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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics
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  #31  
Old 04-08-2012
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Re: Garmin GPS 72H and VMG?

Quote:
Originally Posted by danielt63 View Post
I'll jump in with a recommendation here... For an upwind/downwind course, you only need one waypoint.

The upwind and downwind marks make a line. Project that line as far upwind as you can and put your waypoint there. Going to the upwind mark then means maximizing VMG and downwind means minimizing VMG, no matter what direction the wind is actually coming from.
I suggest you reread this topic. I earlier mentioned that there are TWO different VMGs that can be used, going upwind.

1 --- The 1,000 mile away waypoint which should be based on wind direction (assuming it is constant, which we all know it is not). The CONCEPT here is to get the best UPWIND VMG based on the wind direction, since the start line is not always exactly perpendicular to the true wind direction, and the first waypoint is not always exactly dead upwind. It alerts you to the favored tack. The 1,000 mile away waypoint should be on the same bearing as the true wind direction.

2 --- The waypoint to the first upwind mark, to be used to judge the layline (currents notwithstanding).

They are two SEPARATE and distinct uses of the VMG function from a GPS unit.
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  #32  
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Re: Garmin GPS 72H and VMG?

SinbadOz. Setting the distant waypoint is easy if you know the lat/long of both marks. Before the race, you should make at least one lap of the course. While making the lap, get the lat/long of both marks.

Now find the difference in longitude between the two marks, and the difference in latitude. Multiply the difference by 1000 and apply that number to the upwind mark's location. This will give you a lat/long pair that is 1000 times further away than the length of the course. (An example might be in order, just let me know.)

Yes, just set the device to goto the waypoint.

This system will also work for round the buoys races. For example if you are sailing an olympic triangle, then you will have three waypoints. One is far up the line between the downwind mark and the upwind mark. One is far down the line between the upwind mark and the reaching mark, and one is far down the line between the reaching mark and the downwind mark. After each rounding, you will have to set the GPS to goto the next target (because you will never actually reach it,) but otherwise you still try to maximize VMG in each case, except when going from the upwind mark to the downwind mark, you set your goto point to the upwind waypoint and try to minimize VMG. Yes, this gives you (or your crew,) something extra to do on each rounding.

The nice thing about this system is that when the wind shifts, you don't need to change any of your waypoints, unless the RC moves the marks.
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Re: Garmin GPS 72H and VMG?

Quote:
Originally Posted by Stu Jackson View Post
1 --- The 1,000 mile away waypoint which should be based on wind direction (assuming it is constant, which we all know it is not). The CONCEPT here is to get the best UPWIND VMG based on the wind direction, since the start line is not always exactly perpendicular to the true wind direction, and the first waypoint is not always exactly dead upwind. It alerts you to the favored tack. The 1,000 mile away waypoint should be on the same bearing as the true wind direction.
And what I'm saying is that the waypoint above should *not* be based on wind direction, precisely because (as you mention) it isn't consistent. Instead, put the waypoint far above the line created by the upwind mark and downwind marks. You don't necessarily want the best upwind VMG (after all, if the wind shifted 60 degrees during your upwind run, making best upwind VMG would be a huge mistake!) What you want is the best VMG up the line to the mark. My waypoint placement will give you that, no matter where the wind is coming from.

As for your second waypoint. Yes a waypoint at the mark to help judge laylines is nice, but few GPS units will give you VMG toward two different waypoints at the same time.

Last edited by danielt63; 04-08-2012 at 12:45 PM.
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  #34  
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Re: Garmin GPS 72H and VMG?

Quote:
Originally Posted by danielt63 View Post
And what I'm saying is that the waypoint above should *not* be based on wind direction, precisely because (as you mention) it isn't consistent. Instead, put the waypoint far above the line created by the upwind mark and downwind marks. You don't necessarily want the best upwind VMG (after all, if the wind shifted 60 degrees during your upwind run, making best upwind VMG would be a huge mistake!) What you want is the best VMG up the line to the mark. My waypoint placement will give you that, no matter where the wind is coming from.

As for your second waypoint. Yes a waypoint at the mark to help judge laylines is nice, but few GPS units will give you VMG toward two different waypoints at the same time.

Thanks for that detailed explanation. Great point.

It seems that there are really three useful VMGs.

I agree about the visibility of those two or three VMGs. One one unit, you'd have to switch back and forth. My GPSMap 76 Cx has a handy and easy to use "Recent Waypoints" funtion. Or, you could use a backup GPS.

