The weapon in the sortie record
Hiryū’s second-counterattack table records nine Type 91 modification 3 torpedoes expended. The formation contained nine attack aircraft from Hiryū and one from Akagi; the note says the Akagi aircraft did not release its weapon. Ten aircraft therefore did not mean ten torpedoes launched. M41
That wartime entry establishes use. For the weapon’s characteristics, a different source is needed: the United States Naval Technical Mission to Japan’s March 1946 investigation, assembled through interviews with Japanese technical personnel and collected information. It is a postwar technical report, not a return from the Midway attack. M60
Keeping the weapon steady
The investigation describes the importance of roll control in achieving a reliable transition into the underwater run. Its account credits the anti-roll system with addressing the tendency to turn off course on entry. This was one part of making an aerial torpedo useful; it could not guarantee a hit against a manoeuvring ship. M60
For modification 3, the report gives an explosive charge of about 240 kilograms. Its narrative uses 528 pounds and Table I rounds the figure to 530. Neither number is the weight of the complete torpedo: the same table separately lists 1,870 pounds for the whole weapon. These are the investigation’s technical particulars, not measurements of the nine individual weapons released at Midway. M60
Which modification matters
The report dates modification 3 to 1942, the Improved version to 1943 and the Strong version to 1944. Later strengthening and different launch limits belong to that development sequence. Applying a later version’s capabilities to every weapon labelled Type 91 would misrepresent what the Midway record establishes. M60
What the target tells us
Yorktown’s damage report describes port-side aerial-torpedo damage, flooding and loss of power. That provides evidence of effects aboard the target, separately from Hiryū’s expenditure and hit claims. The subsequent submarine attack belongs to another episode and must not be counted as the aerial weapon’s achievement. M18
Read Hiryū Fights Back for the attack and the Mark 13 profile for the American weapon’s difficulties. Reliable running, tactical opportunity and damage caused are separate parts of the comparison.