for each carrier assigned to the Pacific Fleet.
This allocation provided killing firepower against the carriers.
Weighting the Attack
The battleship anchorages might expose six battleships along battleship row. The Japanese assigned 24 aircraft to attack this group, four per potential target. The other group of 16 bombers would be attacking, at most, three carriers, more likely only two, for five to eight torpedo bombers per potential target. They maintained this overweight even after they received an intelligence report that none of the carriers were in port, in the hopes that the carriers might return.
Carriers did not have the built-in anti-torpedo protection of a battleship, and could sustain fewer hits before sinking. The Japanese felt that three to four torpedo hits were
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