Always nice to learn something new.

Thanks again.
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  #35  
Old 04-09-2012
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Re: Garmin GPS 72H and VMG?

Thanks guys - it's becoming clearer.

As mentioned I sail on a Viper 640 sports boat (really a 21ft open transom dinghy but with a 120kg bulb keel) where all three on board have their hands full, so being able to do the lat/long math's and key into the GPS in the short time available - from course laying to warning signal - will be a tad difficult. (I'll need a calculator as well!!) But an example may be useful for other times, so if you can provide one, that would be appreciated.

By round-the-bouys races I mean our regular club races (4 courses of 10-15 Nm from which RC choose one just before the warning signal based on wind direction & strength) around a combination of 10+/- fixed marks (navigation pylons and permanently tethered bouys), sandbanks, headlands, etc. - so finding the next mark, and knowing the current angle to it, are useful bits of info when you can't actually see it. Hence the point at the end of my last post which danielt63 stated as:
Quote:
...few GPS units will give you VMG toward two different waypoints at the same time.
Cheers
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Re: Garmin GPS 72H and VMG?

Since you say the marks are fixed, why not just go out one day and mark each into the GPS and make them named waypoints?
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Re: Garmin GPS 72H and VMG?

Stu

Instead of doing that time-consuming task I hope to upload all required waypoints and routes from my PC once I get a copy of the data from a friend who already has them on file (or upload them from his PC). That's the easy bit.

The issue I was pursuing (and what led me to this thread) was how to get accurate VMG to the current mark (as I'd read elsewhere previously about the different VMG's), which is a critical issue for us sports boat sailors using asymmetric kites downwind. Does heating up the AWA for a speed gain improve VMG to the mark, versus sailing deeper but slower - especially in lighter winds?
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Old 04-10-2012
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Re: Garmin GPS 72H and VMG?

GPS VMG to a waypoint is almost useless in normal short course racing. GPS VMG has a little bit of use when reaching and there is a current, but for a typical windward-leeward race course, VMG relative to the wind is much more useful information.

On upwind and downwind legs there is always a trade off between a little more speed through the water, vs a higher or lower angle. It is the wind derived VMG that is the critical analytic that helps make a determination of the most advantageous target speed/angle in any particular wind and sea state. To obtain a wind related VMG requires a wind instrument and a knot meter. Speed over the bottom does not come into play.
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  #39  
Old 04-13-2012
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Re: Garmin GPS 72H and VMG?

I just discovered an easy method for creating the far away waypoint (as discussed in this thread) on my new Garmin 72H to assist in seeing VMG to windward.

The 72H has a neat feature under Waypoints called "Project waypoint", wherein from the CURRENT location (but I think you need to initially mark/create a waypoint for that spot) you can "relocate" that waypoint (it doesn't actually create a new/additional waypoint but just changes the position of the one you're projecting!) by XX distance (say 2000 Nm) on XXX bearing. The Save.

Simple!
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Old 04-15-2012
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Re: Garmin GPS 72H and VMG?

Quote:
Originally Posted by SinbadOz View Post
I came across this thread as I too am trying to understand how to get accurate VMG on a 72H (using far, far away point) whilst still seeing info about next waypoint on the course. (I've used both Garmin 76 and 72H models of skippers I've sailed with, but have just bought my own 72H for AUD$149 from Whitworths)

Re using Routes on round-the-bouys races, the trick is to:
  1. create a waypoint that is the midpoint of your starting line (difficult if it's a variable position mid-river start!)
  2. Starting navigation of the selected route as you go across the start line - must be within proximity (50m+/-) of the StartingLine waypoint created in 1 in order to trigger the navigation to the first mark.
  3. The unit should then:
    1. automatically switch focus to the waypoint for the first rounding mark
    2. thereafter switch focus to each successive waypoint after passing through the proximity circle around the current waypoint (i.e. enter the circle and then leave it.)
I agree that the route should work this way, but if you have a course where you round the same mark multiple times the poor little 72H gets confused and starts going to incorrect marks. I have spoken to and had numerous correspondance with Garmin Oz tech support, but finally gave up with them. I have tried several other Garmin units and many have the same issue.

I now have each mark position stored as different waypoints, ie mark A is stored as A1, A2 & A3 and use this within the route. This works much better but very infrequently jumps to the incorrect mark.
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