Memory a little off but I believe it was a one or two man submarine that also acted as a human driven torpedo if necessary. US military called them, I think, "coffin boats".
What was the name of the weapon that the Japanese used so effectively in the naval battles of the Solomon Islands?
In the Solomon Islands campaign, I am finding description of aerial torpedoes...torpedo bombing of American ships from Japanese dive bomber airplanes...
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...and different topic, but this statement was on the website also, regarding the Battle for the Eastern Solomons: "Strategically, the Japanese had an opportunity for a decisive victory but failed to achieve it."