Terminology -VMG & VMC - disambiguation

[rgleason]

Please read and try to understand this.
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Terminology | Official OpenCPN Homepage
FS#2050 : Instr VMG (existing) Change name to "VMC" - Also Dial version bug
FS#2051 : Instr VMG (new) Calculate "Speed achieved directly towards, or away from, TWD"

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1. The top of the line Manufacturers of Tactical Performance software like Ockam recognize that there several types of Velocity made good and have chosen:

VMG to mean Velocity made good to upwind/downwind, which is a far more meaningful value if you are a sailor concerned about optimal sailing angle.
VMC is used to mean Velocity made good to course.

What is Vmg - Ockam Sailing Instruments
Performance Functions - Ockam Sailing Instruments

America's Cup: Velocity Made Good (to wind).

https://www.youtube.com/watch?v=_AAlymBNRTQ

Sailing Blog by NauticEd
VMG - Velocity Made Good | Sailing Blog by NauticEd

"In a sailboat – VMG is the velocity component of the boat in an upwind (or downwind) direction. The distinction is made because sailboats inherently can not head in an upwind direction but it is important information to know. And as you’ll know soon a sailboat can reach a downwind destination faster if it bears off a little rather than heading dead downwind. Thus some manufacturers make the distinction between power and sail by using the defined terms VMC and VMG.

"VMC is the Velocity Made good on Course. i.e. Speed in the direction of the mark.
VMG is the Velocity Made Good i.e. Speed in the upwind of downwind direction.

"Some manufacturers don’t bother with us sailors and some manufacturers do but don’t use VMC . The trick is to just know the difference and know what information you are looking at.

"Since Manufacturers don’t distinguish – in this post we have attempted to point out each time if VMC or VMG is being used."

2. Thomas Rauch is making a Tactics_pi which names and uses VMG acording to these manufacturers (as velocity made good to upwind/downwind.

3. Additionally, we have had a terminology page for Weather_routing for a very long time, with VMG defined as Velocity made good to upwind/downwind.

4. However Dashboard and an Active Route Console use the powerboat term "VMG to course" for an equally long time. Perhaps that value should be called "VMGc(course)". I do not think VMGw(ind) would be good because the "w" can be confused with "w(aypoint) or course" and so "w" is not unambiguous.
Therefore the only way to disambiguate this while keeping "VMG" is to add "c" to become "VMGc".
I would prefer using "VMC" as is widely recognized, but "VMGc" better than nothing.

5. Even Polar Navy has it wrong. Why continue the problem?

https://polarnavy.wordpress.com/category/faq/
VMG – speed (velocity) made good. Speed with which a vessel is moving towards its destination, as measured on a line between current vessel position and destination.

3. We need to disambiguate these values using commonly recognized terms.

If you are a sailor you are first interested in Target Boatspeed, then VMG (upwind/downwind), then VMC (to course or waypoint).

Dashboard uses (VMG to course) - which in my opinion it should be simply renamed VMC and also renamed in the Active Console. This is a more accurate term that is widely used. For those messing with instruments we can make a note that many Instrument manufacturers use an unofficial Nmea sentence "vmg" to actually represent "VMC".

Instrument manufacturers including Raymarine and Simrad use an unofficial sentence with "vmg" to course or waypoint rather than using the more accurate vmc" designation.

Well known performance software manufacturers (Ockham for example) recognize Velocity made good to Course/waypoint as "VMC" and assign "VMG" to the more meaningful upwind/downwind VMG.

Why have confusing nomenclature that is not precise? This is Navigation Software, that has many knowledgeable powerboats and sailors. We should be able to understand these concepts and name them properly, even if Instrument Manufacterers (technical gearheads) can't.

5. Consider
Sailboat Instruments: True wind, VMG and current calculations
// Calculate VMG (velocity made good)
vmg = stw * cos((-twa + leeway) * DEG_TO_RAD);

[wdkester]

There are probably 2 different camps. Those who have heard of VMC and those who haven't. Until now I considered VMG to be velocity made good toward a waypoint. I guess this really turns out to be VMC. I'm not sure how many people use VMG improperly; but my guess is a lot, maybe even a majority. I thing we would have to educate a lot of users about VMC. I also disagree that VMC is a widely used term among OpenCpn users or any users (although I have no evidence).

