Could someone explain vmg to me

[lw395]

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?

I think it's pretty over rated.
But if you don't trust the wind instruments to be accurate enough, or want to use an average or predicted wind direction instead of the instantaneous it could be useful.
I suppose you could dream up uses for it when sailing in lots of current too?

 

[lw395]

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!)

VMG calculation can be done without GPS or Desmond or Loran.
Use the log and wind-ometer to calculate true wind relative to the boat. Then VMG (to windward) is log speed x cos( true wind angle relative to the boat).
I had a Stowe system which did this with no connection to the Decca. I never got it to read the same on the two tacks. Trying to calibrate it whiled away a couple of dull channel crossings.

 

[KINGFISHER 9]

Would somebody please explain it in Chinese? It would be easier to follow!

 

[johnalison]

VMG calculation can be done without GPS or Desmond or Loran.
Use the log and wind-ometer to calculate true wind relative to the boat. Then VMG (to windward) is log speed x cos( true wind angle relative to the boat).
I had a Stowe system which did this with no connection to the Decca. I never got it to read the same on the two tacks. Trying to calibrate it whiled away a couple of dull channel crossings.

Next time I go sailing I must take my cosine tables and work out my VMG, or not as the case may be. I do know that VMG(w) is derived from the boat speed and wind information but my aside about Decca was just a whimsical reminiscence about the old days when few people had co-ordinated instruments, or much in the way of instruments at all. I wasn't aware of the Stowe system, but remember the B&G adverts at a time when boat speed was usually calculated by chucking a beer can in at the bow and timing it (not really).

 

[lw395]

Next time I go sailing I must take my cosine tables and work out my VMG, or not as the case may be. I do know that VMG(w) is derived from the boat speed and wind information but my aside about Decca was just a whimsical reminiscence about the old days when few people had co-ordinated instruments, or much in the way of instruments at all. I wasn't aware of the Stowe system, but remember the B&G adverts at a time when boat speed was usually calculated by chucking a beer can in at the bow and timing it (not really).

You may laugh, but a cosine table for every 10 degrees is well handy!
If you've got a cruising chute, knowing you need to go 6% faster to justify heading up 20 degrees saves a lot of blind guesswork.

 

[johnalison]

You may laugh, but a cosine table for every 10 degrees is well handy!
If you've got a cruising chute, knowing you need to go 6% faster to justify heading up 20 degrees saves a lot of blind guesswork.

Don't get me wrong, but I'm all in favour of getting things right, but in the end I'm much more likely to put the engine on. I haven't got a cruising chute, mainly because I think they look ridiculous, but I do hanker after the days when we sported a spinaker. I have used the old spelling deliberately to make myself appear ancient.

 

[reeac]

You can see the VMC effect when driving a car towards one of those roadside speed indicators .... the indicated speed decreases when the angle between your course (ie the road) and the line between the indicator and your car becomes appreciable. Just commenting.

 

[richardh10]

Have these responses answered you question, Richardh10?

Sorry for the delay, I've been out sailing!

Have they answered the question? To be honest, I'm not sure. I think what I'll have to do is just try different ways, and keep an eye on the vmg and see what causes it to change, and whether or not it is for the better (if that makes sense!)
So, if I have entered a waypoint on the plotter, and the autopilot is taking me in a direct line towards it, then if I change course slightly presumably the vmg figure will change, which then shows me whether I can reach my waypoint quicker on this course with a tack thrown in at some stage.
I have to admit that this is all largely theoretical, as I have a cutter rig, and try to tack as little as possible!

 

[howard easton]

I know what velocity made good means in principal, but how does it actually work?

Velocity is a vector which is to say that it has both magnitude ( mph, or knots or km/hr etc) and direction. If the velocity isnt already in the direction you want, pointing either towards the waypoint or maybe the wind, then you can resolve it into two components one of which is in the direction you want and the other at right angles. Thats the made good bit. So your boat might be doing 6 knots but how many of that six are towards the waypoint?

Simple bit of trigonometry really.

 

[[70521]]

I am struggling to think of a situation where VMG to waypoint would be useful.

So what am I missing?

Last orders at the bar?

It is simply the speed you are making towards the pub. If the road is straight you travel at the speed you are making over the ground for the distance you need to travel. If the road is twisty and takes a number of long sweeps round the glen the speed over the ground is the same, but the distance travelled is much further and thus takes longer.

In my view it is the most useful piece of time/speed/distance data in the know universe.

 

[capnsensible]

Its the measurement you use when you are explaining to your crew why it took twice as long to get somewhere as you originally said it would.