Looks great. Another hydro-tracking tool.
If you also allow for an option to set the sub's course while at the first listening position you could make a true-bearing plot instead of the relative plot now. Then you could also require the player to mark the old first listening location and the new listening location on the game map, have them just measure the (average) distance and course inbetween and feed that to your Solver. Then you'll have the full situation in a true bearing plot automatically. I preffer to do everything on the game map, but I guess alot of other users would find it tricky to compare this relative bearing plot against what they need to plot on the map (the little green line). But what you have sure makes things easier for them allready.