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Captain Seafort wrote:If they were ominidirectional, surely higher yields would produce brighter flashes.

Maybe, but if they were shaped, quite possibly the 'flash' could only be seen from one angle - behind the bulkhead of the surface the torpedo just hit.

Rather, though, who's to say? We have no experience of WMD explosions in complete vacuums.

We have no evidence to support, or indeed go against, any theory, other than that QTs are more advanced and effective, whether that be by higher yields, shaped charges or a combination, or even otherwise. It's up to you individually to decide which you 'want' to be true, and personally I just go along with the TM and say it's a higher yield, perhaps with a shaped charge also.