Before all the electronic RADAR collision avoidance wizardry. We used grease pencils to plot on the scopes PPI and Maneuvering boards to determine what you seek.
If you know your own course and speed and take a range and bearing to target say every three minutes you can easily determine its true and relative course and speed, even in games like Silent Hunter.
You can find reprints of the U.S. Navy Maneuvering Board Manual Pub.217 and a pack of Maneuvering Boards on Amazon. All you need then is a watch and a pencil.
PUB. 217
Maneuvering Board
Oh ya forgot. In addition to the watch and pencil you'll need a set of dividers, compass with sharp lead and a Weems plotter will be very helpful for accurately transferring information to the "Mo" Board.
But I'm sure there's some App out there that does all of this for you