Need GPS Speedometer advice

[bill 22]

I want to upgrade to a GPS speedometer to replace my pitot tube speedometer that is not working. I do a lot of skiing so, I want an accurate speedometer. I do not have a tournament ski boat so, I don?t need a Perfectpass or other type of cruise control. I have been using my Garmin nuvi but in the sun it is very hard to see. I am almost always on the same river and don?t have much need for GPS navigation. I?m looking at the Teleflex P#781-684-035P GPS speedometer but with the GPS receiver it is 2x what a GPS cost! Any advice is appreciated.

Thanks,
Billy

 

[JB]

Re: Need GPS Speedometer advice

Hi, Bill.

Welcome to iboats.

I don't understand your question. Any GPS can give you accurate mph, kph or knots, depending on user programming. It doesn't even need to be a chartplotter. I don't understand how a GPS receiver can cost twice what a GPS costs. . . .??

I would check out Garmin for a simple, alpha-numeric display unit.

You might also consider some GPS apps for your smart phone.

 

[bill 22]

Re: Need GPS Speedometer advice

JB, I was talking about the complete package.

The cost of the Teleflex gps receiver P#68755P $150. + gps speedometer $100 = 2x the cost of a new Garmin ($100). I am looking for a simple gps speedometer that is easy to read in direct sunlight. The Garmin I have has a dashboard type of mode with larger numbers but the screen is still hard to read in sunlight while driving and keeping an eye on a skier. Does anyone know of a simple digital gps speedometer for around $100 or less.

I don?t have a smart-phone but I think it may have the same problem as a gps (dim screen in bright light).

I may need to stick with the garmin and mount it below the dash and try to get it out of the sun.

Thank you,
Billy

 

[dingbat]

Re: Need GPS Speedometer advice

The problem with the cheaper units is the displays are not "daylight" viewable. The problem goes away once you start throwing money at it.

 

[bill 22]

Re: Need GPS Speedometer advice

Yes, that is why I was asking for help...I am trying to avoid ?throwing money at it.? I could find other toys to spend money on.

 

[Hank496]

Re: Need GPS Speedometer advice

I'm not a skier so pardon me if this is a dumb point. A GPS speedometer is going to give you Speed Over Ground (SOG), meaning the speed the boat is moving across the face of the earth including the water speed. Your pitot tube speedo gives you speed through the water. SOG also includes the speed of the current, so wouldn't a pitot, or paddle wheel speed pick up be better for skiing?

 

[jhebert]

Re: Need GPS Speedometer advice

Actually, a GPS receiver does not measure speed at all. A GPS receiver only tries to deduce speed by comparing the distance between the boat's current position and a recent in-the-past position and the time elapsed, a method known as the time and track point method. GPS deduced speed accuracy, about 0.5-MPH plus or minus, is generally good enough for most uses. Also, as mentioned, speed deduced by a GPS is speed over ground, and for water skiing this may not be a good replacement for a pitot tube speedometer.

There are a few very specialized GPS receivers that directly measure speed using Doppler shift techniques. These devices are more accurate than the standard GPS receiver which does not measure speed directly.

For more on GPS receiver speed accuracy see

http://continuouswave.com/ubb/Forum6/HTML/002179.html

 

[JB]

Re: Need GPS Speedometer advice

I don't recall ever disagreeing with you (in about 12 years), Jim, but please allow me to nit pick a little. We might disagree a just a bit.

The basic, repeated calculation done in a GPS receiver is location, in 3 dimensional space, based on measured distance from 3 or more satellites (as many as 12).

Far simpler calculations determine distance between measured locations over time. The calculated speed is based on the distance between measured locations, including X,Y and Z axis movement. So changes in altitude would be included (I agree that wouldn't be very likely when afloat. ). I am not sure whether your "deduction" and my "calculation" are the same thing.

There is a common misunderstanding:

The "built in" error in location (for civilian use) of up to 3 meters (with WAAS) translates to error in speed calculations. Not so, because the error is constant from one measurement to the next (or the displayed track would be a sawtooth), so the distance calculation is accurate to a few micrometers and speed calculation to millimeters per hour.

No, I didn't lift this from some techno-blog. I deduced it from what tech data I know and some grade school fizix ( ).

 

[craze1cars]

Re: Need GPS Speedometer advice

JB, I was talking about the complete package.

The cost of the Teleflex gps receiver P#68755P $150. + gps speedometer $100 = 2x the cost of a new Garmin ($100). I am looking for a simple gps speedometer that is easy to read in direct sunlight. The Garmin I have has a dashboard type of mode with larger numbers but the screen is still hard to read in sunlight while driving and keeping an eye on a skier. Does anyone know of a simple digital gps speedometer for around $100 or less.

I don?t have a smart-phone but I think it may have the same problem as a gps (dim screen in bright light).

I may need to stick with the garmin and mount it below the dash and try to get it out of the sun.

Thank you,
Billy

Do you have the backlight on with the Garmin? Backlight on full bright makes a substantial difference in visibility in full sun. Not sure which specific model you have, but usually this means you cannot be running it on internal battery or the backlight goes off after about 10 seconds to save battery...but if you have it plugged into cigarette lighter or similar, and then the backlight will stay on full time.

I have 2 different Garmins, and that's how they work. Have no problem seeing them in the sunlight....use them on my motorcycle, snowmobile, and sometimes even on the boatm, but the backlight MUST be on.

I also have a Humminbird fish finder that has GPS capabilities, and one of the screens provides speedo number BIG AS DAY....impossible to miss. But they ain't cheap either...unless you also need/want a nice fish finder.

All that said, I have GPS Perfectpass Stargazer on my I/O for skiing. Hands down the best investment I have EVER made on my boat. And only $1,500 or so....

Probably not the price you were looking for.

 

[jhebert]

Re: Need GPS Speedometer advice

...the error is constant from one measurement to the next (or the displayed track would be a sawtooth), so the distance calculation is accurate to a few micrometers and speed calculation to millimeters per hour...

You can see the position error magnitude of the deduced position if you just record the track of your GPS receiver position while it sits in one place for a few hours. The track will show a random error pattern with variations of many meters.

 

[bill 22]

Re: Need GPS Speedometer advice

Do you have the backlight on with the Garmin? Backlight on full bright makes a substantial difference in visibility in full sun.

Thank you! I should have thought about this. I have a Garmin nuvi 1200. I know it was made for a car and I am only using it on my boat for speed and not navigation. I have not been plugging it in on the boat.

Sometimes it is the simple things we overlook!

 

[JB]

Re: Need GPS Speedometer advice

You can see the position error magnitude of the deduced position if you just record the track of your GPS receiver position while it sits in one place for a few hours. The track will show a random error pattern with variations of many meters.

AHA!! You are correct, as usual, Sir. I should have known better than to doubt it.