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VMG display, app ?

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getafix View Drop Down
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Post Options Post Options   Quote getafix Quote  Post ReplyReply Direct Link To This Post Posted: 16 Dec 21 at 4:55pm
how relevant is this in the vast majority of club or open races?  
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JeffStineSailing View Drop Down
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Post Options Post Options   Quote JeffStineSailing Quote  Post ReplyReply Direct Link To This Post 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 Sail2WIN app on my web site or at the Garmin App Store.


Edited by JeffStineSailing - 16 Dec 21 at 7:08pm
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Post Options Post Options   Quote JeffStineSailing Quote  Post ReplyReply Direct Link To This Post 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 Sail2WIN app on my web site, or at the Garmin App Store.

Here's a screen shot:

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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post 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.
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Mozzy View Drop Down
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Post Options Post Options   Quote Mozzy Quote  Post ReplyReply Direct Link To This Post 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. 



Edited by Mozzy - 18 Aug 21 at 8:08am
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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post 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.
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Mozzy View Drop Down
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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/


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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.
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Mozzy View Drop Down
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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. 
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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
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