GPS Max Speed.-

Sea Rider

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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.

Display Speed.JPG

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

Max Speed.JPG

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

Happy Boating
 

dingbat

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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.
 
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Sea Rider

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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
 

dingbat

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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 ;)
 
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garbageguy

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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.
 

Sea Rider

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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
 

JimS123

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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.
 

Sea Rider

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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
 

bruceb58

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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.
 

dingbat

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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%
 

JimS123

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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.
 

QBhoy

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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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