VMG / SOG / ETA Open CPN

[stephanguy]

I am working with the latest version 4.8.8. I have addressed this question before. Maybe there has been some changes since then.
I use OPENCPN extensively for my work. I noticed that ETA (for the total route) is not consistent with ship's speed and course. Since I work on a motor boat, I am interested with the ETA in relation to the route, taking into account my SOG (over the intended route). VMG is not relevant for me. I believe that VMG is the value that is taken for ETA calculation, which makes sense for a sailboat.

If the option do exist to switch from VMG to SOG, let me know. If not, maybe such an option could be developed? If you are familiar with ECDIS systems, they include various options to calculate ETA: with current speed, with average speed, with custom speed, with last 24h speed, with VMG.

Open CPN is the system I recommend to the company I work for. The company is old fashion and do not intend to do ECDIS. OpenCPN is well developed and very stable.

Many thanks and carry on the good work.

[EngNate]

VMG - Velocity Made Good, is the correct value for calculating ETA. It is the speed at which you are closing on your next waypoint. SOG includes all variations off your course line and will always produce an ETA that is sooner than actual. The mode of propulsion is not relevant.

[stephanguy]

Originally Posted by EngNate

VMG - Velocity Made Good, is the correct value for calculating ETA. It is the speed at which you are closing on your next waypoint. SOG includes all variations off your course line and will always produce an ETA that is sooner than actual. The mode of propulsion is not relevant.

Thanks for the answer. I understand what you are explaining but I believe I do not explain myself well...Many thanks anyway.

[nohal]

Originally Posted by stephanguy

Thanks for the answer. I understand what you are explaining but I believe I do not explain myself well...Many thanks anyway.

You explain yourself well and what you ask for makes sense. The only question is when someone will find the time and motivation to implement it. I likely will when I have nothing I personally consider more important...

[rgleason]

Not to start a WW here, but VMG as you have defined it becomes a less and less accurate measurement for sailboats tacking, the closer the boat gets to the layline.

I would prefer to call the VMG to the mark as "Velocity made good to Coarse" (VMC) or Course made good (CMG). For sailing efficiency VMG to the wind is perhaps better, but Target boat speed or "PolarSpeed" is even better.

See Terminology and the Terminology in the Tactics Plugin


However if Pavel were to include VMG in the selection it would be very useful.

[Dockhead]

Originally Posted by rgleason

Not to start a WW here, but VMG as you have defined it becomes a less and less accurate measurement for sailboats tacking, the closer the boat gets to the layline.

I would prefer to call the VMG to the mark as "Velocity made good to Coarse" (VMC) or Course made good (CMG). For sailing efficiency VMG to the wind is perhaps better, but Target boat speed or "PolarSpeed" is even better.

See Terminology and the Terminology in the Tactics Plugin


However if Pavel were to include VMG in the selection it would be very useful.

The terminology is confusing, but I think the Terminology page uses these terms consistently with the instrument makers.


"VMG" is a generic concept which should mean velocity made good in any given direction -- towards a waypoint, towards the true wind direction, whatever.


But that's not how it's used -- coming from racers, I guess, it's used by the instrument makers to refer to what is more precisely "VMG to windward". Hence the necessity of a different term for VMG to a waypoint, and that is "VMC".


So to EngNate -- VMG to the waypoint is absolutely the right value for calculating ETA, but NOT just "VMG", because O means by that VMG to windward, which is not the right value. You meant "VMC".

[rgleason]

Thank you Dockhead. Yes it may seem confusing.

As you point out, as a sailboat tacking gets closer and closer to the Layline, the (VMC) Velocity made good to Course or Mark, gradually decreases to "0". So it never is consistent and is not a very good reference for sailors if they are tacking to the mark. If they are headed directly to the mark it is useful, but I believe it should probably be called VMC to clarify the conditions.

The instrument makers of Wind Indicators generally have VMG calculations for "upwind" or "downwind" (minus) whichever way you are going. This is not VMC or CMG to a mark.

Power boats generally don't have wind instruments, so they simply use "VMG" in their world, as if it were a single value for progress towards a point or mark. It would be less confusing if we all used the same terminology, and called that (VMC) in our community, but that is unlikely to happen.

I believe the sailors will still hold to the distinction of these two values, and consider VMG to the wind, just as the wind instrument manufacturers and use VMC to represent Velocity made good to a mark or course.

[Dockhead]

Originally Posted by rgleason

. . . Power boats generally don't have wind instruments, so they simply use "VMG" in their world, as if it were a single value for progress towards a point or mark. It would be less confusing if we all used the same terminology, and called that (VMC) in our community, but that is unlikely to happen.

I believe the sailors will still hold to the distinction of these two values, and consider VMG to the wind, just as the wind instrument manufacturers and use VMC to represent Velocity made good to a mark or course.

I think we're stuck with VMG as meaning VMG to windward, so with apologies to power boaters, who are not wrong (!), I think they will need to get used to "VMC".


I would certainly not have chosen these terms. Much less confusing would be something like "VMG-W" and "VMG-C". But I think we would create even more confusion if we used terms differently from how Navico, Garmin, Raymarine, et al use them.

[Jeremy 5426]

Here's my 5 cents on VMG. When racing a sailboat and heading upwind you are usually tacking to the next mark. If you're going in a straight line then you've either reached the layline or the wind has shifted. VMG is very useful in that it tells you if you are making ground to the mark and at what speed and this can be used to help you know when to tack and which tack is preferable. eg is vmg better on port or starboard tack.

Downwind VMG is also useful as you don't usually sail directly to the downwind mark but run slightly hotter angles and gybe towards the mark - so you can also compare vmgs on the different tacks.

Also don't foget you're not only sailing with the wind but the tides, currents and seastate will vary along the course and these also affect vmg.

Have I missed anything?

And I just realised this thread is 5 years old - oh well!

[rgleason]

In a word, yes I believe you have missed something.