Print Page | Close Window

VMG display, app ?

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=13816
Printed Date: 05 Jul 22 at 3:07am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com


Topic: VMG display, app ?
Posted By: 2547
Subject: VMG display, app ?
Date Posted: 08 Aug 21 at 6:32pm
I’m 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?



Replies:
Posted By: Sam.Spoons
Date Posted: 09 Aug 21 at 9:21am
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)

-------------
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: PeterG
Date 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


Posted By: L192444
Date 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 ... 



Posted By: eric_c
Date 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 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.


Posted By: Riv
Date 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.


Posted By: Riv
Date Posted: 11 Aug 21 at 11:38am
Vortex aerodynamics in Olympic Sailing

-------------
Mistral Div II prototype board, Original Windsurfer, Hornet built'74.


Posted By: Mozzy
Date 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. 


Posted By: eric_c
Date 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.


Posted By: Mozzy
Date 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/




Posted By: eric_c
Date Posted: 17 Aug 21 at 10:48am
This iis from Garmin, illustrating that VMG can be either wind or waypoint related:

Understanding How Wind VMG and Waypoint

VMG Appear in the Data Bars

The chartplotter automatically switches between displaying Wind
Velocity Made Good (VMG) and Waypoint VMG in the data bars.
Waypoint VMG appears under these conditions:
The Route Leg data bar displays Waypoint VMG when you are
navigating a route or an automatic guidance line.
The Sailing data bar displays Waypoint VMG when you are
navigating a route or an automatic guidance line, and you turn off
the Route Leg data bar.
Wind VMG appears under these conditions:
The Sailing data bar displays Wind VMG when you are not
navigating a route or an automatic guidance line.
The Sailing data bar displays Wind VMG when the Route Leg
data bar is on.


Wind VMG really needs to be caclulated using wind instruments and a log to measure the boatspeed through the water, to keep everything in a water/true wind frame of reference rather than a GPS/Ground Wind frame.

Unfortunately this means it's full of errors and delays, very hard to measure/calculate to any useful accuracy without a lot of averaging. I've sailed on a few yachts with VMG displays, mostly they tell you how hard it is to get the instrument calibration good enough.


Posted By: Mozzy
Date Posted: 18 Aug 21 at 8:08am
Well, it won't come as a shock, but I don't think Garmin's definition is very helpful and are probably born more out selling instruments to both motor boat and sailing boats, or yachts which frequently motor. 

However, anyone I have ever talked who races has used wind direction to define VMG. And unless you plan on doing some long distance route with a motor on your dinghy then it's fair assumption to discuss VMG in reference to the OP under the most common use of the term. 

Anyway, back to the topic in hand. As this is dinghy forum, many of us will be sailing on lakes where current isn't an issue. Here GPS is actually better than traditional instruments for calculation of VMG as it includes leeway. The other thing about dinghies is we tend to sail our boats flatter and on flatter water, so a lot of the issues with calibration from heave, roll etc. are minimised. Additionally, unlike yacht, use of many electronic is illegal during racing, so often dinghy sailor would be more concerned about a system which works during training; which opens up more elaborate mounting options to get around the some of the other calibration issues. 

Once in the tide, however, it does get very messy, and that's why I've not spent out on the equipment. One of the driving factors, beyond daylight and winter storms for INEOS going to Cagliari was tide in the solent messing up their instrument readings. 

But, I don't think we're that far away. Reliable apparent wind data I think in many ways is easier on a dinghy now that instruments are miniaturised and is really just a problem with selecting the appropriate smoothing. Heading has been sorted for a very long time already. So it's just mostly about getting an accurate speed through water and leeway. 

I would probably spend £1-2k if I could get real time VMG and post session analysis for my dinghy in a tidal venue. I am already very tempted on getting what is presently available and going to other venues to avoid the current.

Back to the apps:
Gamin connect has a dinghy racer app for the smart watches (I have it on my fenix 5+) that gives VMG but it relies on a single TWD number , however, it uses an assumed tacking angle of 90 degrees. Which cause a couple of issues, firstly, I am not sure if the watch can tell the difference between pinching up 5 degrees and a 5 degree lift. If both happened without a change in speed the former should increase VMG and later leave it unchanged. 

