VMG display, app ?

Loads of apps that will display speed, are there any I can enter the wind bearing into to display VMG upwind?

Originally posted by Sam.Spoons I think you need a waypoint to get a VMG readout, the wind direction alone won’t work (and will vary from minute to minute)

VMG means different things to different people.

On some yacht GPS systems you can set up readouts for VMG to 'windward'.

I have a little old Magellan GPS which reads out VMG to a waypoint.

VMG to windward sounds more useful than it is. It's a trap because you can always get a higher VMG in the short term, by heading up a bit, it then collapses as the boat starts to slow down.

Also the calculation of 'true wind is usually not accurate enough due to errors in wind sensors, compasses etc etc.

And then there's the question of what kind of true wind you want your VMG relative to?

Instantaneous?

Average over the beat (in retrospect?)?

Average over some other period?

And if the wind has a bend in it so it's a few degrees different further up the course?

Because we are interested in small differences, a boat length or two over a beat, we need VMG readouts to be very accurate if they are to be at all useful.

Then, if you are using a GPS based VMG instead of water-referenced speed, you have all the complication of tide making VMG different on either tack and hour by hour.

My Laser seems to tack through about 85 degrees. So VMG to windward is about (boat speed)*(COS 42.5) i.e. 74% of boat speed. That's crude bur probably more accurate than you'll get from an app or most instruments.

Originally posted by Mozzy Velocity made good is in to or away from the true wind direction. Course made good is to a fixed point. .....

It's easy to make a definitive statement like that, but there are tens of thousands of yacht instruments which use the NMEA 'wcv' (waypoint closure velocity) data tiem and display it as 'VMG'.  Thanks for the info about more dinghy oriented instrumentation, it looks as if they may be thinking differently from the yacht people, but this kind of sailing by numbers in yachts has been rife in yachts since at least the 90s and you will have a tough time changing defintions now.

Course made good is not 'to' anywhere, it's basically COG, course over the ground, i.e.allowing for leeway and tide. Change the GOTO waypoint and the COG won't change.

Basic Yachtmaster stuff. Don't forget that a lot of the nav terms pre-date GPS and such like, when I started sailing cruisers, CMG was something you have to guesstimate after consulting a tide atlas and an almanac, using 'wishful thinking' to estimate boat speed through the water.