Awesome, I have no problems with getting out my calculator and doing some
cosines.... while coaching the crew, checking on sail trim, and holding on for
dear life to the wheel.
I think we are getting a bit overboard on this stuff. If the jib is luffing,
you are sailing too high! Or, on my less than $20K boat, spending the same
amount to get some screens which will tell you that you are screwed up.....
Now, if I were sailing from here to (a long distance away) that may be
important, but when I can see the next mark.....
Gary
----- Original Message -----
From: Dennis C.
To: cnc-list@cnc-list.com
Sent: Friday, October 11, 2013 8:19 PM
Subject: Re: Stus-List calculation of VMG upwind
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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