Dennis I think is the cos of true wind angle, not apparrent
On Fri, Oct 11, 2013 at 9:19 PM, Dennis C. <capt...@yahoo.com> wrote: > Eric, > > For simplicity, think of wind relative VMG as the boatspeed through the > water multiplied by the cosine of the apparent wind angle. > > If you're sailing at 40 degrees to the wind at 6 knots, the wind relative > VMG is 6 * 0.766 or 4.6 knots. That is, you're making 4 knots dead upwind. > > In the extreme, if you were motoring dead upwind at 6, the VMG would be 6 > * cosine (0) or 6 * 1 = 6 knots. If you were sailing at 90 degrees to the > wind, it would be 6 * cosine (90) or 6 * 0 = 0. > > The calculation is independent of GPS input. > > Hope this helps. > > Dennis C. > Touche' 35-1 #83 > Mandeville, LA > > > > On Friday, October 11, 2013 5:22 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 > > > > _______________________________________________ > This List is provided by the C&C Photo Album > http://www.cncphotoalbum.com > CnC-List@cnc-list.com > >
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