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GPS Max Speed.-

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  • GPS Max Speed.-

    In order to get the most accurate speed numbers dialed on a GPS Garmin 72 model during wot runs on flat calm water cond is there a minimum distance to take into consideration ?

    There are several speed readouts throughout the different displays selected (1) usually reads non fixed +,- variable speeds.

    Click image for larger version

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    When at wot the max displayed reading does not match that of the lower Trip Computer Readout (2) which is usually bit higher.

    Click image for larger version

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    Which reading is the one to consider when checking max wot rpm and speed ?

    Happy Boating
    Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

  • #2
    Can't speak specifically about this model but generally, GPS location is updated every second then speed is calculated between the two points.

    The plus and minus variances (errors) in location related to signal strength, atmospheric disturbances, satellite geometry, etc. all of which translates into errors in the speed calculation.

    GPS speed is calculated by the time it takes for you to move from point one to point two. You move 1 mile in 60 seconds your going 60 mph. You move 1 mile in 120 seconds your going 30 mph.

    Not sure what your question is about the speed. The display shows your max. speed at 42.6 mph. (based on 1 second update)

    "Moving average" is total distance / time at speed above zero. "Overall average" is total distance / total time.

    Top left is calculated speed at that moment, usually averaged to give a better sigma value. The manual might give you the mean update rate of the speed display.
    Last edited by dingbat; September 26th, 2017, 12:20 AM.
    ....

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    • #3
      Which reading is the one to consider when checking max wot rpm and speed ?
      Top left in the first picture.

      Comment


      • #4
        Originally posted by Fed View Post
        Top left in the first picture.
        OK , fist pic shows the speed moment say when running wot for 1/2 mile, What about the Max Speed shown on lower left side of the Trip Computer when wot run finished or at complete combo stop ?

        If you want to state my OB runs say 6000 wot revs at "XX" speed, which speed should you use, the max achieved seen on upper pic or the one seen on lower one ?

        Happy Boating
        Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

        Comment


        • #5
          Originally posted by Sea Rider View Post

          OK , fist pic shows the speed moment say when running wot for 1/2 mile, What about the Max Speed shown on lower left side of the Trip Computer when wot run finished or at complete combo stop ?

          If you want to state my OB runs say 6000 wot revs at "XX" speed, which speed should you use, the max achieved seen on upper pic or the one seen on lower one ?

          Happy Boating
          Good trick question.....neither

          As stated above, the first pic gives you the calculated average speed based upon the average of the last two (one second) readings. The display below gives you the highest speed calculated during time period X.

          Example.....calculated one (1) second values = 30.2, 33.3, 30.1, 32.2, 30.5, 30.4

          Max. Speed = 33.3
          Pic 1 speed = 60.9/2 = 30.45
          Moving average = 31.11

          The moving average is the best representation of your speed over the 1/2 mile distance.

          In reality, it doesn't matter which figure you use... the 0.6 difference between the two relative values is within your margin of error
          Last edited by dingbat; September 26th, 2017, 12:55 PM.
          ....

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          • #6
            Originally posted by Sea Rider View Post

            If you want to state my OB runs say 6000 wot revs at "XX" speed, which speed should you use, the max achieved seen on upper pic or the one seen on lower one ?

            Happy Boating
            Maybe I can't see the pic clearly, or don't understand that unit's readout (hence, good suggestion to consult the manual), but is it giving you kilometers per hour?
            I think you just need to be running at WOT til speed reaches a somewhat constant value, note your rpm's (if you're confident that tach is right), and note your speed at that time. Consider the speed of the water's current, if there is one.

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            • #7
              Yep, the setting is in kilometers same as my car, feel more comfy reading kilometers than miles/knots per hour. Tomorrow will do another wot test on a flat no current rowing water course with Trip Computer data set to 0 and check if both dialed "speeds! are same or near same. Thanks. BTW, the Garmin Manual at least on this particular model is in Limboland, won't find that tricky info there that's why asking..

              Happy Boating
              Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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              • #8
                One thing nobody mentioned is that GPS is a poor instrument to measure the speed of your boat in the water. GPS shows SOG, or speed over ground. If you are floating in a river with current (with the engine off), it will resister the speed of the river.

                Use the instantaneous speed readout and mentally average it, or use the average speed after you clear the register once you get on a constant throttle. Write the number down. Then, repeat the test going in the opposite direction, and average the 2. The difference / 2 is the speed of the wind and / or current. The 2 tests should be done opposite of the current or wind direction.
                2017 Boston Whaler 150 Montauk & Mercury 60 ELPT Command Thrust & Merc 3.5 Kicker
                2015 Yamaha FX Cruiser HO
                1984 Stingray SVB190SS & Mercruiser 3.0 Liter 140 & Evinrude 9.9 Kicker
                1964 Sea Nymph 14R & 1970 Evinrude Sportwin 9.5
                1960 Mulray 100 Dinghy, equipped with Beaver Oars
                1952 Lyman 15' Mid Steer & Evinrude Big Twin 25
                68 Outboards: 1919-2017, representing 11 manufacturers
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                Member ACBS, LBOA, AOMCI

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                • #9
                  Thanks Jim,

                  Does the GPS installed on dash board on boats have same issues, are they marine type if the term is applicable and don't have issues ? The Garmin 72 model is a handheld portable one..

                  Happy Boating
                  Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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                  • #10
                    All GPS receivers work the same way. They get location data from the information from the satellites. They use the location data over a period of time to calculate the speed.

                    They all work basically the same.
                    1998 Wellcraft Eclipse 24 Cuddy
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                    "Common sense is not very common"
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                    • #11
                      Originally posted by Sea Rider View Post
                      Does the GPS installed on dash board on boats have same issues, are they marine type if the term is applicable and don't have issues ?
                      People often get stuck in the weeds over thinking things. How accurate of a speed reading do you need for your calculation? Plus or minus 5 -10%
                      ....

                      Comment


                      • #12
                        Originally posted by Sea Rider View Post
                        Thanks Jim,

                        Does the GPS installed on dash board on boats have same issues, are they marine type if the term is applicable and don't have issues ? The Garmin 72 model is a handheld portable one..

                        Happy Boating
                        I have both and they both report the same answer. The only thing "marine" about them is that they probably include a sonar component as well.
                        2017 Boston Whaler 150 Montauk & Mercury 60 ELPT Command Thrust & Merc 3.5 Kicker
                        2015 Yamaha FX Cruiser HO
                        1984 Stingray SVB190SS & Mercruiser 3.0 Liter 140 & Evinrude 9.9 Kicker
                        1964 Sea Nymph 14R & 1970 Evinrude Sportwin 9.5
                        1960 Mulray 100 Dinghy, equipped with Beaver Oars
                        1952 Lyman 15' Mid Steer & Evinrude Big Twin 25
                        68 Outboards: 1919-2017, representing 11 manufacturers
                        -------------------------------------------------------------------
                        Member ACBS, LBOA, AOMCI

                        Comment


                        • #13
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                          Maybe you just haven't actually physically witnessed the exact highest speed recorded, but the instrument has ?
                          It happens fairly often with my boat and the vessel view.
                          Sometimes on a trimmed up top speed run, I only see late 50's on the real time gps read out, but when I check the max speed recorded it will be over 60.
                          Just that at that speed there is too much to concentrate on, other than the speed, haha.

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