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Board » General Discussion » VMG

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Posted by
larx


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2008-08-31 21:38:48
2008-11-30 10:56:05 - VMG
the VMG you can see (under Boat speed) in polar diagram, is that generated against last destination or which waypoint?

Pls explain how this VMG is calculated.

--- Last Edited by larx at 2008-11-30 10:59:49 ---
Posted by
Pergite


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2008-10-07 12:19:07
2008-11-30 13:21:35 - Re: VMG
The VMG value is the boat speed in relation to the direction of the wind.

Its calculated with this formula:
"speed*(cos(twa))"

//Göran
Posted by
NOR184


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2008-09-07 20:28:31
2008-11-30 14:58:41 - Re: VMG
Yes, ok. But to which point? Next waypoint to follow?. I mean in order to calculate VMG you need two points to calculate to and from. One is my currnt position, but the second?
Posted by
Pergite


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2008-10-07 12:19:07
2008-11-30 15:07:58 - Re: VMG
As I understand it and according to the formula, you don't. VMG is "way made good" in relation to the wind direction or the angle to the wind, twa. So you don't need two positions, only boat speed and twa.

Though I guess you need two positions or a course to calculate VMC, "way made good to course" or something like that, but I don't know the formula for it.

//Göran
Posted by
NOR184


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2008-09-07 20:28:31
2008-11-30 15:16:57 - Re: VMG
OK. What I (and maybe larx) don`t understand is if the VMG is 2kn and my boat speed could be 4kn (if different course). Then I mean this has to refer to a certain waypoint.
Posted by
kalle


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2007-06-01 09:58:04
2008-11-30 15:41:03 - Re: VMG
VMG is velocity made good _against_ _the_ _wind_.
Posted by
6times7


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2008-10-11 16:26:11
2008-12-05 19:12:36 - Re: VMG
kalle said @ 2008-11-30 15:41:03:
VMG is velocity made good _against_ _the_ _wind_.
.
.
.
Think that clears it up... but if you really want, imagine a mark in the direction of the wind at your current location, VMG is speed towards that mark. -ve VMG is towards a mark downwind, +ve VMG towards a mark upwind.

--- Last Edited by 6times7 at 2008-12-05 19:13:05 ---

--- Last Edited by 6times7 at 2008-12-05 19:13:26 ---
Posted by
HolidayTours


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2007-08-12 16:24:04
2008-12-09 13:19:31 - Re: VMG
Do any have the formula for VMC? Or could you calk. it by change the TWA to the dir. to mark.

Rolf

EDIT: I found it. Its COS(angel of your boats directions) * Speed
or COS(60) * 5 = 2.50 knots made north (vmg)
if mark is N (360) and you sail 60 deg. in 5 kn.

And BTW VMG is "Velocity made good"

--- Last Edited by HolidayTours at 2008-12-09 13:44:57 ---

--- Last Edited by HolidayTours at 2008-12-09 13:47:36 ---
Posted by
jakob


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2007-09-04 10:05:30
2008-12-09 14:08:33 - Re: VMG
Hi
If you want to calculate your speed component parallell to the wind direction, here called VMG you simply use VMG=boat_speed*cos(TWA). Yes, you need the cosinus and you need to make sure that you and your calculator use the same units, e.g. degrees for the TWA. You can test that by verifying that cos(90)=1.

The boat speed relative to course VMC is somewhat more difficult to calculate. However, if one simplifies things and assume that YOU know what Course from boat To Target (CTT) you are interested in life is easy again. You can e.g. get CTT by using the ruler-tool in the game.

Now you need only to calculate your speed component parallell to CTT, lets call it VMC, which is almost as simple as VMG. If we define the boat course as CC you simply calculate VMC= boat_speed*cos(CTT-CC). Piece of cake :-) ... as long as you remember that the precision of these calculations decay with increades distance between boat and target. Also, negative VMG & VMC means that you are heading away from target.

Note:
The advanced reader might observe that the VMG-calculation is nothing but a special case of the more general VMC-calculation, where CTT=-TWD and thus the TWA=-TWD-CC.

/Jakob
...one of the guys behind the game...
Posted by
HolidayTours


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2007-08-12 16:24:04
2008-12-09 22:02:34 - Re: VMG
Thanks Jacob

Now we know ho to calculate the VMC. But to find out the best one you need to do a lot of calculatens in different directons (or). As your boatspeed is different (as the polar shows)at differents angel. I.e if you do 5kn with 55 deg to mark or you do 6.5 kn at 60 deg you had to calc both to find hos is fastest VMC.

Any easy way to do that?
/Rolf

--- Last Edited by HolidayTours at 2008-12-09 22:08:09 ---

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