Tactics Plugin

[fgd3]

Originally Posted by tom_BigSpeedy

Rick,

VMG versus CMG :

It is the same formula, but instead of using the bearing to a waypoint for CMG, the VMG uses the (bearing to the) true wind direction ...

Thomas

It can't be the same formula since course doesn't have a time factor and velocity does.

Fabbian

[fgd3]

Originally Posted by rgleason

Yes, CMG requires a waypoint. Your excellent diagram could additionally show a WPT and CMG

As I understand it there are several types of VMG one is to a Waypoint and the other is VMG to the wind.

The Dashboard terminology does not currently acknowledge anything but VMG (to a waypoint). I want to make this distinction clear and with unified terminology in OpenCPN and across all plugins.

The VMG to waypoint is much less useful for determining optimal sailing than VMG to upwind/downwind.

I don't agree. The only time VMG directly upwind is important is if you are trying to see how high your boat can point (maybe if you're trying to construct a polar diagram). Otherwise, you are always sailing to a destination (waypoint). VMG to the waypoint allows you to determine if falling off to increase boat speed will get you to your destination faster or slower. An "upwind" destination is rarely exactly upwind, so VMG to the destination is more important than VMG to the true wind. You really don't care about the wind direction if you know you are making the fastest possible progress to your destination.

As a practical matter, the true wind is rarely steady and seldom known, so relating VMG to true wind is problematic. On the other hand, VMG related to a waypoint is easy to compute.

The glossary (Terminology | Official OpenCPN Homepage) needs a little work. The definition of SOG (speed over ground) makes sense, but SMG (speed made good) and VMG (velocity made good) should be synonymous and should refer to the component of speed towards a destination (waypoint). That's not the same as SOG.

Instead of a single entry for VMG there are two, VMGC and VMGW. That's fine if you find it useful, but the long accepted term is VMG for velocity made good to a waypoint.

"Velocity Made on Course (VMGC) – Speed achieved directly toward the active waypoint. The value of VMC..." If the abbreviation is VMGC shouldn't that abbreviation be used in the body of the definition instead of VMC?

"The average value of VMG is an indicator of optimum sailing angle, and should be used as a general indicator, however the skipper should be using Target Boat speed to sail at the optimum tacking angel." VMG is something which can be measured in real time and used to determine the optimum progress toward the destination in current conditions. Target Boat Speed is an estimate based on polar diagrams. Which do you think will help you get the best performance? "Tacking angel" should be "tacking angle."

It's really important that the software author and end users agree on how these terms are defined. While you can come up with completely new terms and definitions if you like, it makes it easier for a broad user base if the terms are consistent with established usage.

Fabbian

[tom_BigSpeedy]

Originally Posted by fgd3

... It's just the bearing to the waypoint at the start of the leg. ...

Fabbian,
yes, you're right, I meant ...
CMG is the component of SOG towards a waypoint, like shown in the drawing
Small but important typo ... It was too late yesterday, sorry...
CMG is the length of the vector, not the bearing.

NKE uses the terms VMG (velocity made good windwards) / CMG (velocity made good on course to a waypoint) on their instruments, and as I'm having NKE on board, I sticked to this terminology.
I can easily change this, as soon as we know what we want

Thomas

[rgleason]

Thomas, earlier I understood CMG to be in units of speed as in SOG and VMG (to wpt/course Or upwind / downwind). I also understood CMG = VMG upwind / downwind, a vector magnitude (speed) and direction.

Your later reference to COG with the "C"ourse of CMG made me think it was simply a direction.

Fabian, we can have a long discussion and disagreement about the usefulness of VMG (to wind) versis VMG (to waypoint) however the facts are that for a sailboat tacking back and forth to reach a mark, VMG to waypoint varies dependent on where the boat is; the value drops to zero as the boat sails to the left or right of the course, even if the boat is being sailed optimally. The extreme example is when the mark is directly abeam, and you have just begun sailing further away from the mark, or "overstanding", the VMG to waypoint will be 0!
This is not a useful value for optimizing boat performance upwind or downwind! I do hope you can agree with that point first.

