There is a mod that gets you reassigned in mid patrol.
http://www.subsim.com/radioroom/down...o=file&id=1502
The bad news is that it only works after you sink a ship, which is the opposite of the way it should work. Not WB's fault, the game won't allow that much sophistication. SH4 does though.
I use a die-rolling system myself. After a week without seeing an enemy ship, I roll a die. On a roll of 1 I move to an adjacent grid. After two weeks I'll move on a 1-2, after three weeks a 1-3 etc. It's not perfect but it works.