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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics > Garmin GPS 72H and VMG?
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Thread: Garmin GPS 72H and VMG? Reply to Thread
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Topic Review (Newest First)
04-17-2012 07:35 AM
SinbadOz
Re: Garmin GPS 72H and VMG?

The 72H I just bought is running v2.6 - but I haven't been on the water with it yet. The one I have used whilst sailing (another skipper's) is a couple of years old, so expect its software is an earlier version.
04-16-2012 08:00 PM
No Drama
Re: Garmin GPS 72H and VMG?

Thanks, that very similar to what we have been doing. My proximity is set lower but does beep at each mark showing that we have reached the mark. Will try it set to 50m.

What software version are you running?

I originally had ver 2.5, but Gramin support got me to rollback to 2.4 and then I couldn't get back to 2.5 as the website didn't know that 2.5 was realeased. This was late last year and I haven't tried to update the software version since.
04-16-2012 07:38 AM
SinbadOz
Re: Garmin GPS 72H and VMG?

Steve

I too gave up on Garmin Support - the solution came by pure chance from another skipper with a 72H!

Several of the round-the-bouys courses we sail use at least one mark several times - but I have not had the issue you mention since I:
  1. Set the proximity distance for waypoints to a practical distance (50m)
  2. Created a start line waypoint (mid-line)
  3. Added the start line waypoint as the first waypoint of each course
  4. Added each mark/waypoint in order (including any repititions, and a Finish line mid-point) to each course
  5. 'Start Navigation' of the chosen course whilst crossing the start line AND within the specified proximity circle of the start line waypoint (If I miss doing this because I'm too busy at the start - or just plain forget! - , I use 'Goto' from the Active Route page to jump to the first mark/waypoint to get the Route navigation back on track)
  6. Ensure that I pass within the proximity circle of the each waypoint when rounding that mark. (If this doesn't happen because I went really wide of the mark, I use 'Goto' from the Active Route page to jump to the next mark/waypoint to get the Route navigation back on track)

Having done all the above, the 72H switches religiously to the next mark/waypoint as I either pass the mark or exit its proximity circle (I can't recall which)

You shouldn't need to do the following:
Quote:
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.
04-15-2012 09:13 PM
No Drama
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.
04-13-2012 07:59 AM
SinbadOz
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!
04-10-2012 09:51 AM
Jeff_H
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.
04-10-2012 08:52 AM
SinbadOz
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?
04-09-2012 03:54 PM
Stu Jackson
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?
04-09-2012 01:08 AM
SinbadOz
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
04-08-2012 08:19 PM
Stu Jackson
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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