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  1. #1
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    Speedometer questions

    My Allison Grand Sport has a pitot style speedometer. I want to replace it with a GPS speedometer. The pitot pickup bracket has broken and I don’t really want the pickup hanging down as they do.

    How accurate are the pitot style pickups? Is it possible that they could be as much as 10-15 mph off?

    Secondly, where do people usually mount the pickup antenna for a GPS speedometer on a grand sport? I’ve read that they need a clear view of the sky.

  2. #2
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    I had a Nordskog that was very accurate. Most pitots are not.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

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  4. #3
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    My SS2000 pitot was dead on with a hand held gps up to 50 mph, and then grew increasingly optimistic. An indicated 94 was actually 90. Not bad.

    I have the gps antenna under the front deck of my SS2000, and my brother’s Grandsport also has the antenna under the front deck.

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  6. #4
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    Nordskog and Gaffrig dry speedos are very accurate with in 1 or 2. The liquid gaffrig was the most accurate. The VDO is the most optimistic when i compared them to radar years ago.

    On antenna placement. They will work thru fiberglass. Mine is under my deck just forward of windscreen on my 28.
    Jim

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  8. #5
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    I have seen Pitot speedometers be off by as much as 15 MPH. They are more accurate at slower speeds. The faster you go the bigger the error can be. My Faria Beede GPS speedometer in my boat is exactly accurate to the GPS speedometer in my phone. It has an internal antenna inside the speedo case. No external antenna needed. Unfortunately, it only goes to 60MPH, so I need to replace it.

  9. #6
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    I like my Monster Livorsi GPS. It runs through a diagnostic/ calibration sequence every time the boat is started and updates very quickly. Antenna is mounted with 2 way tape on the dash.

  10. #7
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    I have a cheapo "eling" GPS speedometer and it is accurate as phone. I also have pro comp marine GPS on another boat and looks better and more quality but same readout

  11. #8
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    my go pro is about 1 mph less than my Pro Comp Marine GPS in the dash of my Allison on top speed. Speedo in the boat says 103 and the go pro only says 102..... Why couldn't it be the other way! haha.
    1973 Viper - sold
    1978 Viking - sold
    1995 XB02

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  13. #9
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    remember there a two speed calcs, OTB over the bottom, and OTW oner the water, 1 example; GPS only measures OTB, if your river running GPS wil show faster with the current, where a pilot tube will show the speed the same, GPS doesnt calculate for the moving water for prop testing, a pilot tube speedo is the way,OTW. A 5 MPH current GPS will show say 55 downstream, the pilot tube will show 50 each directi,on. BOTH gauges are correct. actually the pilot tube measurement is a true OT W speed. went to USN classes ArtK. its hard to compare gps to pilot tube because of that variable.
    Last edited by KIRCHNER; 09-30-2024 at 04:30 PM.

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  15. #10
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    Quote Originally Posted by blackmaxonly View Post
    I have a cheapo "eling" GPS speedometer and it is accurate as phone. I also have pro comp marine GPS on another boat and looks better and more quality but same readout
    I have the same brand. I use it the analog gauge as reference since my Garmin has GPS speed on it as well.

    22R

  16. #11
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    For $60 it's a steal, especially considering other proper brands go for $300+ without the GPS module. Mine is 3 years strong

  17. #12
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    Kirchner is correct. except it is SOG (speed over ground) vs STW (speed through water) Here is a thing on it.


    Click image for larger version. 

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    Attached Thumbnails Attached Thumbnails Screenshot_30-9-2024_193455_www.simrad-yachting.com.jpg  
    Last edited by skialot2; 10-04-2024 at 12:55 AM.

  18. #13
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    hey ski, you pretty smart guy i say OTB/OTW thats what us old mariners say, lol when comparing props, always use OTW if your splitting hairs
    Last edited by KIRCHNER; 10-04-2024 at 03:41 PM.

  19. #14
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    Quote Originally Posted by KIRCHNER View Post
    remember there a two speed calcs, OTB over the bottom, and OTW oner the water, 1 example; GPS only measures OTB, if your river running GPS wil show faster with the current, where a pilot tube will show the speed the same, GPS doesnt calculate for the moving water for prop testing, a pilot tube speedo is the way,OTW. A 5 MPH current GPS will show say 55 downstream, the pilot tube will show 50 each directi,on. BOTH gauges are correct. actually the pilot tube measurement is a true OT W speed. went to USN classes ArtK. it's hard to compare gps to pilot tube because of that variable.
    So, that means back when Michael Dixon ran the 151 mph upstream in the Rocket, verified as the same by two GPS devices, and given the river current was at least 3 mph, he actually ran 154, or was it 152.5?
    Last edited by AlliStan; 10-04-2024 at 07:27 PM.

  20. #15
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    Quote Originally Posted by AlliStan View Post
    So, that means back when Michael Dixon ran the 151 mph upstream in the Rocket, verified as the same by two GPS devices, and given the river current was at least 3 mph, he actually ran 154, or was it 152.5?
    154 over the water

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