A fun story to go with this just a few days ago.
In the Atlantic...
Small passenger/cargo merchant leading a small merchant, no escort...1941.
Late afternoon. Track them on the surface for some time, get ahead of them by near sundown for a submerged attack. Time the lead ship (the passenger cargo) several times as it goes by me to check my work. Use that speed for an attack on the trailing ship: the small merchant.
Hit it with one torpedo and eventually it sinks. The passenger/cargo is going off into the distance...
Catch up to it an hour later. One torpedo dead-center. It doesn't sink.
Catch up with it again a half-hour later. 1 torpedo misses to the stern. Quick followup without checking my work, next torpedo hits the tip of the bow.
Damaged but still doesn't sink. 3 torpedoes invested in this one ship. Surface...now nighttime. Pace it a while... it sees me, opens fire (later found out it was a twin 40mm on the stern). The previous ship was unarmed.
Crash dive, get back up to periscope depth. Get some distance. Catch up, with it on the surface. It is now zig-zagging. Next torpedo fails. Next torpedo, they see the wake, on a zig and keep going in their turn. Miss. Burn up the battery a bit to cut it off. One electric torpedo: hit... and sinks.
Grand total for two ships: 4444 tons.
At least in your case, you were hitting the target.
