http://www.subsim.com/radioroom/showthread.php?t=89910
It depends on the Campaign.mis in use.
An Edited/modded Campaign.mis' may totally differ from the Vanilla one.
But you can use the SH3Mission Editor located in the game directory to open the .MIS files and check for the positions and times.
If the Nelson is scripted to sail from Scapa Flow to Loch Ewe, it depends on the set probability to spawn the journey.