What does VMG measure on a ...

[petery]

Raymarine ST60 wind instrument when you press the VMG button?

It's hooked up to a Seatalk network and it indicates values even if no waypoint is entered. ..and if you enter a waypoint, the values don't correspond to the velocity made good towards the waypoint.

When you are heading directly into the wind, the VMG appears to indicate the apparent windspeed.

... so what does it mean and what use is it? ... or is the instrument faulty?

PS event though I am a 'techie' I have read the manual and it's no help.

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[snowleopard]

i believe the vmg of a wind instrument should show your velocity directly to windward as opposed to a gps which shows velocity towards waypoint. does that make sense?

it assumes of course that the instrument is getting course and speed through the network.

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[vyv_cox]

That's my understanding, too. In the past I recall that VMG was known as "Velocity made good to windward". The instrument is computing boat speed and wind angle relative to zero, or "speed cos a"

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[petery]

Thanks for the replies ..makes sense nm`

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[qsiv]

Not necessarily to windward - it does the relevant calculation downwind as well as up - which can be most useful when trying to establish what sort of angle to gybe through.

There is a school of thought that says only the navigator, nnot the helmsman, should make use of VMG. It is too easy to be seduced by trying to increase the VMG figure - which is easily done by luffing a little - at which point windspeed increases, boatspeed increases and so does VMG. Unfortunately the laws of physics catch up - the sails stall, boatspeed drops, so you bear away and apparent wind drops and VMG drops like a stone. For the Navigator VMG is useful figure - but for the helm it flatters to deceive.

VMG's first cousin VMC is also output by most integrated NMEA systems and is even more useful as it gives you VM to waypoint, which is usually what we are all really interseted in - particularly when the final waypoint is inside the Club bar....

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