Then there is the issue that not all boats tack through 90 degrees anyway. Then the issue of tide. So the VMG is useless for me. 



Posted By: eric_c
Date Posted: 18 Aug 21 at 9:24am
Originally posted by Mozzy

...... So the VMG is useless for me. 



It really depends what the pupose of measuring/calculating it is.


If you want to use to get a rough idea of how long a windward leg will take, then accuracy of a few % is fine and you can do useful things like predict the tide when you arrive at the windward mark.

If you want to use it for handicapping, then accuracy of maybe 1% would be useful?

If you want to use it for tuning or training, then very small differences matter (except at total novice level or 'apparent wind asy sailing lesson one' perhaps?). To take two extremes, my club Laser sailing is greatly influenced by my arch-B***-rival being able to pull away a couple of boat lengths in a drag race to the corner of the bay. Thats maybe 1% and to chip away at that, I'd need an indication down to 0.1%. Ifyou think about foiling AC boats, the apparent wind is so close to the boat speed vector that small errors will have a big influence on where the 'true' wind is calculated to be.

To get near to the sensitivity of two boat tuning is a very big ask, even in perfect wind and flat water. Real wind and the sea make it more difficult, many variables and constant change.


Posted By: JeffStineSailing
Date Posted: 12 Dec 21 at 10:47pm
Hello Peter,
A fair number of GPS systems do require a waypoint or mark to navigate to before they provide a number for "VMG". What they are actually providing is velocity toward the mark, not upwind or downwind VMG and not velocity along the rhumb line. 

With these systems, if you enter the location of a mark that's upwind of you and begin navigating to it, they show the component of your course and speed that's taking you toward the mark. The result is that they display a "VMG" value that decreases to 0 as you approach the layline. That's really misleading and it's been misunderstood by a lot of folks I sail with.

If you want upwind / downwind VMG you'll either need a system with wind sensor, or a smart app that determines true wind direction as you tack. Since I wanted that and a lot more and couldn't find it in a handheld device or a GPS watch, I developed an app that provides me what I want while I'm racing. It provides upwind / downwind VMG from a good estimate of true wind direction along with VMC (velocity along the rhumb line), tacking angles, angle to the mark, start timing, etc.
It runs on fairly inexpensive Garmin watches (~$200 and up).
You can check out the http://www.jeffstinesailing.com/Sail2WIN" rel="nofollow - Sail2WIN app on my web site , or at the http://apps.garmin.com/en-US/apps/791191d4-0593-426c-838d-ee71558a967d" rel="nofollow - Garmin App Store .

Here's a screen shot:



Posted By: JeffStineSailing
Date Posted: 12 Dec 21 at 11:13pm
Originally posted by Mozzy

 
I would probably spend £1-2k if I could get real time VMG and post session analysis for my dinghy in a tidal venue. I am already very tempted on getting what is presently available and going to other venues to avoid the current.

Back to the apps:
Gamin connect has a dinghy racer app for the smart watches (I have it on my fenix 5+) that gives VMG but it relies on a single TWD number , however, it uses an assumed tacking angle of 90 degrees. Which cause a couple of issues, firstly, I am not sure if the watch can tell the difference between pinching up 5 degrees and a 5 degree lift. If both happened without a change in speed the former should increase VMG and later leave it unchanged. 

Then there is the issue that not all boats tack through 90 degrees anyway. Then the issue of tide. So the VMG is useless for me. 

The single TWD number is obviously a problem as is assuming a tacking angle of 90 degrees.
I developed an app for Garmin watches that handles both of those issues. As for pinching up 5 degrees vs a 5 degree lift, GPS systems without wind instruments won't be able to tell the difference. That said, GPS based VMG can be quite useful if it detects your tacking angles and updates it's understanding of TWD as you go.

You can check out the http://www.jeffstinesailing.com/Sail2WIN" rel="nofollow - Sail2WIN app on my web site or at the http://apps.garmin.com/en-US/apps/791191d4-0593-426c-838d-ee71558a967d" rel="nofollow - Garmin App Store .


Posted By: getafix
Date Posted: 16 Dec 21 at 4:55pm
how relevant is this in the vast majority of club or open races?  

