richardh10
Well-Known Member
I know what velocity made good means in principal, but how does it actually work?
Could someone explain vmg to me
- Thread starter richardh10
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lw395
Well-Known Member
A lot of talking at cross purposes.
You can have VMG towards a waypoint. That is effective the rate at which you are getting closer to it.
Or you can have VMG to windward. Which is your speed time the cosine of your true pointing angle. So if you are 45 degrees off the wind doing 6 knots, your VMG to windward is 6 x 0.707 = 4.62 knots.
Unless you are directly downwind of the waypoint, the two won't be the same.
You can have VMG towards a waypoint. That is effective the rate at which you are getting closer to it.
Or you can have VMG to windward. Which is your speed time the cosine of your true pointing angle. So if you are 45 degrees off the wind doing 6 knots, your VMG to windward is 6 x 0.707 = 4.62 knots.
Unless you are directly downwind of the waypoint, the two won't be the same.
GHA
Well-Known Member
Scuse the units, just happened to have CAD open in MM, but as i understand it - it's like this -
If you are on a heading 45deg one side or the other of a waypoint then at 5K, then your VMG towards the waypoint is about 3.5Kts.
So if you throw a tack in halfway and the waypoint is 7Nm away you'll sail 10Nm to get there.
If that's wrong hopefully someone will be kind enough to make it right
If you are on a heading 45deg one side or the other of a waypoint then at 5K, then your VMG towards the waypoint is about 3.5Kts.
So if you throw a tack in halfway and the waypoint is 7Nm away you'll sail 10Nm to get there.
If that's wrong hopefully someone will be kind enough to make it right
lw395
Well-Known Member
Scuse the units, just happened to have CAD open in MM, but as i understand it - it's like this -
If you are on a heading 45deg one side or the other of a waypoint then at 5K, then your VMG towards the waypoint is about 3.5Kts.
So if you throw a tack in halfway and the waypoint is 7Nm away you'll sail 10Nm to get there.
If that's wrong hopefully someone will be kind enough to make it right
As the boat leaves point x, its VMG to windward and to the waypoint are both 3.5 knots.
As it progresses on that tack, the VMG to windward stays the same, but the VMG towards the waypoint falls off.
Eventually the waypoint is abeam, and the VMG towards it has fallen to zero. But the VMG to windward is the same.
The boat then tacks. The VMG to windward is 3.5 knots again, but the VMG to the waypoint is 5 knots, you're sailing straight at it.
In some ideal (?) world where you can sail for two hours without the wind shifting.
So you need to be clear whether your VMG is relative to the waypoint, wind at this instant, average wind etc.
William_H
Well-Known Member
It seems to me that Vmg has changed in meaning a little with the use of GPS. It now truly means the rate at which you are approaching (or not) your target. (way point or windward mark) So it is a culmination of many factors. In practice it will depend on boat heading versus wind direction and possibly current direction. In a smaller boat easily tacked one might be willing to observe Vmg on GPS on each tack to squeeze the best Vmg. Although as Iw395 points out as you approach a way point it will become more obvious that one tack will give less Vmg or indeed take you away from way point.
So on a larger boat where you don't want to tack unnecessarily you need to have a diagram on paper chart or in your head so you keep heading for target at best angle. (tack). It is what makes racing a lot of fun and a challenge to cope with varying wind shifts etc. Especially when you have a similar performance boat going to the same mark and can compare resultant performance (Vmg) Windward sailing sorts out the sheep from the goats. olewill
So on a larger boat where you don't want to tack unnecessarily you need to have a diagram on paper chart or in your head so you keep heading for target at best angle. (tack). It is what makes racing a lot of fun and a challenge to cope with varying wind shifts etc. Especially when you have a similar performance boat going to the same mark and can compare resultant performance (Vmg) Windward sailing sorts out the sheep from the goats. olewill
Sybarite
Well-Known Member
The basic principle is that sometimes sailing at angle to the line between where you are at and where you want to go means that you sail further but faster. The Vmg is the optimal compromise between direction and speed towards your destination.
knuterikt
Well-Known Member
Velocity Made Good (VMG) to windward
Velocity Made Course (VMC) to waypoint
Furuno use this terms on their plotters
Velocity Made Course (VMC) to waypoint
Furuno use this terms on their plotters
scrambledegg
Well-Known Member
Have these responses answered you question, Richardh10?
lpdsn
Well-Known Member
Does anyone else use that? Never seen VMC. NMEA seems to use VMG as VMG to a waypoint.
johnalison
Well-Known Member
New to me, though I like the idea of making a distinction. VMG to wind has been use as far as I recall since the '70s, possibly B&G first, and is so established in older sailors' minds that we have trouble adapting to the change. I now invariably specify which term I am using, but a change like the Furuno one would help everyone.
