VMG how to calculate

[simonfraser]

short of spending loads with a top end instrument maker and getting all the gadgets, assuming access to speed / wind / basic gps, is there a rough way of working this out?
i sail alone, so long calculations whilst sailing are out of the question.

 

[duncan]

I am afraid I don't understand the question (not the words!) - even a basic handheld GPS nowadays will give you VMG to a waypoint; in fact it's what they do best!

 

[Anonymous]

How are you defining "VMG"?

 

[stevebirch2002]

Buy any GPS as it will give you VMG towards a waypoint. Garmin are my favourite but they all do it.

 

[simonfraser]

ooops,
about time i read the manual!
only used mine to get a bearing and distance to go . . . . .

 

[scarlett]

I wait to be corrected but it is possible to calculate VMG but our challenger, without instruments, could need to do some maths every time something changes, eg wind speed or track or tide.

If he wants an easy task, let's start with a simple, voyage with a known departure time from a known point to target similar. The distance run in nm divided by the time taken in hours will give a basic VMG in knots.

After confidence has been gained the future voyages could be sub divided as known points passed are measured and timed.

I have two 'mains' GPSs and two battery models. I think you can see where my heart is.

 

[duncan]

no problem.

I have a lot of buttons on mine I never seem to find a reason to push - and even more on the humble video!

Which model do you have?

 

[JAYEL]

What you need is a Astro- Navigation calculator. Included in its programs are Dead Reckoning, Course and Distance Mid Lat and Great Circle Sailing, Altitude and Azimuth, Celestial Navigation and Most Probable Position. All done at the touch of a few buttons. Used it regularly until I acquired my GPS, but now gathering dust in one of my boats lockers.

 

[MedMan]

As others have said, even a basic GPS will give a read out of VMG updated every few seconds. However, if you are a glutton for punishment or just like maths, consider the following:

Velocity is what is know as a vector measurement, i.e. one in which the direction must be specified. This gives us the definition:

Velocity Made Good (VMG): The speed of a yacht relative to the waypoint it wants to reach, or toward or away from the wind.

Thus, you cannot just ask the question: "What is my VMG?" as that question has no meaning and therefore no answer. You must ask: "What is my VMG towards my next waypoint" or "What is my VMG towards the wind?" Thus:

VMG in a given direction = the distance travelled in that direction divided by the time taken.

Me? I've retired. I don't care how long it takes so long as I get there. If SWMBO objects I change the destination.

 

[graham]

If you record the exact time you leave and the exact time you arrive VMG is then simple time over distance arithmetic.....

Sailing is becoming so complicated and technical I think I will have to jack it all in .

 

[tsmyth]

What about the tacks involved?

Ray

 

[macca]

VMG to a waypoint is easy with a GPS. VMG to windward requires some maths. Try this (presumably with a programmable calculator):

awa = apparent wind angle from your instrument
aws = apparent wind speed from your instrument
bs = boat speed, from your GPS

twa = true wind angle
tws = true wind speed
vmg = vmg to windward

twa=arctan((aws*sin(awa))/(aws*cos(awa)-bs))
tws=(aws*sin(awa))/twa
vmg=awv*cos(twa)

Doesn't compensate for leeway.

 

[Anonymous]

Yes, that's a more interesting 'VMG' than the one given by a GPS but I suggest that 'Boat Speed' in your example would be best derived from the log (i.e. speed through the water) and not from the GPS (i.e. speed over the ground). But then it all depends on how you define 'VMG' and what you want to use it for. Personally, I think that velocity through the water in the direction of the wind is the most interesting figure because it tells you how well you are beating. When you are not beating, then boat speed through the water tells you most of what you want to know about sail trim. But the original questionner did not state what he wanted to know, or even why he was interested, so who knows? /forums/images/graemlins/smile.gif

 

[oldsaltoz]

I don't know of a very short and fast way to calculate VMG; but a simple (and rough) estimate can be done as follows.

Mark your position on the chart and your speed and heading plus some for leeway.
Mark the location of your destination, add a little if rounding a mark or headland.

Assume a tacking angle of 85* and draw a line above the mark to intersect the line you are on.
Measure the distance and divide it by your speed.

The whole thing takes a minute or two provided you have your position and the destination marked.

You also have to take sea state into consideration and adjust the length of your tacks to get full advantage.

It's years since we used this method but good to know if your GPS goes blank.

Hope this helps..........Avagoodweekend.

 

[graham]

I was ,shall we say ,not thinking too straight when I made the first reply.It is obviously a gross over simplification of the problem. Calculating an ETA directly to windward I tend to measure straight line distance then double it for a rule of thumb answer. Then stay on the tack that is closest to the mark while trying to leebow the tide as much as possible.

No real point in calculating to the nearest decimal point any slight wind shift or increase/decrease in tide will soon make a nonsense of your hard work.

 

[duncan]

[ QUOTE ]
Personally, I think that velocity through the water in the direction of the wind is the most interesting figure

[/ QUOTE ]

whilst I can understand this I have yet to find any SWMBO who takes that view..... /forums/images/graemlins/crazy.gif

 

[Anonymous]

[ QUOTE ]
whilst I can understand this I have yet to find any SWMBO who takes that view..... /forums/images/graemlins/crazy.gif

[/ QUOTE ]I agree totally - SWMBO want's to know whether we will be in time for a shower and change before going out to dinner but to make matters more complicated mine prefers the silence of not having the engine running. /forums/images/graemlins/laugh.gif So, if we need to beat to reach the destination then if I can make the best windward progress possible then I can run the engine for less time and get into harbour at an 'acceptable' hour. Meanwhile, SWMBO is happily reading or doing what SWMBOs do /forums/images/graemlins/wink.gif

 

[peterb]

[ QUOTE ]
VMG to a waypoint is easy with a GPS. VMG to windward requires some maths. Try this (presumably with a programmable calculator):

Doesn't compensate for leeway.

[/ QUOTE ]

The leeway question is important. If the boat speed falls significantly then leeway angle can increase dramatically, particularly when close-hauled. That's the major penalty for pinching.

If I'm really interested in making the best VMG to windward, then I set a waypoint a long way away (thousand miles or more) directly into the wind. Then I can use the VMG to waypoint as a measure of VMG to windward, and that does allow for leeway. Can be quite revealing; frequently sailing five degrees further off the wind brings the speed up and reduces leeway so that the VMG increases.