...on how you want to play the game. As mentioned you can use the "look into the future" method to find out where major activity will occur or you can use the old Mark 1 Brainbox and figure out where the merchant routes might be given "existing" maps/routes etc.
Personally, I have also experienced patrols where I return to base empty handed to refuel and go out again. I consider this realistic and a part of the RSRD gameplay. Yes it's frustrating but when you do come across a target it makes it all the more imperative to sink the bugger, LOL.
