I think a lot of how you search depends on what tools your boat has.
If you only have sonar best to stick close to the coast within sonar range and find single merchants trying to avoid you.
If you have radar run the convoy lanes in a pattern so you don't overlap your radar coverage. If you are in wide open water I perfer a higher speed to cover more ground, in tighter spots or near chock points I really slow down.
If you have radar and are near land draw lines out from the land that equal your radar range and connect your way-points to the end of the lines and follow this pattern. This will guarantee that no ship is missed on your land side.
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