TackTracker

VMG stands for Velocity Made Good, and represents the effective speed along the rhumb line direct to the next mark.

A good article on VMG can be found here.  Note that VMG is the effective speed along the rhumb line - not the current direct line to the mark for your boat. 

TackTracker measures current VMG to the course, average VMG for each tack and leg and also has a VMG chart.  TackTracker draws the rhumb line, to help you visualise VMG.

 VMG is most useful on the reach and run legs, where it can help determine the most effective sailing angle.

As well as measuring VMG to the Rhumb line, TackTracker also measures VMG to the prevailing wind direction.  This is useful for training scenarios, or where the course is not true to the wind.