I have the Raymarine ST 60 instrumnet package,which measure boatspeed, apparent wind speed, apparent wind angle and true wind angle as well as many other parameters unrelated to the calculation of VMG upwind
Check this page to see how VMG upwind is calculated from true wind angle and boat speed. http://en.wikipedia.org/wiki/Velocity_made_good On Fri, Oct 11, 2013 at 7:22 PM, Eric Frank <efran...@mac.com> wrote: > 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 > > > _______________________________________________ > This List is provided by the C&C Photo Album > http://www.cncphotoalbum.com > CnC-List@cnc-list.com > >
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