I'd like to use my GPSmap 76C for clocking VMG. I've no problem putting VMG up as one of the data fields, but I don't know how to activate it. I couldn't find any specifics on it in the manual either. Anyone willing to educate me?
VMG on GPSmap76C
- Thread starter mpaxman
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Waypoint...
I think you have to be navigating toward a waypoint or route? I'll check my B/W version
I think you have to be navigating toward a waypoint or route? I'll check my B/W version
True Bearing
The speed displayed is the VMG for your true heading. On my 76s it projects a line in the direction I am heading so if this line intersects my destination I am reading vmg. The other way is as Bill suggests is you have a way point entered.
The speed displayed is the VMG for your true heading. On my 76s it projects a line in the direction I am heading so if this line intersects my destination I am reading vmg. The other way is as Bill suggests is you have a way point entered.
VMG is not always VMG
Sailors use the term VMG as Velocity Made Good parallel to wind. As in toward or away from the true wind direction. GPS units display VMG as Velocity Made Good to a waypoint. The two are not the same unless your waypoint is directly up or down wind. A GPS cannot display VMG in sailors terms because it does not have the correct information.
Sailors use the term VMG as Velocity Made Good parallel to wind. As in toward or away from the true wind direction. GPS units display VMG as Velocity Made Good to a waypoint. The two are not the same unless your waypoint is directly up or down wind. A GPS cannot display VMG in sailors terms because it does not have the correct information.
Moody, Why would they do that?
When I go sailing I am sailing to a place not to the wind. The wind is only the means of propelling me towards my destination. So VMG is for me the speed towards my destination. Just as COG and SOG is all that counts. I have sailed down river on an ebb tide and made 8 kts over the ground flying only a jib. But my speed through the water was probably less than 5 kts.
When I go sailing I am sailing to a place not to the wind. The wind is only the means of propelling me towards my destination. So VMG is for me the speed towards my destination. Just as COG and SOG is all that counts. I have sailed down river on an ebb tide and made 8 kts over the ground flying only a jib. But my speed through the water was probably less than 5 kts.
Only racers ...
really care about VMG parallel to wind. Race courses are typically upwind/downwind these days. Your VMG parallel to wind is the fastest angle to sail on a upwind/downwind course. VMG or more properly VPW - Speed - Measured Parallel to Wind (the relevant NMEA sentence) is a measure of a boat's performance, a monohull may point higher than a multihull so the assumption is that it is faster to weather (since it sails a shorter course). In many cases the multi is sailing enough faster at a lower angle to more than make up the difference and have a higher VMG (VPW) than the mono. For cruisers, sailing to get maximum VMG is good feedback when your destination is generally upwind. You might spend more time on one tack than the other but while on a tack, trimming to maximize the VMG gets you to the bar faster.
For navigation, VMG on course or the NMEA "WCV - Waypoint Closure Velocity" is what is important. Speed towards where you want to go is the main concern. VMG (WCV) can mislead you. Most boats have a weak spot in the polar curve somewhere in the 80-100 degrees off the true wind range. If you steer a direct course you are at a high WCV at the time, but sailing part of the distance on a closer reach, above 80 degrees true until the waypoint bears over 100 degrees off the wind, then turning to a broad reach with the spinnaker up will get you to the waypoint the quickest. The higher VMG (WCV) on the second part of the leg will more than make up for the low VMG on the first part.
really care about VMG parallel to wind. Race courses are typically upwind/downwind these days. Your VMG parallel to wind is the fastest angle to sail on a upwind/downwind course. VMG or more properly VPW - Speed - Measured Parallel to Wind (the relevant NMEA sentence) is a measure of a boat's performance, a monohull may point higher than a multihull so the assumption is that it is faster to weather (since it sails a shorter course). In many cases the multi is sailing enough faster at a lower angle to more than make up the difference and have a higher VMG (VPW) than the mono. For cruisers, sailing to get maximum VMG is good feedback when your destination is generally upwind. You might spend more time on one tack than the other but while on a tack, trimming to maximize the VMG gets you to the bar faster.