As far as terminology goes at this point, I am looking for a clear and consistent distinction that is uniformly applied. I would have been comfortable with CMG vs VMG as NIKE uses, but I now fear others may make the same mistake we just made?

I will look again at terminolgy in the User Manual to make it as clear as possible. It would be helpful to agree on the short form abreviations that we all should be using!

[rgleason]

Fabbian,
"The only time VMG directly upwind is important is if you are trying to see how high your boat can point (maybe if you're trying to construct a polar diagram)"

Would you mind explaining exactly what you're thinking when making this statement? Perhaps I will be able to respond more succinctly if you can elaborate.

Also you said this below:
"Velocity Made Good (VMG) is useful in telling you if you are going to reach an upwind destination sooner by sailing as close to the wind as possible or falling off a bit to get an increase in speed. It also applies to a downwind destination, telling you if it's better to sail dead downwind or to tack downwind."

Which value of VMG are you referring to here?

I might add, that some time ago I tried to have a meaningful discussion in the dashboard thread about the very ambiguous term VMG, trying to sort out acronyms, and no one really responded, except TransmitterDan offered suggestions to make the Dashboard naming user selectible in the future.

[RogerD]

Thomas/Rick/Fabian - You are mixing terms refering to different times - past present and future

If you start at Point A, at time 1 and intend to get to Point B you can lay out a Course between the 2 points, with an estimated speed from which you can estimate a travel time and an arrival time. You can include estimates of current set and drift and leeway to improve the accuracy of the plot to determine the estimated Course to steer to arrive at your destination. This is all future stuff - what you are trying to do.

Once you start travelling you will be steering a course - your Heading according to your corrected compass, with the boat going thru the water at a speed shown on your speed sensor - Speed Thru the Water. If you have a GPS you will also have Speed over the Ground and Course over the Ground which will both be different from STW and Heading if you have current or leeway. You will also have a Bearing from your present position to Point B. These are present values.

You will also have your present position wrt Position A. This is the history of your journey so far i.e. the past. You have the straight line plot from Point A to your present position, independent of an course deviations due to tacking or steering or current. This is the Course Made Good to this point. You can measure the distance between Point A and your present position along the Course Made Good. With the time it took to get from Point A to your present position you can calculate the Speed Made Good. This is the past!

Velocity Made Good is usually a present value looking at your present position to where you are trying to go and calculating how fast you are achieving the goal. With a Waypoint set I believe most boaters would consider this to be a measure of how fast you are gaining the waypoint. (I am aware that the value changes as the angle to the Wp changes. How this info is used is another discussion.) This is reflected in the Route table that OpenCPN displays when a Route is active - XTE, BRG, VMG, RNG, TTG all refer the the active WP. However racers and some cruisers also use this term to refer to boat speed wrt the true wind. They just have a different definition of "where you are trying to go". There should be a different unique term for this speed.

My opinion only
Roger

[rgleason]

Roger, I think this is an excellent basic description which is needed whether powerboat or sail. Tacking and consideration of optimal performance bring in VMG to wind, Target Boatspeed and other measurements as in Tactics_pi. For sailors these factors are important and cannot be dismissed, although sailors sense and adjust for them intuitively.

Roger, if you would read Terminology under opencpn.org User Documentation and suggest appropriate modifications where we have mixed past and present. Thanks!

[rgleason]

Your vote for a separate term is noted

[fgd3]

Originally Posted by rgleason

Fabian, we can have a long discussion and disagreement about the usefulness of VMG (to wind) versis VMG (to waypoint) however the facts are that for a sailboat tacking back and forth to reach a mark, VMG to waypoint varies dependent on where the boat is; the value drops to zero as the boat sails to the left or right of the course, even if the boat is being sailed optimally. The extreme example is when the mark is directly abeam, and you have just begun sailing further away from the mark, or "overstanding", the VMG to waypoint will be 0!
This is not a useful value for optimizing boat performance upwind or downwind! I do hope you can agree with that point first.

