Garmin GPS 72H and VMG? - SailNet Community
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post #1 of 43 Old 06-15-2011 Thread Starter
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Garmin GPS 72H and VMG?

I was thinking about purchasing a Garmin 72H for use in a dinghy (primarily for distance, speed and VMG since I will remain in sight of the dock.) I downloaded the owner's manual from Garmin's website and found the following:

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To receive VMG data, you must be connected to a NMEA 0183- compatible water-speed sensor.
This seems a little ridiculous since the owners manual of the eTrex (a less expensive unit) says it can display VMG and I see no reason why speed over water would even be necessary to calculate VMG.

Could people who own one of these units comment? Say it isn't so!
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post #2 of 43 Old 06-15-2011
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I think the etrex points out that for vmg you need a waypoint in the GPS up ahead on your course from which the measurements are taken.

Think about it, VMG is always to some point other than your boat. In which case any GPS that you can set a waypoint and navigate to, will work for vmg.

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post #3 of 43 Old 06-15-2011 Thread Starter
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Think about it, VMG is always to some point other than your boat. In which case any GPS that you can set a waypoint and navigate to, will work for vmg.
I would think so too. It makes sense that a waypoint needs to be selected to calculate VMG (since VMG means "velocity made good" toward some point.) However as I said in my OP, the owner's manual for the 72H specifically says that a water speed sensor is needed. Why on earth would such a sensor be needed and how on earth would it help calculate VMG?

I am hoping someone can assure me that such a sensor is not needed for the 72H to calculate VMG (toward the selected waypoint.) Surely, the manual must be wrong...
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post #4 of 43 Old 06-15-2011
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For a sailboat under sail: No GPS can always calculate VMG to a waypoint if that waypoint is the thing that's marked as a destination and it's close-by. The reason being that sailboats tack. The only way that VMG works with a sailboat is if the waypoint is a long way away (as in thousands of nms), if the waypoint is more or less on your heading or if you tack or gybe back-and-forth frequently.

If you think about it for a while, you'll see it.

VMG on GPS' is for powerboats and is primarily for use in setting a course to compensate for current. There's a Garmin GPS DVD that explains it quite clearly.

I'll have to think about the water speed transducer and how it would be used in VMG calculations. I'm not seeing it, either.

Jim
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post #5 of 43 Old 06-16-2011 Thread Starter
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Originally Posted by SEMIJim View Post
For a sailboat under sail: No GPS can always calculate VMG to a waypoint if that waypoint is the thing that's marked as a destination and it's close-by. The reason being that sailboats tack.
If you are close-hauled and your VMG is zero, then you know that you are on the layline. If your VMG is negative, then you know you have passed the layline. That sounds pretty useful to me. Also as you point out, if you extend the waypoint to very far away to windward, you can basically use VMG to map out your polar diagram.

However, you can't do any of this if the unit refuses to display VMG because of some unneeded sensor. So I ask again, can somebody here who owns one of these devices confirm that the 72H shows VMG even if a water speed sensor is not connected?
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post #6 of 43 Old 06-16-2011
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The 72H does VMG as long as your going to a selected waypoint and the 7# series in general supports it going back to at least the year 2000 firmware

To receive VMG data, you must be connected to a NMEA 0183- compatible water-speed sensor.

I wondered about this also and its wrong

You will need to wonder through the menu and do a custom data field as VMG is one of a dozen or more display options

If your able to do the math and figure a waypoint FAR away from your real one it will do much better VMG

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post #7 of 43 Old 06-16-2011
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1,000 miles away to get a VMG? C'mon, that seems just wrong, as much as I think about it. To calculate VMG, the GPS simply needs a waypoint, regardless of how near or far away.

VMG is just as valid for sailboats as it is for powerboats, if not more so.

I'm a tad unclear as why VMG would be needed when "still in sight of the dock."

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post #8 of 43 Old 06-16-2011 Thread Starter
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Originally Posted by Stu Jackson View Post
1,000 miles away to get a VMG? C'mon, that seems just wrong, as much as I think about it. To calculate VMG, the GPS simply needs a waypoint, regardless of how near or far away.

VMG is just as valid for sailboats as it is for powerboats, if not more so.

I'm a tad unclear as why VMG would be needed when "still in sight of the dock."
The idea is that if you have a waypoint set that is very far away (1000 to 2000 miles for example,) to windward, then you can effectively gauge windward performance. Sailing lower will gain actual speed, but will it increase VMG upwind? Sailing higher will loose you speed, but it may increase VMG upwind.

If your goal is to sail fast to the upwind mark, this is useful information even if you are still in sight of the dock.
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post #9 of 43 Old 06-17-2011
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Guys,

Any racer will tell you that VMG is what it's all about. We use VMW all the time to determine how quickly we are moving TOWARDS WHERE WE WANT TO BE. If the mark is 1 mn away or 1000 nm away you want to maximize VMG. SEMIJim, the VMG information is helpful in determine WHEN to tack. If your VMG goes negative, you usually want to tack, because now you are sailing away from your destination.

When sailing downwind, VMG can be used to determine if you are better off sailing right down to the mark,. or 'heating it up' and sailing higher. With an Asymmetric, you sail faster when you sail higher, but if your VMG drops (even though your boats peed increases) you are not helping yourself.

Anyway, back to your original question, why not just get an Etrex? I have an old one that I bought back in 2004. I have all the race marks saved and I set up a screen with heading, bearing and distance to the mark, boat speed and VMG and estimated time to the mark. It's very helpful.

Barry

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post #10 of 43 Old 06-17-2011 Thread Starter
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Anyway, back to your original question, why not just get an Etrex?
It's on my list, but it doesn't seem to have an easy way to transfer data to my computer for later study. (Seriously, what computers have serial ports anymore?) The 72H has a USB port.

I'm looking for the least expensive solution that gives me speed, distance, VMG and allows for easy data transfer to a computer. Do you think something else will fit the bill better?
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