For navigation, VMG on course or the NMEA "WCV - Waypoint Closure Velocity" is what is important. Speed towards where you want to go is the main concern. VMG (WCV) can mislead you. Most boats have a weak spot in the polar curve somewhere in the 80-100 degrees off the true wind range. If you steer a direct course you are at a high WCV at the time, but sailing part of the distance on a closer reach, above 80 degrees true until the waypoint bears over 100 degrees off the wind, then turning to a broad reach with the spinnaker up will get you to the waypoint the quickest. The higher VMG (WCV) on the second part of the leg will more than make up for the low VMG on the first part.GPS VMG
Even if you put the weather mark into your GPS as a waypoint the VMG will only be a geometric VMG. Such that (assuming a hard on the wind tacking situation) ... as you approach the laylines, VMG declines and becomes zero at the layline ... if your boat was on the rhumb line and aimed directly at the mark(waypoint) VMG would equal SOG ... likewise if your boat is on the layline and aimed directly at the mark VMG would equal SOG ... In other words if you tack right on the layline VMG will go from zero on the one tack to whatever your SOG is on the other tack. If you see negative VMG, you've overstood and probably should tack. You might try turning on the "track" feature of your GPS ... setting the track interval to a low number (depending on how many points you can keep in memory) ... say one or two boatlengths ... will give you an interesting post race graphic view of where you should have tacked ... you can see every lift and header in your trail of breadcrumbs. A sophisticated racing computer would take inputs from wind speed/direction, water flow speed and direction, GPS speed and direction and calculate your true sailing VMG ... and given historical inputs, local weather foreasts, etc. predict when you should tack ... I'm quite sure that at the top levels (Americas Cup) of racing they use such computers. It would also be possible to interface a radar so that the relative performance of competitiors could be measured as well ... Might as well just make a video game out of it at that point. One thing I've often tried but found it to be less than ideal is to "mark" the starting line and then select it as a "go to" ... then when in the starting sequence you can see exactly how far you are from the line ... "ETE" could then be compared to the starting timer and tell you how much to speed up or slow down. It would be nice if the GPS would allow you to "Go To" a line instead of a point ... racing computers probably do.
Even if you put the weather mark into your GPS as a waypoint the VMG will only be a geometric VMG. Such that (assuming a hard on the wind tacking situation) ... as you approach the laylines, VMG declines and becomes zero at the layline ... if your boat was on the rhumb line and aimed directly at the mark(waypoint) VMG would equal SOG ... likewise if your boat is on the layline and aimed directly at the mark VMG would equal SOG ... In other words if you tack right on the layline VMG will go from zero on the one tack to whatever your SOG is on the other tack. If you see negative VMG, you've overstood and probably should tack. You might try turning on the "track" feature of your GPS ... setting the track interval to a low number (depending on how many points you can keep in memory) ... say one or two boatlengths ... will give you an interesting post race graphic view of where you should have tacked ... you can see every lift and header in your trail of breadcrumbs. A sophisticated racing computer would take inputs from wind speed/direction, water flow speed and direction, GPS speed and direction and calculate your true sailing VMG ... and given historical inputs, local weather foreasts, etc. predict when you should tack ... I'm quite sure that at the top levels (Americas Cup) of racing they use such computers. It would also be possible to interface a radar so that the relative performance of competitiors could be measured as well ... Might as well just make a video game out of it at that point. One thing I've often tried but found it to be less than ideal is to "mark" the starting line and then select it as a "go to" ... then when in the starting sequence you can see exactly how far you are from the line ... "ETE" could then be compared to the starting timer and tell you how much to speed up or slow down. It would be nice if the GPS would allow you to "Go To" a line instead of a point ... racing computers probably do.
mpaxman...to answer your question...
you must be in navigation mode (goto) to activate the VMG feature. You need a waypoint/destination. Don't get too confused by all this argument, VMG is simply a tool to measure your progress toward a given point by comparing your boat's speed over ground and its direction relative to your destination. With respects to Moody, I use VMG on the DOWNWIND leg quite a bit. Downwind you always have the rhumbline decision, whereas upwind, the dead zone restricts your choice of sailing angles towards the mark.
you must be in navigation mode (goto) to activate the VMG feature. You need a waypoint/destination. Don't get too confused by all this argument, VMG is simply a tool to measure your progress toward a given point by comparing your boat's speed over ground and its direction relative to your destination. With respects to Moody, I use VMG on the DOWNWIND leg quite a bit. Downwind you always have the rhumbline decision, whereas upwind, the dead zone restricts your choice of sailing angles towards the mark.
