VMG data incorrect

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Rafiki

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I have a full set of Raymarine ST60 instruments ( depth, speed, wind, compass, graphic, and autopilot). Up until yesterday VMG was showing correct figures, today the reading is complete rubbish and also negative. ie motoring towards a way point at 5kts VMG shows negative 3.7. All the data to calculate VMG is shown by one or more of the instruments ie waypoint position, COG, SOG. What on earth is going on? Any ideas? All worked perfectly in the past.
 
Rafiki said:
I have a full set of Raymarine ST60 instruments ( depth, speed, wind, compass, graphic, and autopilot). Up until yesterday VMG was showing correct figures, today the reading is complete rubbish and also negative. ie motoring towards a way point at 5kts VMG shows negative 3.7. All the data to calculate VMG is shown by one or more of the instruments ie waypoint position, COG, SOG. What on earth is going on? Any ideas? All worked perfectly in the past.
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Are you getting an accurate STW too Andrew ? The VMG calculations use true wind, true wind is calculated using STW.
 
PaulRainbow said:
Are you getting an accurate STW too Andrew ? The VMG calculations use true wind, true wind is calculated using STW.
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STW, not sure what this is, I've never used it. I will check in the manuals, which are with me. I do have True Wind and Apparent Wind, I thought both were OK, but I will check this afternoon, as currently anchored Hamford Water.
 
Rafiki said:
VMG to a waypoint.
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First, I would check this! If your display has switched to displaying VMG to wind, it would explain everything.

VMG to a waypoint only requires the plotter/display and a GPS signal.
If the GPS position and speed and direction are correct, the plotter has no excuse for not calculating VMG properly.
If the GPS is feeding it duff position data, then VMG will be wrong.
 
dunedin said:
In which case the wind (true or apparrent) has nothing to do with it - it is pure GPS data to calculate VMG to Waypoint.

But a negative VMG does sound awfully like VMG to wind when heading downwind.
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This explanation sounds encouraging, our salingthis week has been pretty much downwind, even the motoring was downwind due to not enough wind. I will look at the other manuals. I hadn't realised there was more than one sort of VMG.
 
If you're going by your "Graphic" screen, VMG to wind is displayed when the "Speed" chapter is selected. VMG to waypoint is in the "Navigate" display chapter.
 
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