When I see a sonar finding of a ship I drag the waypoint marker and change course to intercept the ship. Don't use TC at more than 32, and zoom in too on the Nav map.
I'm more than a complete idiot than Sober on navagation intercepts, but this does work 90% of the time for me. Becareful tho, the ship you intercept might not be one you want to mess with.