VMR What is it?

[Quoddy]

VMG What is it?

Someone said that VMG on a GPS doesn't take into account many factors such as currents.
We have a C70 with GPS, no wind or other inputs except radar. I though VMG was calculated by comparing your past position with your present position in relation to your Go To/Waypoint . Then comparing it with your speed to tell you how fast you are closing on your target in relation to how fast you are going over ground in general. If this is the case it would seem by default ,it blindly accounts for all influences?

 

[Todd Smith]

I don't know what VMR is. But in a sailing context, VMG is typically shorthand for "Velocity Made Good to windward". Calculating VMG requires wind speed and direction and boat speed and heading. A GPS can calculate "Velocity Made Good to a waypoint" without any other information.

Todd

 

[RichH]

Yes it is 'blind' to all other influences. It is a trigonometric calculation that is comparing the angle/velocity of your present course to the course angle between the 'start' of the leg and the waypoint. The calc. is a ratio of the hypotenuse (length) of the angle TO the ETA of the 'leg' (length) .... multiplied by the ETA (time) to the waypoint.

 

[Stu Jackson]

VMG again?

Blindly calculates? ...Made Good to windward?

C'mon, the machine calculates based on the numbers it has: speed and direction. VMG is just a vector calculation.

And to windward? Nope, VMG works in every direction. I think I understand your windward reference to sailing, but even in sailing VMG works, for upwind or downwind legs.

 

[Ross]

Re: VMG again?

My gps displays speed on the surface of the earth and it shows the direction of travel and if I have a goto point it tells me how far off course I am. It doesn't seem to matter if I am walking, driving my car, sailing my boat or motoring. It simply keeps track of the changing position relative to a gang of satellites. VMG is a factor determined by your off course direction.

 

[tsmwebb]

Someone said that VMG on a GPS doesn't take into account many factors...

Oy! Okay, here is are a couple of sketches that I hope will help with one of the places where GPS and instrument VMG differ. They may be hard to see as I have a slow link to the net and need to keep them as tiny as possible. If they're too small I can fiddle with them... Anyway, the set-up is this:

You are at waypoint 1 going to waypoint 2 which is 1 mile upwind upwind. You are traveling at 1 knot and your course is 45 degrees off the true wind (including leeway).

At way point 1 you read your VMG to waypoint 2 off your GPS. Since the distance is short we can assume the Earth is flat and that VMG = cos(45) * your speed or .707 * 1 or .707.

At point 2 (second sketch) your course and velocity and wind angle are the same and you read your VMG off the GPS. But your course to way point 2 has changed to 77.2 degrees. Your GPS shows that VMG = cos(77.2) * your speed or .222 * 1 or .222.

At point 2 your VMG with respect to the wind will still be .707...

Notce that at point 2 you have not yet reached the layline (the line at which you can make it to the mark on one tack). It is evident that you must reach a layline to reach waypoint 2. Your VMG to each layline is the same as your VMG with respect to the wind. If a tack costs nothing then you can tack as often or as little as you want within the two laylines without affecting the time it takes you to get to waypoint 2. In effect, your veloctiy made good to the top mark is the same as your VMG upwind anywhere inside the laylines. But, that isn't what your GPS will report.

A point that I think worth keeping in mind is that you will be ill served by attempting to maximize your GPS VMG to a waypoint if you will need to tack or jibe to reach that waypoint particularly if you are close to the waypoint or a layline. If you are sailing to someplace that you don't need to tack/jibe to then your boat speed and course as reported by your sailing instruments, because they report speed and course without accounting for any movement of the water, may be misleading and the GPS that reports speed and course with respect to the Earth more useful. In short, both sets of information are useful in some contexts but they are different and not interchangeable.

--Tom.

 

[John]

KISS

For us math retards, maybe there is a simpler way of explaining it:

Course made good (CMG) is the course sailed (or motored) from point A to point B, regardless of how many turns you may have taken or headings you have gone on in between. For instance, if B is due north of A, your CMG is zero degrees, but you may never have sailed on this heading; you may have sailed at 45 degrees and then at 315 degrees.