A thought
I have no idea why VMG (WCV) is a display option on GPS's. The only time WCV and SOG are not the same is when you are off-course. Joe is right, with nowhere to go, the GPS can't tell yoiu how fast you are getting there. (No Waypoint = No Waypoint Closure Velocity [VMG})
I have no idea why VMG (WCV) is a display option on GPS's. The only time WCV and SOG are not the same is when you are off-course. Joe is right, with nowhere to go, the GPS can't tell yoiu how fast you are getting there.
(No Waypoint = No Waypoint Closure Velocity [VMG})VMG
when I race (or cruise) I want to know the best course to get to my mark (or home). My GPS 75 uses a simple geometry to get the VMG. Even if everything stays the same (wind,etc) the VMG drops as you get closer to the waypoint. At the layline you get 0 VMG. One trick I use is to project the waypoint 100 miles to windward and use that phatom waypoint to allow the GPS 75 to give me good VMG all the way to the layline. In practise this is good as you can see the difference in sailtrim and pointing in real time. During a race I may not have time to mess with this stuff. Hope that helps
when I race (or cruise) I want to know the best course to get to my mark (or home). My GPS 75 uses a simple geometry to get the VMG. Even if everything stays the same (wind,etc) the VMG drops as you get closer to the waypoint. At the layline you get 0 VMG. One trick I use is to project the waypoint 100 miles to windward and use that phatom waypoint to allow the GPS 75 to give me good VMG all the way to the layline. In practise this is good as you can see the difference in sailtrim and pointing in real time. During a race I may not have time to mess with this stuff. Hope that helps
Thanks all, again this is a GREAT forum
Thanks all, and you were right, Joe, my real question was how to get the VMG feature turned on. I finally got it; didn't realize I had to not only add or 'mark' a waypoint but then to get at the 'go to' function you have to do a search for it and select it, THEN you can use it. Seemed a little counter-intuitive to me that I can create a waypoint but then I have to pretend I don't know where it is and use the 'find' function to get at it. Oh well, as a programmer I'm always grousing about other people's software And thanks Moody and others for the technical discussion, I understand the geometry perfectly and found the comments very interesting, confirming things that I suspected. Seems like WCV still can be quite useful, especially if you place the waypoint many miles off in the distance (like bob suggested) for trimming and/or then use the real mark for testing for the rhumbline. Again, I learn so much reading this forum - THX
Thanks all, and you were right, Joe, my real question was how to get the VMG feature turned on. I finally got it; didn't realize I had to not only add or 'mark' a waypoint but then to get at the 'go to' function you have to do a search for it and select it, THEN you can use it. Seemed a little counter-intuitive to me that I can create a waypoint but then I have to pretend I don't know where it is and use the 'find' function to get at it. Oh well, as a programmer I'm always grousing about other people's software
And thanks Moody and others for the technical discussion, I understand the geometry perfectly and found the comments very interesting, confirming things that I suspected. Seems like WCV still can be quite useful, especially if you place the waypoint many miles off in the distance (like bob suggested) for trimming and/or then use the real mark for testing for the rhumbline. Again, I learn so much reading this forum - THX 
One more thing
that was tangentially covered above (how's that for techno speak?) Once you 've got it set up for reading, witha waypoint, we use it to determine the long tack leg going upwind. The VMG will show a better # on the long tack to any waypoint.
that was tangentially covered above (how's that for techno speak?) Once you 've got it set up for reading, witha waypoint, we use it to determine the long tack leg going upwind. The VMG will show a better # on the long tack to any waypoint.
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