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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics
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  #11  
Old 06-17-2011
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Well, I've got my eye on a Garmin Montana, but they're kinda spendy. Then again: I want a GPS that'll do boat, car and trail. (I don't think it's been released, yet, either.)

We've got a 498C (fixed mount) on Abracadabra. The next GPS will be more-or-less my "personal" GPS, and will be used on the little powerboat, as well. I'll also try to use it in the "polars substitution mode" (i.e.: waypoint a couple thousand clicks off, on the bearing to the mark) on Abracadabra.

Jim

Last edited by SEMIJim; 06-17-2011 at 11:32 AM.
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Old 06-17-2011
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"and I see no reason why speed over water would even be necessary to calculate VMG."
Purely semantics is what it is. They are defining VMG as "Velocity made good through the water" which is not the same as "velocity made good over ground". I won't argue which definition is or should be the "right" one but "velocity" is always a relative term, and without specifying the context it is meaningless.
Apparently Garmin have hired at least two writers, or changed the definition of "what is is" if they are using VMG in one place to mean "Velocity Made Good over ground To Mark" while in another place, arguably more correctly, indicating that your VELOCITY also may reflect the current bucking against you.
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Old 06-17-2011
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Just to clarify, you guys were talking about two completely different VMG issues.

The thousand miles away waypoint VMG is to determine the favored tack.

The "local" VMG is to determine either the layline (as the OP correctly noted) to a nearby waypoint and can be used to determine favored tack if the waypoint is a reasonable distance away.

Two separate uses of the VMG function.
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Old 06-17-2011
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Quote:
Originally Posted by Stu Jackson View Post
Just to clarify, you guys were talking about two completely different VMG issues.
True, but...

Quote:
Originally Posted by Stu Jackson View Post
The thousand miles away waypoint VMG is to determine the favored tack.
You don't need VMG to determine favored tack. You should know that before you start or round a mark.

For most boats on either tack on any windward leg there is a best course to steer that is usually a compromise between as high as she can point and as fast as she'll go. Racers sometimes pay big bucks to have polars generated to tell them what that course is for any given set of conditions. The thousands-of-clicks-distant-waypoint method is used to determine the same thing, on-the-fly, much less expensively.

Jim
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Old 06-23-2011
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Quote:
Originally Posted by danielt63 View Post
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:



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!
I am also in the market for a basic handheld and had the same question re VMG but also another question. The downloaded 72H manual doesn't list "heading - the direction you are travelling" in the list of data field options (see pg 31).

Can anybody please confirm this is a mistake in the manual & the unit will display the heading on the various pages.

My idea was to use the Trip Computer page to display the following fields as an aid to around the bouys racing. Can anybody confirm that this is possible?
• Bearing*—the compass direction from your location to your destination.
• Heading—the direction you are traveling.
• Next Dist*—(Next Distance) the distance between your current location and the next
waypoint.
• Speed—shows how fast you are going.
• VMG*—(Velocity Made Good) the speed you are closing in on a destination along a desired course. Also referred to as the ‘vector velocity’ to your destination.
Fields marked with an asterisk (*) only display data while you are actively navigating.
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Old 06-24-2011
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Quote:
Originally Posted by No Drama View Post
I am also in the market for a basic handheld and had the same question re VMG but also another question. The downloaded 72H manual doesn't list "heading - the direction you are travelling" in the list of data field options (see pg 31).
The GPS can't know your 'heading' without input from a flux gate compass. Your heading is the direction the bow is pointing.

The data field you're seeking will be something like 'COG' or 'CMG' (Course Over Ground, or Course Made Good) This will essentially be your TRACK (another possible label), or the path you are on over the bottom - which is actually what you're looking for anyway.

EG in a 2 knot cross current you might be 'heading' 180 but your COG could be 175 or less depending on your speed and the amount of current. At the end of the day you'll end up where you're going, not necessarily where you're pointing.....
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Last edited by Faster; 06-24-2011 at 12:12 AM.
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Old 06-24-2011
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Thanks for prompt reply.
I had realised it's based on over the ground movement. The data fields I listed was taken straight from the etrax v3 manual and thus the "heading" terminology was garmin's. I would also normally refer to it as COG.

I tried 6 retail outlets yesterday (west australia) and none will allow the unit to be switched on to confirm it's operation.
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Old 06-24-2011
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Ok. For starters, I actually have a GPS 72H. It's essentially the same beastie as any of the others in the Garmin range but you can't download maps (if you could, it'd be a GPSMAP see?)

We use it for round-the-buoys racing, by entering in the co-ordinates as waypoints using Mapsource and downloading over USB. If you knew the course in advance, you could setup a Route, but we usually don't so I just pick the next mark.

1. The 72H basically gives you actual, average and max (since turned on) Speed-over-ground in kts to one decimal place. The 'compass' heading is direction to next waypoint and it's really easy to see at a glance if you're off course; you also get distance to next mark and ETA.

2. Although there is no map, the 'Track' it records is enough to show you if you are lifting or being knocked toward the next mark. It will tell you if you are off-track, but, no, it doesn't tell you which direction you're traveling; that's what a compass is for.

3. It will display depth (for the anchor alarm function) and speed-over-water with the appropriate NMEA sensors as suggested - but I think of those as being a 'future upgrade' and not essential.

I'm quite happy with it. It powers up quickly, is salt-resistant, tells me all I need to know and holds plenty of data, although Mapsource is awkward to use and I wish there were better software out there..

If there is anything specific that you'd like to know - ask away!


EDIT: I find that the 'compass' display is the most often used one since it shows current speed, time to next mark and - more importantly - the direction!

"Where's the mark?" "Over THAT way!!!"
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Last edited by Classic30; 06-24-2011 at 02:38 AM.
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Old 06-24-2011
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Quote:
Originally Posted by No Drama View Post
I am also in the market for a basic handheld and had the same question re VMG but also another question. The downloaded 72H manual doesn't list "heading - the direction you are travelling" in the list of data field options (see pg 31).

Can anybody please confirm this is a mistake in the manual & the unit will display the heading on the various pages.

My idea was to use the Trip Computer page to display the following fields as an aid to around the bouys racing. Can anybody confirm that this is possible?
• Bearing*—the compass direction from your location to your destination.
• Heading—the direction you are traveling.
• Next Dist*—(Next Distance) the distance between your current location and the next
waypoint.
• Speed—shows how fast you are going.
• VMG*—(Velocity Made Good) the speed you are closing in on a destination along a desired course. Also referred to as the ‘vector velocity’ to your destination.
Fields marked with an asterisk (*) only display data while you are actively navigating.
My 6 year old Garmin eTrex Legend does all of this. I would assume any current model will as well.
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Old 06-24-2011
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"I tried 6 retail outlets yesterday (west australia) and none will allow the unit to be switched on to confirm it's operation."
I wonder if that is a Garmin warranty issue, since some of their Nuvi units "remember" when they were first turned on, and that gets transmitted as part of the warranty/registration information to Garmin-US here?

Some higher-end handheld GPSes have separate flux compasses and altimeters built into them, perhaps some handheld or marine units with a built in "real" compass also have different abilities to show real heading, as opposed to track?
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