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GPS Speedometer

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Old 05-17-2004, 09:40 PM
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Default GPS Speedometer

I finally decided to dump the old fashioned water pickup and add a GPS speedometer to my boat. I purchased a gaffrig 90MPH Analog GPS speedo. The documentation led me to belive that I could simply interface it with my existing Garmin 162 GPS unit.

I've connected the unit according to the instructions and enabled the NMEA port on the garmin. I believ ethat it is communicating because the speedo has a red LED that is illuminated when there is no GPS signal. Once I power-up my Garmin GPS, the red LED goes off, as it should. The problem is that the speedo does not operate.

I'd rather not add a separate receiver for the speedo if I don't have to. I like the fact that my Garmin has an internal antenna already.

Does anybody know of a small unit with an internal antenna that definately works with the Gaffrig GPS speedo? I would even buy th enew Garmin GPS276C color GPS with th einternal antenna if I knew that it would interface correctly.

If I really have to add a separate receiver for the speedo, can anyone suggest a white unit that would blend with my gelcaot better than the black Gaffrig unit?

Thanks
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Old 05-17-2004, 10:33 PM
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Default Re: GPS Speedometer

I have the ANALOG version. I didn't get the digital unit for exactly that reason.
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Old 05-17-2004, 10:53 PM
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Default Re: GPS Speedometer

I don't understand why everybody wants a GPS speedometer for their boat. I WANT TO KNOW MY SPEED OVER WATER, NOT MY SPEED OVER GROUND. A GPS reads speed over ground not speed over water. You can take your boat to a river with a 6 knot current and go down current and pick up 6 knots on your top end speed, but if you turn around, you will lose 6 knots on your top end speed. I ALWAYS want to know my speed on the water vs my speed over the ground. Water is always in motion, and your boat is "ON" the water, so, wouldn't you want to know your speed on the water.

Robert Tanner,

Owner of one of those lumpy ass crooked resin buckets.
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Old 05-17-2004, 11:14 PM
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Default Re: GPS Speedometer

Tonto, I've always aske dthe same question. But the truth is that I just added superchargers to my boat and the speedo with the pitot tube bounces all over the place over 80 MPH. And that's the Gaffrig liquid filled unit.

I figured that I would rather have the consitency of A GPS speedo in the long run.
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Old 05-18-2004, 12:34 AM
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Default Re: GPS Speedometer

Tonto, doesn't your mechanical speedo with a pitot tube do the same thing on not as accurate? If you go up that same stream you are talking about with a mechanical speedo until it registers 6 knots are you getting anywhere? I bet the guy with the GPS unit set on 6 knots gets to the beach before you.
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Old 05-18-2004, 05:33 AM
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Re: GPS Speedometer

scarabmike,
I have a gaffrig gps speedo with a white receiver, paid $400.00for the
speedo & receiver works good & looks great.
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Old 05-18-2004, 06:02 AM
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Default Re: GPS Speedometer

BobBarker,

Where did you get the package? I only purchased the head unit because the receiver was black. Plus, As I said earlier, I thought that I coul dget away with using my existing GPS.
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Old 05-18-2004, 07:56 AM
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Default Re: GPS Speedometer

I've got a Gaffig/Livorsi GPS analog speedo hooked to a Garmin 176c. It works great! Even the recall button works. Sounds like you hooked up everything the same way i did. Maybe digital has another step somewhere.
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Old 05-18-2004, 09:36 AM
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Default Re: GPS Speedometer

Robert, get the hell outa Florida before you lose all your common sense Speedo's are suppose to tell ya how long before ya get someplace ! Ya know point a to point b, how the hell would you estimate your ETA without knowing the exact wind and current set ? When the next stop ( cocktail ) is 30 miles away and I'm running 60 mph gps I know i've got 30 minutes till Now thats what speedo's are for.

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Old 05-18-2004, 01:56 PM
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Default Re: GPS Speedometer

Quote:
Originally Posted by Tonto
I don't understand why everybody wants a GPS speedometer for their boat. I WANT TO KNOW MY SPEED OVER WATER, NOT MY SPEED OVER GROUND. A GPS reads speed over ground not speed over water. You can take your boat to a river with a 6 knot current and go down current and pick up 6 knots on your top end speed, but if you turn around, you will lose 6 knots on your top end speed. I ALWAYS want to know my speed on the water vs my speed over the ground. Water is always in motion, and your boat is "ON" the water, so, wouldn't you want to know your speed on the water.

Robert Tanner,

Owner of one of those lumpy ass crooked resin buckets.
Actually it doesn't measure speed over ground, it measures the speed at which the antenna moves in relation to a fixed position (sattelights)

The GPS cares not whether you are on land, sea or air..it measures the speed at which the antenna is moving and so you know the actual speed you are moving. If you relater this to aircraft your speed through the air is your indicated airspeed or for a boat the indicated waterspeed. The GPS measures true speed ie True Airspeed or true Waterspeed, not counting corrections for density of air or water which also affect your indicated speed by a pressure insturment. Aircraft have a pitot tube and a static pressure port to help with the indicated airspeed and correct for air density changes. This would relate to moving your boat between fresh and salt water as well as different water temperatures and therefore different pressure readings on your pressure speedometer.

With aircraft there are reasons why you look at both. Indicated relates to things like V speeds and how for example the airplane can be maneuvered. True speed is used to measure arrival times and precision approaches. I can see the same for a boat, ie I don't want to make a sharp maneuver in my twin step boat when my waterspeed is above XX, however I may also want to know that it is going to take me XX minutes by the GPS speed to reach my destination.

Neither is right or wrong..but useful for different puropses.

Last edited by Von Bongo; 05-18-2004 at 01:59 PM.
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