I like trashing ports on my way past. Though there was some merchant traffic so I decided to stay near one. 2 tankers came at me but as they got close to port they vanished, I was under 1200 yards too, this must be a auntomatic de-spawn point or something, as they didn't stop at the port. Later I returned to find another small old freighter parked in the same berth I had sunk one in 3 days (game time) earlier. I sunk it too, but didn't get credit or an enemy destroyed notice from 900 yards. I suspect it has something to do with an object already being sunk at that exact location.

Also it'd be nice when the liferafts launch form sinking ships that they show up on your map. Those euro-liners lunch tons, good for target practice.

Oh I guess I should mention I am using an unmodded 1.4 version.

You are right on both. That was a "de-spawn" point and you cannot get credit for re-sinking the same ship during the same patrol. You can sink all the ships in a port, leave the area, come back (they will "re-spawn"), but you won't get credit.

It's too bad there wasn't live ports, IE if you sit in one too long the subchasers or whatever are likely to fire up and come after you, fresh merchants arrive ect. There are always different things in ports depending on when you arrive at a given dock (recently I found 3 ships sunk in wake island that I didn't put there..so must have been the enarby friendly taskforce that did it) so they should count as different ships even though they are in the same spot. Once a ship is sunk it will be cleared for a new ship to dock (or raised and reused) and that is historicly acurate so you should be able to sink the new one and get credit. Ships shouldn't "despawn" in visual range either.

Oddly enough I had a small DD type ship ram a cargoship off the coast of pearl and sink itself, that was funny to watch.