Not what I meant.
You could in theory just draw a line for the sonar bearing every 3 minutes (even if your own ship is moving) which would result not in a "fan" of bearings but a bunch of more or less parallel/convergent/divergent lines.
Then get the ruler out and see if you can draw a line across all bearing lines that has the same distance between each - for which a better drawing gadget would indeed be needed.
Not sure why movement of ownship would be a problem, as long as you are marking down every bearing line directly on the map?
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