In the days of Decca, the speed shown didn't fully update for 10-20 minutes. It was interesting to see the speed change when making long boards, when the minimum speed shown was near enough one's VMG over the ground (there's another one!)
TLouth7
Well-Known Member
I am struggling to think of a situation where VMG to waypoint would be useful.
Upwind (within the laylines) VMG to wind is correct, and VMG to waypoint incorrect unless you set a waypoint upwind of you at an arbitrarily large distance. The same is true downwind (largest negative VMG to wind).
On a reach it might be that luffing or bearing away from the straight line would increase your closing speed, but then you would end up sailing in a curve, which must be slower than a constant heading?
So what am I missing?
Upwind (within the laylines) VMG to wind is correct, and VMG to waypoint incorrect unless you set a waypoint upwind of you at an arbitrarily large distance. The same is true downwind (largest negative VMG to wind).
On a reach it might be that luffing or bearing away from the straight line would increase your closing speed, but then you would end up sailing in a curve, which must be slower than a constant heading?
So what am I missing?
lpdsn
Well-Known Member
Maybe the way point is where you want to go.
VMG to windward is not really that practical with real winds that shift back and forth on a frequent basis. It's merely academic, telling you how well your boat can go to windward at that time, but it is not telling you how quickly you're getting to where you want to be. Not even if your waypoint is a windward mark in a race.
knuterikt
Well-Known Member
From HTTPS://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn
pencpn_user_manual:terminology [FONT="]VMC[/FONT][FONT="] Velocity Made Good (to Waypoint, course)[/FONT]
[FONT="]VMG[/FONT][FONT="] Velocity Made Good (to Wind)[/FONT]
johnalison
Well-Known Member
"When are we getting there?"
johnalison
Well-Known Member
VMG to windward is very helpful in knowing how well you are sailing, and very useful in training. I play with occasionally, and getting the best figure requires a lot of concentration, looking at the sails, speed log, wind direction and VMG, as well as taking into account wind speed and wave pattern - all without hitting anything. I find ten minutes or so quite enough before I go bonkers. The one thing you learn is to be extremely delicate and to use absolutely minimum movements of the helm.
lpdsn
Well-Known Member
I agree with that. I've found presenting the rudder angle to a less experienced helm actually helps.
Trouble with using VMG for training is that it is GPS derived so hard to compensate for tide.
lpdsn
Well-Known Member
From HTTPS://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn
[FONT="]VMC[/FONT][/COLOR][COLOR=#333333][FONT="] Velocity Made Good (to Waypoint, course)[/FONT]
[FONT="]VMG[/FONT][/COLOR][COLOR=#333333][FONT="] Velocity Made Good (to Wind)[/FONT]
So not a completely isolated usage them. Not sure how they argue that it's Made Good is it isn't to where you want to go, but I guess there was some evolution of terminology in the early days. Every GPS I've used uses VMG to the waypoint so I guess that is the de-facto dominant use of the terminology these days.
TLouth7
Well-Known Member
This is true, and a very important question, especially with regard to closing times. On the other hand we have Time To Waypoint for this*. I guess my point was that steering to maximise VMG to waypoint does not inherently result in getting there soonest.
lpdsn said:
Absolutely. It tells you how well you are sailing to the wind you are experiencing right now, but does not tell you which tack is favoured due to shifts. VMG to the 'average' wind would be best, if such a thing could be achieved. Then you have the problem that you don't know what your VMG would be on the other tack without going there to find out.
*I realise that time to waypoint is derived from VMG
PuffTheMagicDragon
Well-Known Member
I do find that "time to waypoint" is very useful when making a passage out of sight of land.
lw395
Well-Known Member
VMG has to be used with great care as a tuning or training aid upwind, because you can get an instant improvement in VMG readout by pinching. Until your speed drops off, then you will have to bear away, reducing VMG further until speed increases.
To be accurate, the wind sensors, log (if that's used) need to be very well calibrated.