-------------
Feeling sorry for vegans since it became the latest fad to claim you are one


Posted By: Do Different
Date Posted: 16 Dec 21 at 5:22pm
Probably not very, given most dinghy classes don't allow any electronic aids beyond a watch and compass.   
This is the dinghy development page after all.


Posted By: JeffStineSailing
Date Posted: 16 Dec 21 at 5:27pm
I use it in all of our club races.
If the use of a GPS is not banned in your class rules it useful in the following ways:
The start timer is useful whether you're sailing a pursuit format race or a normal 5,4,1 start.
The practice timer, separate from the starting timer, lets you take timed practice runs at the line to get yourself dialed in before the real start.
If you want to maximize upwind / downwind VMG it's useful.
If you're sailing in current you'll see your actual tacking angles which helps with layline calls.

If you have mark locations and a list of possible courses before the race, the navigation helps you nail laylines even in current, helps you maximize VMC (velocity along the rhumb line), and tells you where the next mark is right after you do a rounding.
If you need to know how long till the next rounding in order to get setup for the next rounding, knowing time to the mark is a real help.
There's plenty there that helps you get around a race course more efficiently.



Posted By: JeffStineSailing
Date Posted: 16 Dec 21 at 5:44pm
Originally posted by 2547

I’m 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?

As for relevance to the discussion, I believe this is the post that caught my eye and prompted my response. 


Posted By: Old Timer
Date Posted: 27 Dec 21 at 11:00am
Originally posted by JeffStineSailing

Originally posted by 2547

I’m 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?

As for relevance to the discussion, I believe this is the post that caught my eye and prompted my response. 

Your response was most relevant and useful. 

Do you have an iPhone app?


Posted By: JeffStineSailing
Date Posted: 27 Dec 21 at 2:44pm
Part of the reason that I wrote the watch app was to have the information I want on my wrist rather than messing with a phone or a hand held GPS..
I did use my iPhone for a while before switching to a watch.
There are a number of apps for the iPhone that do some of what Sail2WIN now does.
I used iRegatta for a while. I generally liked it and got some of what I was looking for.
It’s inexpensive and the user’s guide is online so you can take a good look at what it does before taking it on the water.
Best of luck.


Posted By: Sam.Spoons
Date Posted: 27 Dec 21 at 2:57pm
I'm finding it hard to discover how much the app costs?

-------------
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: JeffStineSailing
Date Posted: 27 Dec 21 at 3:50pm
It was a one time charge of $20 for the “pro” version when I was using it.
The free version has some useful features and at that time you could pay around $5 each to add features that you wanted to use. In he App Store, scroll down to “In App Purchases” and there’s a list of add-on features & their cost.
I see that the basic app costs $9.99


Posted By: Sam.Spoons
Date Posted: 27 Dec 21 at 7:49pm
Sorry I'm referring to Sail2Win, my post must have crossed with your previous one (took me a while to complete it as I was trying to get it to download. I'm trying, unsuccessfully, to download it now...

-------------
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: JeffStineSailing
Date Posted: 27 Dec 21 at 8:31pm
Ahhh. 
Sail2WIN is $35/yr.
 Sorry about mixing up who I was responding to.  
The price is near the bottom description on the App Store.
http://https://apps.garmin.com/en-US/apps/791191d4-0593-426c-838d-ee71558a967d" rel="nofollow - Here's that link again .  
There’s a free trial period that I’m happy to extend to 60 days given we’re in the middle of winter.
During the trial period the app is fully functional. No restrictions, so you can really get to know what it provides and how it will help your sailing before deciding if you want to subscribe.
If you have a Garmin watch that will run it, you can download it to your watch using the Garmin App Store and then sync your watch using Garmin Connect or Garmin Express.
Not all Garmin watches will run Sail2WIN, so check the “Compatible Devices” tab on the App Store page for Sail2WIN to see if your watch will run it. If not the App Store probably won’t allow you to attempt to download it.
Do you have a Garmin watch and if so, which model ?
Jeff

p.s. Very strange formatting going on with these posts.
Is there some solution to that ??


Posted By: Sam.Spoons
Date Posted: 27 Dec 21 at 11:01pm
Thanks

-------------
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"



Print Page | Close Window

Bulletin Board Software by Web Wiz Forums® version 9.665y - http://www.webwizforums.com
Copyright ©2001-2010 Web Wiz - http://www.webwizguide.com