[Eric Frank]

I also got this working, using an Arduino to calculate true wind speed and direction and speed upwind. The program uses RMC from a GPS unit to provide SOG, COG, and Magnetic Variation, MWV from a wind instrument to provide apparent wind speed and direction relative to the boat heading, and compass input (from an electronic tilt-compensated compass) to provide magnetic heading. The Arduino then computes the true wind speed and direction (relative to earth, not the water) in a second MWV sentence, and true and apparent heading and speed upwind in a VHW sentence. All this is displayed in MacENC and iNavX automatically.
Happy to provide the details for anyone interested.
Eric