Slitherine

Run one Sonar search and then immediately run the next (new ship of course).
Often the second search fails to spot the sub revealed by the first search.
But if you do something else with a different unit between the two sonars, the second search is fine.

In the screenshot the 8-strength DD has launched depth charges against the sub.
The 10-strength DD's sonar do nothing, but the unit's turn is spent.

what is the range of the sonar search? I guess it is just two hexes away.

Ah, my bad. The Crop circle stays at 3 hexes during the sonar sweep, but I notice the tool-tip says 2 hexes.
Thanks
You would think I had learned this after playing for nearly a year...