With a paper bag over my head :)

[ichso]

So it was late '42 and me and sargo boat approached a single merchant in the Bismarck Sea, west of Rabaul. Zero winds, excellent visibility I had no choice but to attack from beneath the surface, even if I had waited for the night to come. It was dawn at the time of the sighting.
I considered myself already in a good position for the approach so I ordered to dive and took my first markings of the target's course on the map...

The topmost markings are the first ones. The red circles encloses the tracking of my boat's movement. We went straight SW for the most part.

I first drew two bearing lines to the ship with a 5min time difference to see where it roughly should be heading. Sonar guy states 'moderately fast' screws, Rec manual says it (medium composite freigher IIRC) has a maximum speed of 17kn, so I assume it to travel at something from 8 to 13 knots.
The blue circle shows the resulting course estimation. The light blue line would be used as approximated target's course at first.

Shortly afterwards the ship got close enough for a precise range estimation. The lines a little bit to the left show the estimations of it's relative position which would give away it's true course and speed. The latter one calculated as 12-13 knots so I went "uh oh, this is gonna be close".
We made a hard turn to course W and brought her up to flank speed.
The bright yellow lines show my estimated travel lines for me and the ship in a time interval of 10 minutes. And they show that it would have gotton away even with our adjusted heading and speed.

Under the given weather and daytime it would have been unfeasable to let it get ahead, surface get around him at flank and attack from a better position again. Rabaul wasn't that far away so this would have become a almost impossible undertaking.

Usually I just would have gruntled at this point, let the boat stay at PD for another half hour and surface then, continuing the patrol. But this time the conditions for a long distance shot seemed almost good.
The ship still was about 3000 yards away, the AoB had to be almost exactly 90° and I was pretty sure about the speed estimation also. So we sent two torpedoes on their intercept course, high speed, shallow depth with the course altered for one 2° to port the other one 2° to starboard.

After about 1.5 minutes they were assumed to hit and only 3 seconds later one of both hit the target at it's stern bringing it to a complete halt immediately. We started a slow approach but it wasn't necessary anymore. Before we could get close enough for a safe finishing shot the target would sink already.
Call this a lucky shot Also I made a memo to myself to not try this to often now, only because it worked this time