Velocity (or speed) made good, is the derived from the time taken to sail this CMG. If the distance of CMG is 10 nautical miles, and you covered it in 5 hours, then your VMG (or SMG) is 2 knots. However, you may have actually sailed (or motored) 20 miles, given that you haven't sailed directly. This is irrelevant.

I think this is right, no?

 

[Todd Smith]

And to windward? Nope, VMG works in every direction. I think I understand your windward reference to sailing, but even in sailing VMG works, for upwind or downwind legs.

The whole point of using VMG, instead of speed over the ground, is to optimize speed toward a waypoint that is upwind (or downwind), where the fastest course is not pointing directly toward the waypoint. If you are pointing directly at the waypoint, you don't need to calculate VMG seperatly.

Todd

 

[tsmwebb]

...
I think this is right, no?

I think you have it... VMC (said velocity made good on course or course made good) is usually used in a little different manner.

I'm attaching some more sketches (tiny again 'cause nobody complained last time ). The first just shows VMG wrt any two points as one might graph it. Math guys would normally rotate it so the triangles would be in the standard position but we sailors think of 0 degrees as "up" so I've drawn it that way. VMG's are mesured along the "up" axis and courses go up from 0 starting at "North" and going either East or West. Mathematically, for plane sailing the calculation is simply VMG = V * cos(theta).

In the second sketch I've placed two waypoints (WP1 and WP2). Velcocity made good on course would be the VMG between those points as defined above. Used correctly, your GPS will read the VMC between waypoint 1 and 2. BUT, you may well notice that that isn't your VMG upwind NOR is it the VMG you will actually achieve between waypoint 1 and 2. I've labled the VMG's assuming a boat speed of 1. The mean of the Port and Stb VMG's reported by your GPS will be your VMG upwind and the VMG you will actually achive all else being equal in this case.

[BTW, just noticed that I made some typo's in the sketches. In the first one I don't know how I got "layline" in there -- cut and paste strikes again -- but ignore it. In the second I messed up the tacks on the GPS VMG lables. The one on the left is for starboard and on the right for port -- sorry ,I'll change them if anyone says they care....]

--Tom.

 

[Joe]

Geez... some pretty complicated explanations for a simple term. VMG is your speed to the waypoint.

 

[John]

Not exactly

Geez... some pretty complicated explanations for a simple term. VMG is your speed to the waypoint.

Maybe I misunderstood what you're saying, but I think that could be a confusing way to say it. You might never travel at the VMG rate; you could be going twice as fast as that rate all along. Speed (or velocity) is distance divided by time. The distance we are talking about is the distance between point A and B. But you might travel twice this distance, since you might not be able to go directly. But this doesn't matter in calculating the "made good"; you are calculating based on this direct distance - distance as the crow flies.

 

[Todd Smith]

VMG for performance

Geez... some pretty complicated explanations for a simple term. VMG is your speed to the waypoint.

Not necessarily. As was mentioned earlier, you can calculate VMG directly upwind or VMG directly downwind. If you are trying to sail ( without turning on the engine ) to an upwind ( or directly downwind ) anchorage or some other destination before dark, or if you just like to get the most efficency out of your sailboat, then you can use VMG to windward to help.


Describing how to use VMG, but others have written it better already. See these links as an example:
http://www.wingsailor.com/html/vmg___gps.html
http://knol.google.com/k/john-navas/why-vmg-matters/qefv1b2dsnit/2#

It's not just for racing, makes cruising more fun too.

Todd

 

[tsmwebb]

Describing how to use VMG, but others have written it better already. See these links as an example:
http://www.wingsailor.com/html/vmg___gps.html
http://knol.google.com/k/john-navas/why-vmg-matters/qefv1b2dsnit/2#

Hmmm... I like the second link. The first link has almost got the right idea of putting the waypoint very far upwind. The author misses out on at lest on important point -- the wind can change and the result will be what the second link calls "cone error". I attahced a little doodle above which graphs out the problem.

I have also just done a quick sketch to show several course and speeds all with the same VMG because I have this feeling that there is still some confusion about the basic concept... I should have labeled the VMG for zero degrees (ie. heading directly to the waypoint). It is 0.71 knots. Does this help anyone at all?

--Tom.