New Posts New Posts RSS Feed: VMG display, app ?
  FAQ FAQ  Forum Search   Register Register  Login Login

VMG display, app ?

 Post Reply Post Reply Page  123>
Author
2547 View Drop Down
Really should get out more
Really should get out more


Joined: 11 Aug 11
Online Status: Offline
Posts: 1142
Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Topic: VMG display, app ?
    Posted: 08 Aug 21 at 6:32pm
Iím after getting speed and vmg display. 

Loads of apps that will display speed, are there any I can enter the wind bearing into to display VMG upwind?
Back to Top
Sam.Spoons View Drop Down
Really should get out more
Really should get out more


Joined: 07 Mar 12
Location: Manchester UK
Online Status: Offline
Posts: 3325
Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug 21 at 9:21am
I think you need a waypoint to get a VMG readout, the wind direction alone wonít work (and will vary from minute to minute)
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Back to Top
PeterG View Drop Down
Really should get out more
Really should get out more


Joined: 12 Jan 08
Location: United Kingdom
Online Status: Offline
Posts: 810
Post Options Post Options   Quote PeterG Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug 21 at 11:18am
I think you need a waypoint to get a VMG readout

Is that true? I agree about rapid changes in wind making it unlikely to be useful as a method of calculating VMG. Realistically you need a wind direction transducer, which is likely to be tricky, and expensive on a dinghy - even if class legal.

But I don't see why you need a waypoint. VMG is velocity made good to windward, not made good to the top mark.

BUT, it's an interesting idea. If you could enter the top mark as a waypoint a phone with a GPS could give a reading of velocity made good towards that, without needing wind direction. That could be interesting?
Peter
Ex Cont 707
Ex Laser 189635
DY 59
Back to Top
L192444 View Drop Down
Really should get out more
Really should get out more


Joined: 30 Apr 12
Online Status: Offline
Posts: 500
Post Options Post Options   Quote L192444 Quote  Post ReplyReply Direct Link To This Post Posted: 09 Aug 21 at 11:50am
Originally posted by PeterG

I think you need a waypoint to get a VMG readout

Is that true? I agree about rapid changes in wind making it unlikely to be useful as a method of calculating VMG. Realistically you need a wind direction transducer, which is likely to be tricky, and expensive on a dinghy - even if class legal.

But I don't see why you need a waypoint. VMG is velocity made good to windward, not made good to the top mark.

BUT, it's an interesting idea. If you could enter the top mark as a waypoint a phone with a GPS could give a reading of velocity made good towards that, without needing wind direction. That could be interesting?



Exactly; I can work out the average wind direction before the start ... type that in to the app and then get a VMG figure ... 

Back to Top
eric_c View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 21 Jan 18
Online Status: Offline
Posts: 359
Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 21 at 9:36am
Originally posted by Sam.Spoons

I think you need a waypoint to get a VMG readout, the wind direction alone wonít 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.
Back to Top
Riv View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 23 Nov 13
Location: South Devon
Online Status: Offline
Posts: 339
Post Options Post Options   Quote Riv Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug 21 at 11:37am
Check out all the instrumentation in this. A really good listen.https://youtu.be/bVfY0yRxmgY
Mistral Div II prototype board, Original Windsurfer, Hornet built'74.
Back to Top
Riv View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 23 Nov 13
Location: South Devon
Online Status: Offline
Posts: 339
Post Options Post Options   Quote Riv Quote  Post ReplyReply Direct Link To This Post Posted: 11 Aug 21 at 11:38am
Vortex aerodynamics in Olympic Sailing
Mistral Div II prototype board, Original Windsurfer, Hornet built'74.
Back to Top
Mozzy View Drop Down
Posting king
Posting king


Joined: 21 Apr 20
Online Status: Offline
Posts: 194
Post Options Post Options   Quote Mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 21 at 8:51am
Velocity made good is in to or away from the true wind direction. Course made good is to a fixed point. 

To get VMG you can plug in a wind direction, but it's next to useless. You need real time wind readings.

I looked at getting the Sailmon MAX + wind. But the wind sensor had no built in compensation for movement of the mast. Sailmon thought this wouldn't be an issue on most dinghies where you sail relatively consistently upright. And with the right amount of smoothing I think it could decent VMG readings. I know Lennon Sails were using this linked to the Clyclops load cells to give customer some solid numbers to work to for initial sail set up. 

However, there is no way to calibrate for tide in real time. So for training against numbers it still wasn't worth the money for me given where I sail. 

If you want to look at VMG post sail then Njord Analytics is probably the best and you can plug in a wind direction to get VMG then. But it's not a live display thing. 
Back to Top
eric_c View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 21 Jan 18
Online Status: Offline
Posts: 359
Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 14 Aug 21 at 9:53am
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.
Back to Top
Mozzy View Drop Down
Posting king
Posting king


Joined: 21 Apr 20
Online Status: Offline
Posts: 194
Post Options Post Options   Quote Mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 21 at 9:14am
https://en.m.wikipedia.org/wiki/Velocity_made_good#:~:text=Velocity%20made%20good%2C%20or%20VMG,the%20direction%20of%20the%20wind.

https://sailzing.com/velocity-made-good-vmg/

https://www.nauticed.org/sailing-blog/what-is-vmg/


Back to Top
 Post Reply Post Reply Page  123>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy