I have seen it hit the prop and get knocked backwards too... but it did this about 3 or 4 times.
as the ship turned the torpedo was able to hit something besides the prop and it finally detonated, i have a feeling this is what heppened to Balz.
3 or 4 or 5 prop impacts followed by a good detonation would have easily added anywhere from 20 to 40 seconds to the "torpedo travel time" depending on how many times it was knocked back
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