What I do is to use the tip of the sonar contact line as a proxy for the ship's location. Get at least 2 (preferably more) plots and do a "best fit" line. You'll then have a rough indication of the ship's course.
Plot an intercept, then after a while (say 15 min or 30 min later, it all depends on the current situation) repeat the process to refine the solution.
Note that sometimes the ship changes course, and that will throw off your solution and you'll have to start all over again.
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