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
>
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