Today I learned about something called the archer's paradox. When an arrow is launched from a longbow the bowstring pulls toward the center of the bow. Because the arrow rests on the side of the bow the closer the string gets to the bow the more it forces the arrow out of alignment. It's pretty much the same problem you're talking about with VMG to a waypoint. So I understand the appeal of VMG to the wind to help you optimize boat speed upwind or downwind.

On the other hand, imagine you are trying to reach a waypoint bearing 0 degrees in wind conditions that prevent you from sailing higher than 35 degrees on the port tack. Maximizing VMG to the wind you sail and sail until the target waypoint bears 270 degrees. You're not very close to your destination, are you?

If, instead, you tried to maximize VMG to the waypoint you would find that value dropping as you move farther east of the waypoint. Time to tack! Now you sail 305 degrees and try to maximize VMG to the waypoint. At some point it will start declining, you tack again, and eventually you will reach the waypoint.

In practice, of course, you won't follow VMG to the wind indefinitely. When you cross the lay line to the waypoint you'll tack so you can reach the waypoint. I think the difference between VMG to the waypoint and VMG to the wind is mostly theoretical but I can see advantages to both.

It's important to remember that both measurements tell you an instantaneous value based on conditions at the time the measurement was taken. We extrapolate our forecast based on the assumption that conditions will remain unchanged. But conditions always change. Wind strength increases or decreases. Wind direction shifts. When the GPS gives you VMG to a tenth of a knot it's hard to remember it isn't really as precise a value over time as it appears.

Originally Posted by rgleason

As far as terminology goes at this point, I am looking for a clear and consistent distinction that is uniformly applied. I would have been comfortable with CMG vs VMG as NIKE uses, but I now fear others may make the same mistake we just made?

We have the same interest here: we want terminology that is well understood and consistently used. It's not easy to achieve but I think it is worth spending some time and effort to get it right.

I've always seen CMG used as an abbreviation for course made good and VMG for velocity made good, just as COG is course over ground and SOG is speed over ground. I hope some other folks will join this conversation so we can ascertain common usage instead of making up new terms.

Originally Posted by rgleason

I will look again at terminolgy in the User Manual to make it as clear as possible. It would be helpful to agree on the short form abbreviations that we all should be using!

I agree with you about that. Your work in this area is a major contribution to OpenCPN.

Fabbian

[fgd3]

Roger, that was an excellent summary of the various navigation measurements. I especially liked your description of course made good (CMG) as the straight line plot from your starting position to your current position. Very well put.

Don't most terms apply to both past history and future expectations? When a sailor looks at speed made good he knows that is a measure of what has happened (past) but it's important to him because he expects to achive the same results going forward (future). Sometimes when we are discussing those measurements we aren't as careful as we should be to specify which way we're looking, past or future.

Fabbian

[fgd3]

Originally Posted by rgleason

Fabbian,
"The only time VMG directly upwind is important is if you are trying to see how high your boat can point (maybe if you're trying to construct a polar diagram)"

Would you mind explaining exactly what you're thinking when making this statement? Perhaps I will be able to respond more succinctly if you can elaborate.

I wish I could remember. By the time I get around to this forum my thought processes tend to be fogged by fatigue. To the best of my recollection I was thinking VMG to the wind is useful for optimizing boat speed when you aren't trying to reach a specific destination. I overlooked other information you would use to redirect the boat to a destination.

Originally Posted by rgleason

Also you said this below:
"Velocity Made Good (VMG) is useful in telling you if you are going to reach an upwind destination sooner by sailing as close to the wind as possible or falling off a bit to get an increase in speed. It also applies to a downwind destination, telling you if it's better to sail dead downwind or to tack downwind."

Which value of VMG are you referring to here?

I was referring to VMG to a waypoint.

Originally Posted by rgleason

I might add, that some time ago I tried to have a meaningful discussion in the dashboard thread about the very ambiguous term VMG, trying to sort out acronyms, and no one really responded, except TransmitterDan offered suggestions to make the Dashboard naming user selectible in the future.

