{Stus-List} Nexus system - VMG vs WCV - report

Robert Mclaughlin rmclaughlin2 at earthlink.net
Tue Jun 30 10:40:34 CDT 2009


I think this thread started asking about RayMarine instruments.  I had an early 90's set of Autohelm instruments and with the autopilot compass and "Multi" display (and speed, wind) it would display "magnetic true wind direction" in degrees (and of course calculate VMG, etc.)  I find keeping an eye on true wind a great way to help track wind shifts)  I had to replace all the instruments last fall due to a nearby lightning strike.  I now have the RayMarine ST60+s.  They would NOT calculate true wind without the GPS input, even though the system had the data it needed (as the old Autohelm's had).  Of course without the GPS you are leaving current out of the equation, maybe why Raymarine requires it.  And the now call it "Ground Wind" in the display.

Regarding racing and using VMG, I find it only an increasing guideline but rely much more on what I can see and feel.  The problem with sailing to max VMG on an instrument is you end up "chasing it".  For example, in a W/L course, and lets say downwind, anytime you head deeper your VMG will increase as you head more directly toward the mark, until your boat speed falls, thenVMG will drop, so you heat up, the boat accelerates and VMG increases until you go too high, then the cycle continues.  You have to be smart and average it out.  I prefer to sail my polars the tweak from there.  VMG becomes just another piece of input but I use it carefully.

(C&C33 II)

Bob





----- Original Message ----- 
From: Dennis C. 
To: billbina at sbcglobal.net;cnc-list at cnc-list.com
Sent: 6/29/2009 12:54:55 PM 
Subject: Re: {Stus-List} Nexus system - VMG vs WCV - report


Bill,

You are correct if you're using WCV (waypoint closure velocity) also known as GPS VMG, not VMG (wind).  If you're racing to windward and set the actual mark as your waypoint in your GPS, as you approach either windward layline, WCV (GPS VMG) will tend to decrease.  When you reach the layline, WCV will equal 0 because you are proceeding perpendicular to your course to the mark.  This may help a racer know when to tack to the mark but won't take current into account.  It also assumes the boat tacks through exactly 90 degrees.  I use the time honored technique of sighting along the wheel.

So, if you want to use a GPS to calculate VMG towards the windward mark, once the course is set by the race committee, set a relative mark on the race course bearing 100 miles out like you said.  :)

Dennis C.
Touche' 35-1 #83
Mandeville, LA






From: Bill Bina <billbina at sbcglobal.net>
To: cnc-list at cnc-list.com
Sent: Monday, June 29, 2009 11:43:15 AM
Subject: Re: {Stus-List} Nexus system - VMG vs WCV - report

I have been told that to get accurate VMG, you need to set the "mark" at an imaginary point about 100 miles beyond, and aligned with, the actual mark.

Bill Bina


Dennis C. wrote: 
Dwight,

In the Nexus vocabulary, VMG is both VMG to windward and VMG downwind.  Both show as a positive number.  We maximize it.  Downwind, we find that we sail a little bit "hotter" than boats which are more windward/leeward designs.  That is, our gybe angles are wider.  

When racing downwind, we set one of our multidisplays to show apparent wind on the top line and VMG on the bottom.  Once we optimize VMG, we can then gybe to the same apparent wind angle on the opposite gybe and the VMG should be close to optimized given no difference in wave angle, etc.  At this point we usually switch to watching the windex on the masthead because it's more responsive.

In shifty conditions, we have been known to put the top line while in wind mode on true wind angle because we feel wind shifts will be seen quicker.  True wind angle (TWA) is the angle between the true wind direction and the boat's heading.

Dennis C.
Touche' 35-1 #83
Mandeville, LA





From: dwight veinot <dwightveinot at hfx.eastlink.ca>
To: cnc-list at cnc-list.com
Sent: Monday, June 29, 2009 11:17:08 AM
Subject: Re: {Stus-List} Nexus system - VMG vs WCV - report


Dennis

VMG (wind) would be helpful in determining the optimum point of sail when going to weather but do you know if it is useful to determine the optimum jibe angle when sailing downwind.  I haven’t checked that out yet with my instruments which are new to me this year but I would expect something like a minus number and that  larger negative numbers would be faster?  Is VMG(wind) a shortened way of saying VMG to windward.  Do you know what the Nexus system outputs for VMG wind on a downwind jibe?

Dwight Veinot
1974 C&C 35, Alianna
Head of St. Margaret's Bay, NS




From: cnc-list-bounces at cnc-list.com [mailto:cnc-list-bounces at cnc-list.com] On Behalf Of Dennis C.
Sent: June 29, 2009 12:58 PM
To: cnc-list at cnc-list.com
Subject: Re: { Stu s-List} Nexus system - VMG vs WCV - report

Dwight,

I think you're correct.  The compass shouldn't be needed.

Dennis C.


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