DarkWraith's MultiUI mod includes the old SH3 officer bar with "follow nearest contact" buttons on it.
You can also plot it out using the map. You can press M to toggle the nav map while on the sonar station, or just use the TAI map (tab key) to plot.
Or simply set your course to the bearing you heard the contact on, flank for a while, then submerge and listen for it again, that way you'll get a general idea of it's direction of travel.
Or use the hydrohunting tutorials so plot it's course, position and speed passively.
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