Virtual Skipper

[UlteriorModem]

Hello,

Id like to get a plain english explination of VMG.

I know its "Velocity Made Good" but made good in comparison to what ?

How is it calcuated ?

I thought it was velocity made good to the next mark but I have seen where Im headed directly to the mark at 14 knots and the VMG is indicated 4 knots. Pretty sure I wasent bucking a 10 knot current.

[Camster]

Velocity made good = VMG = speed of travel directly into the wind, when going upwind; and directly away from the wind when going downwind. It's a trigonometry calculation based on your boatspeed and the angle you are sailing to the wind.

[UlteriorModem]

[Camster]

Yes that's right - and regardless of where the mark is.

[eMOTION]

I'd figure this definition to be almost close. Yet, it is more about relative speed as approching an intended destination, or waypoint. As approaching a next mark in VSK.

Variables fighting against this or aiding this would be considered as relative wind direction with sail trimmed for best, currents, hull & ruddder drag/maneuvering. The intent is to quickly achieve, maintain & enhance VMG as from where you need to get to; the next mark or finish line.

VMG is a true indication of the speed & efficient progress being made to the selected waypoint against relative times. Bear off for some speed & you add mileage, head-up to reduce mileage & you may be giving off some speed, consider this as the VMG formula SOG is another, 'Speed On Ground' is more like the speedometer on a car, and can be metered to sailing, yet what makes sailing readings more complex than road racing is all the environmental variables. So VMG on the water is more valuable to read than SOG, since current drifts, etc. mess with the SOG. yep.

[UlteriorModem]

Oh boy... now Im confused.

Camster says "regardless of where the mark is"

eMOTION says "relative speed as approching an intended destination"

How does the computer select this "intended destination" ? Is it just an arbitrary point on the course.

[CANKnot]

I think both are right!

VMG can be calculated to any waypoint or destination, but in racing it is usually calculated as the next mark. So, if a leg is 5 nautical miles long and it takes you one hour to sail the leg then your VMG was 5knts. So your VMG should be calculated as (total leg length - distance to mark) divided by the elapsed time on that leg.

However, that is in the real world. In the VSK world I believe VMG is actually your velocity dead up/down wind. For example, if you are sailing directly towards the next mark your VMG should be identical to your boat speed, but in VSK it is not.
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In theory there is no difference between theory and practice, but in practice there is.

[Stoker]

In my experience VMG is more usually used (at least in racing sail boats) to mean speed towards or away from the wind.

Several instrument manufacturers and navigation software companies use the term "Waypoint Closure Velocity" or WCV to indicate spreed towards a mark. Interesting thing here is that WCV is not the same on both tacks unless you are exactly in the middle of the course, it actually reaches a minimum as you approach a lay-line, until you tack when WCV will become equal to your boat speed.

WCV is a lot less useful on a sailing boat than VMG. Its a commonly held view amongst the tactitians that I know that you shouldn't let the helmsman see the VMG instrument whilst going to windward as it tends to lead to too much pinching
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--ian

[admiral]

VMG should be the SOG (speed over ground) in the direction of whatever, as the destination is not mentionend

In a sailrace the destination usualy is a mark. If a greater VMG can be reached by not sailing in the direction of the mark, SOG will deviate from VMG, then the VMG towards the wind will be equal to the VMG toward the mark. VMG becomes irrelevant when the fastest course can be sailed directly to the mark then SOG becomes VMG toward the mark, becomes WCV.

In VSK VMG is always the speed towards wind or away from wind.

@Stoker a good helmsman will trade speed for hight and visa versa as needed to reach the highest DMG ( Distance Made Good) per schedule, or watch If you can it helps to have the VMG (towards whatever) set as an average over different time intervals (on as many displays as you can spare) depending on wavelangth etc.

[eMOTION]

Admiral,
YES! Take a breathe.

You gave a good dictonary-worthy definition, and the correct variables to what matters with this. Thanks. So does VSK display a good VMG reading or is it not correctly simulated?

[CANKnot]

As Admiral said, in VSK VMG is always your velocity into, or away from, the wind regardless of where your destination actually is. So the VMG is only useful for the brief periods when the next mark is directly to windward, or leeward, of you. Once you get out to the laylines the VMG isn't measuring your velocity in the direction you want to go. VMG is completely useless (in VSK) on any kind of reach.

Cheers,
Keith
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In theory there is no difference between theory and practice, but in practice there is.

[UlteriorModem]

[maitai]

[M-1]

[admiral]

For those having trouble with the math and the physics but brave enough to read so far
To get around the course as fast as possible you aim for

- higest VMG when in between laylines or a tack and or gybe is needed to get to your destination



- highest BS when beyond the laylines and you can sail directly to your destination.

! like in real life in VSK also it's not not your topspeed that counts but the least time needed to cover the distance.

cheers

Theo