There was one US sub that fired 8 torpedoes over a five-hour period, all at one maru. The target took six hits before it finally sank, and one of the two misses was fired while the target was sitting still.
On the other hand there was one maru that was hit once, and it was a dud. The dud ripped open a hull join, between two plates. The force of the ship's movement opened the rip more and more, and the ship finally sank.
We all have our bad days...