It seems that to calculate VMG you need true wind. I wonder if a majority of users have the proper equipment? And if they do they've probably been educated to believe that VMG=Velocity Made to a point.

If you really must be unambiguous, how about VMG(toWP)? And when Raymarine ( who has a lot more weight than we do) changes we change?

[Hakan]

The two different camps are obvious. The limits between them are also rather clear; those, like me, sailing "comfortable" without bothering very much of performance but more of convenience and those having interests in racing and touching limits.
I suppose Rick's very well done investigation is connected to "Tack and laylines" plugin where it's essential to have a correct terminology in this matter.
And - if that plugins will use one meaning for VMG and the rest of OCPN another we are creating confusion. As we do if OCPN use VMC and a Simrad gear in the same boat use VMG for the same purpose.

My vote is that we change the route consol window, and Dashboard's instrument, to VMC in order to keep it correct inside OCPN. Those sailing with comfort will anyhow not bother much? ( "This VM-something value is very low" - "Should I maybe do something???") The racers will know the differences and understand what's going on.

Håkan

[rgleason]

David & Hakan,


Thanks for your responses, all good.


When we do change VMG to VMC I think we should have a short paragraph in the documentation which explains the differences and why VMG (upwind/downwind) is a better value to use for optimal sailing angle.

Please note that for Powerboaters and Sail alike, the the Active Route Console would not change other than to have VMG change to VMC, and it would remain "Velocity made good to course (or waypoint)"

The designation would simply become more refined, and if a powerboater or sailor wanted to understand why the "G" was changed to "C" for "Course" we could simply show in the User Manual "VMC(to course or waypoint)" which it ALWAYS HAS BEEN! Nothing changes except the "G" to the "C".


Of course the Dashboard will need to have a similar change.


It could be VMGwp, but VMC is commonly used by Ockam and other performance sailing software. Of course, I do not really care which is used, and VMGwp would be fine with me because it will achieve the same purpose. Only consideration really is VMC is a shorter and more succinct acronym that is defined and used commonly in the sailing world.

[FSMike]

I used to think I knew what VMG was.
Now I'm totally confused.
Congratulations.

Carry on lol.

[barnakiel]

Irrelevant mumbo jumbo pseudolinguistics.

Why not, for simplicity and usability sake, use STT and STW (speed towards tagret and towards wind)? After all, the base data is called SOG not VOG.

And why use something so obscure and far from the real world as Ockham as a benchmark? How many cruisers here ever use Ockham?

My Garmin 72 uses VMG for how fast I am approaching the target. So now I will go into OpenCPN and see VMC there? Why? And are you about to talk Garmin into writing software patches to all their units?

Assigning VMG to a new term will be at best puzzling to people who mostly spend time sailing, then they look up the screen at times and do not waste their lives reading instruction manuals.

Leave it as is, or introduce new terms that do not have opposite meanings in other maker's interfaces.

STT, STW, anything that sounds nice and iz not puzzling

Racers can always write their own plug ins that name things according to their tastes.
b.

[rgleason]

Mike and Barnakiel, Thank you for your posts.

You certainly should be confused. "VMG" has been used to designate many totally different values! We are trying to get our Dashboard, Active Route Console and the upcoming Tactics plugin all working together so you guys understand what the different values are, have good clear designations for them, that are pretty much industry standard. Keeping the value the same but changing the "G" to a "C" is not so complicated.

If you are really sailors, then I believe VMG to mean "Velocity made good upwind/downwind" as that is the most meaningful value for optimum sailing angle (other than Target Boatspeed), which then leaves VMC for velocity made good to course.

You can invent your own designations such as
STT Speed towards target
but STW is Speed thru the water, so that cannot be used.

Furthermore aren't there going to be questions about these terms?

You can have a cacophony and continuous discussion with no resolution and nothing done to unify the terms used within OpenCPN, leaving the developers a whole lot of work to untangle it later, or you can help us figure this out NOW, so that we can move forward with unified terminology across OpenCPN and ALL of its plugins.

