Capsizing of the USS Oklahoma
by Parks Stephenson
Copyright © 2010 Parks Stephenson
During our analysis of the 3-section midget submarine wreck found by HURL in the Defensive Sea Area, we arrived at a profound conclusion. Not only did we find that the sub had been scuttled inside the harbour, but also that her torpedoes had been fired before she was sunk. This begged the question: at what did the I-16tou fire? What was the real meaning behind Sub-Lieutenant Masaharu Yokoyama's "Mission Success" radio message from the I-16tou at 2241 (local) later that night?
Independent of other analyses going on elsewhere in parallel, my colleague Tom Taylor and I began examining reported torpedo damage or unexplained explosions around the harbour. Specifically, we were looking for indications of a torpedo with a much larger warhead than the ones dropped by aircraft.
We could soon discount unexplained explosions in the harbour. We looked for, but could not find, any accounts of mysterious explosions that could not be explained by some other means; e.g., falling anti-aircraft shells. We therefore concentrated on damage and salvage reports for an assessment on torpedo hits.
The Nakajima B5N2 torpedo bomber (Kanjyo Kougeki-ki, or "kanko," later nicknamed "Kate" by the Allies) carried the Type 91 Mod 2 torpedo during the Pearl Harbor attack. This torpedo carried the Type 97 warhead with an explosive charge of 452 lbs. equivalent TNT. By contrast, the special submarines carried two Type 97 torpedoes with an explosive charge of at least 796 lbs. The number most cited in reference books is 772 lbs., but I was not able to determine the primary source for that number. Instead, I used as a reference the physical and chemical analysis performed by the US Naval Powder Factory at Indian Head, Maryland, on the two torpedo warheads removed from Ensign Sakamaki’s I-24tou. That analysis reported that the explosive material in one warhead was 796 lbs.; the other, 786 lbs. and that was by weight alone. In addition, the report noted that the explosive material was a 60/40 Hexa-TNT mix (hexanitrodiphenylamine and trinitrotoluene), which generally increases the explosive power of pure TNT by approximately 7%. The true explosive power of the Type 97 torpedo, then, could be as much as 850 lbs., give or take a little. For purposes of this monograph, I will refer to the Type 97 torpedo warhead as being 800+ lbs.
Essentially, we were looking for torpedo damage that was about twice as severe as the norm, the norm being the typical damage caused by the Type 91 Mod 2 aerial torpedo.
Just when we started in our search, we happened upon an interesting clue. Although it was no proof in and of itself, the message conveyed a point worth considering. In a 1942 Castle Films "The News Parade" short, entitled, "Bombing of Pearl Harbor," moving footage of the upturned Oklahoma was shown with the following narration: "It is believed that the small two-man Jap submarines, carrying dual torpedo tubes, were responsible for these two losses to our Pacific Fleet." The "two losses" in the narration referred to the USS Utah and Oklahoma. Could our midget sub actually have fired on one of these two ships? We looked first at Utah.
Matching up the reports of aerial torpedo drops against the USS Utah’s damage report, we could soon account for all torpedoes and therefore, eliminate Utah as a submarine target. Likewise for USS Helena, another torpedo target that was tied up at 1010 dock. By process of elimination, we were thus left with the ships of Battleship Row.