Lesson learned: stationing crew to best effect
 

After missing ahead and astern of a composite freighter with 2 stern torpedoes at about 1,000 yards, I checked my crew-management screen and saw that I had only 2 men stationed in the stern-torpedo compartment in each watch, and all the qualified torpedo men were in the bow.

I positioned my crew more appropriately, reengaged, and sank the ship with a single shot beneath the keel at midship.

More and more, I appreciate the level of gameplay detail in SH4!