I don't care what we do, except its got to be unified and not conflict within itself, and be easy to understand. I researched and made my suggestions, and quite frankly I don't care what Garmin uses because they are an airplane company and are only just now really getting into sailing displays, whereas B&G has been at it a long time. (PS: I do recall the great screens and VMG on my Garmin 48 which finally failed despite my soldering in a new battery, one of the best in the nascent GPS industry). Garmin no longer sets the standard, and instruments and sailors are still learning, so terminology evolves.

So come up with a viable alternative that we can all consider.
By the way, you might want to read the posts and links below carefully, and look at this Terminology link before responding, so that you can understand the context that we are working within.

Thanks, Looking forward to your considered recommendations.

PS: Ockam wrote the book on sailing performance and is not obscure.

[wdkester]

In 55 years of sailing (not racing), this is the first I've heard of Ockam. I thought he made razors.

VMGwp is fine with me.

On a more general topic, Rollovers. If you make a big change (VMC) how about a rollover explaining the change or pointing to the Help File.

[rgleason]

If you want to continue to be like Polar Navy, defining VMG in this way (incorrectly IMHO) then VMGwind or VMGwd would work.

Alternatively we could use acronyms such as

VMGwp for velocity made good to waypoint
VMGwd for velocity made good to upwind/downwind.

Or

STC for Speed to Course (waypoint)
STWD for Speed to Wind

We can then spend months considering these terms and end up with something totally non-standard.

It does not matter to me, except we need to work it out now and move forward now, before we end up with total confusion.

[Hakan]

Hi-Hi... Engineers and humanists rules the world, but each in its own court. Do we foresee a kick off or not? -Fellowship or chaos?

Sorry -- Sunday evening laid back philosophism .

[rgleason]

Ok David, I can certainly vote for VMGwp. That would solve the problem.
Who else can vote for that?

I can also vote for your rollover message idea, where/how would the message appear?
I think it would be a very useful help tool for new users too.

Ockam: think America's Cup

[barnakiel]

Yes.

As said, same terms used in various ways by various makers. Impossible to say whom to follow given the vast array of toys we have onboard.

I think, in practical terms, if possible, a four-letter code could work,

e.g.
VMGW - velocity made good to wind,
VMGT - velocity made good to target
VMGC - (an alternative to VMGT).

Should a four letter code not be an option, I would bet my sensory attention on a new three-letter code,

e.g.
VTW - velocity to wind,
VTT - velocity to target,
VTC - (an alternative to VTT).

Many ways to skin a cat. A mono too.

b.

[rgleason]

Barnakiel,
I've said enough, but any of those work. I'm going to listen,

[rgleason]

Just a note of explanation:
VMGwp, VMGC, VTC, VMGT, VTT measures the boats speed going towards a waypoint or target, it is not associated with any of the parameters that create a boat's polar file, and it will vary depending on a number of factors: position relative to the starting rhumb line, 0 just before overstanding, also 0 when abreast of the mark and ready to tack onto the layline, negative if overstanding. If you use this value to guide your sailing, you will pinch and not sail optimally. You will be much slower than a boat sailing to optimum sailing angles, using VMGwind and Target Boatspeed. You will turn the mark well after this faster boat despite having maintained a faster VMG to wpt.

VMGwind, VMGW, VMGwd or VTW or Speed towards or away from the wind, uses the wind direction which is a similar basis for the creation of your boat's polar file, therefore it is a better indicator of sailing at the boat's optimum sailing angle. However the skipper generally should not look at this value because it is not sensitive enough, and it has been found that when sailing upwind or downwind it is best for the skipper to look at Target Boatspeed which is derived from the boat's polar files (which also has to be adjusted to wave and hull conditions and sails).

If you have more interest in this topic I would suggest this link http://l-36.com/polars.php and just below the title "Background" there is a pdf file called "Using Target Boatspeed". It is an excellent explanation of all the factors involved in sailing optimally.

[skipperearly]

why not using internally VMG and VMC and allow the user under options to name the cat a dog or whatever?

BTW for me expedition is the benchmark, and they have it all vmg,vmc,stt,stw etc.


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