I know you have tried to get input from forum members on a variety of subjects and the response is usually pretty minimal until you make a decision. Then everybody jumps on you to change the decision. It's frustrating, but that's the nature of public service.

Fabbian

[rgleason]

Fabbian I've been away sailing a bit. You've been busy here.
I like your bow & arrow analogy for VMG to wpt. You are correct that it is often better to stay towards the center of the race course and to tack in the headers when you have oscillating winds (veer and backubng) in order to maxmize VMG to the mark, but don't let that confuse you about the best values to use for optimizing your boat's performance upwind. Also I might add that that is also the fastest way to get to a waypoint!

Think about VMG to wpt when you've taken a flyer and gone to one side or another expecting a huge structural wind shift. Normally you would tack before the mark was abeam, and then ride the wind shift up to th mark (if you anticipated winds correctly), but lets say you tack when the mark is abeam when VMG to mark is 0, once you complete the tack you will be headed directly to the mark and VMG to mark will be positive and with the exception of Drift (current + leeway) will equal Speed through the water STW, and your boat will be headed to the mark at the maximum possible speed, provided the sails are trimmed right, you are sailing to the boats polars using best VMG to wind (Target Speed) and the wind does not shift or change velocity.

Note: I honestly thought you were describing VMG to wind when you wrote "VMG is useful in telling you if you are going to rech an upwind destination sooner by sailing as close to the wind as possible or falling off a bit to get an increase in speed. It also applies to a downwind destination...telling dead downwind or to tack downwind.."

I'd say that's a darn good description for VMG to wind!
That's why I asked.

Perhaps tom_bigspeedy can help explain the difference between VMG to wind and VMG to wpt. Maybe looking carefully at Tactics_pi User Manual and definitions will help. The diagrams are excellent. I intend to get some diagrams into OpenCpn Terminology page after I get Thomas's Tactics_pi user manual upload to the website.

[LeaseOnLife]

Originally posted in "Plugin: Dashboard", but there seems to be more recent traffic here.

On my Raspberry Pi 3 the (dashboard) wind history graph uses excessive CPU power. It starts all well, graphing TWD and TWS, but the more data are shown, the more CPU is used, until one CPU pegs out at 100%, used by process Xorg. Side effects are NMEA parsing seems to stop, loss of GPS position, AIS signals, etc. Turning off the dashboard restablishes normal behavior.

Above is true for openplotter on Raspian with opencpn 4.4.0, as well as self compiled OpenCPN from git today (4.5.0). Using the experimental VC4 GL and enabling opengl in OpenCPN makes no difference. I then switched to tactics_pi, which shows the same issue when displaying wind history.

Anyone else seeing this?

Dirk

[rgleason]

Tom_r I've made an External Plugin page for Tactics_pi.
Since the User Documentation is 27 pages, I just used the beginning pages and linked to the Tactics_pi.odt and Tactics_pi.pdf on Github.

I am wondering if it would be possible to add the documentation to your "Wiki" page and I would also add that link? - Is that a reasonable way to do this, or should I ask Bdbcat to upload your pdf to the website and give me a linking address, so the docs are with the website? The problem with that is we have to bother him each time the documentation is updated.

Tactics_pi | Official OpenCPN Homepage

PS: I like your terminology for VMG and VMG best, but it appears to be a tough sell because of powerboaters who just care about VMG to mark (or VMC) even though that data is not as meaningful to sailors who care about data for optimizing upwind/downwind VMG.

[rgleason]

Tom,
I have a question regarding calculation of True Wind while using COG/SOG data.
Isn't it necessary to modify the ground track (COG/SOG) with DRIFT (current + leeway) data to get the accurate True Wind Speed and Direction? (The reason I say this is, so often when there is current, the ground track is severely impacted! For example to see this just look at your GPS track while tacking upwind.)

See Dashboard post/discussion here please. http://www.cruisersforum.com/forums/...ml#post2195920

Correction: Set & Drift are the compass direction and speed that water is moving.
Leeway is not a part of Set & Drift.