How is VMG upwind calculated? Earlier on this list, someone made the good point that VMG calculated to a waypoint (from GPS data) is not the best way, and my exploration of Google hits confirm that. One should focus on VMG upwind. But I am struggling to figure out how that is calculated. To get true wind direction, the wind devices I am familiar with require input from GPS data, (usually RMC, which includes SOG and COG). But they don't require knowledge of the orientation of the boat, i.e. HDG (on my boat, from the fluxgate compass connected to the Raymarine autopilot). If the boat were really going in the direction it is pointed, then HDG would be the same as COG, so the calculation of VMG upwind would be straightforward. But if the boat is not moving exactly in that direction (current, or (importantly) leeway because of heeling when going upwind), I don't understand how sufficient information is being sent to the wind device to make the calculation. Wouldn't it be required to know the actual compass heading of the boat, so that could be compared to the COG? Do higher-end wind devices also require input from the compass (which is not always exactly right in my experience). Google-searching so far has not provided an explanation, and my high school trig is so far in the past that I can't figure this out.
Input welcome. Eric Frank Cat's Paw C&C 35 Mk II Mattapoisett, MA
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