WildWarrior

Yesterday we decided to compare mph readings between my mounted Garmin 192 GPS with antenna mounted just front of small Cock pit wind screens in front of driver side, and a hand held Garmin GPS.
Smooth water, no wind!
They were dead on until we hit the mid 80s, soon I saw 92mph on the small unit,later with a max speed of 94.4 mph during several miles of trimming her up, while my 192 only recorded a max speed of 88.7mph.
We were spinning 29s with a 1.35 ratio at 5500 rpms!
What could give us a 6 mph difference between the two units?

zt260

I've seen the small hand held units give readings that are off quite a bit. I would think your 192 is the accurate one. You could test with another unit or test it against radar.

Wobble

Ditch the 192

Much better bragging rights with the handheld

Dave M

My handheld and my GPS speedo read almost dead on. I wouldn't think the 192's method of recieving the signal and calculating speed is any different than the handheld. I think with the higher end GPS units you are paying for the built in functions and mapping rathing than the quality of the receiver. Maybe a malfuntion?

d1mbu1b

there are two different techniques used to derive speed from GPS.
The first technique takes consecutive position measurements and calculates delta distance vs delta time to arrive at speed.
The second technique actually measures the Doppler off the carrier waves from the GPS and calculates "instantaneous speed"
Given the error associated with the GPS position it is quite possible that the calculated velocity can jump momentarily.
The instantaneous technique is much more accurate and will not have as much error associated with the measurement.
Furthermore, a momentary jump in velocity can occur due to the constellation of the GPS satellites. If one of the satellites used by the GPS receiver is no longer in view than the position and subsequently velocity will jump. and conversely if new satellites come within line of sight the calculated position and velocity will also be affected.

Two different GPS receivers can also have differing sensitivities causing one unit to see more satellites than the other, thus, yield a more accurate reading.

in summary, GPS receivers capable of providing instantaneous velocity are more sophisticated then the derived velocity GPS units and most likely more expensive. I doubt any of the GPS receivers we are talking about actually do this. The problem is most likely a combination of the error associated with GPS position and changes in the GPS satellites the unit can see.

Scott

Sorry don't want to be Mr. Obvious but just borrow a third unit find out wich one is broke and get rid of it.

DiamondPerformance

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Glenn, I have a hand-held you can try, 94.4mph that Gun is flying! Thanks for the everything at Rock The Bay. Bill

mmwalters

Glen If you were to do the math 89mph is 20% prop slip 95 mph is 15% prop slip. 95 seems about right. I know you hull is kevlar, do you know what it weighs?

WildWarrior

My Top Gun is approx 9250lbs dry with 800+ hp @ 5600rpms per side.
We have pics from this weekends poker run and @ speed the only thing in the water are her drives, after 80 or so she is light as a feather but if it's rough enough she is very stable.
As I don't often run her trimmed up and wide open I would like to really know what she will honestly do .
If I can get my hands on a third unit, Fri eve we will give it another shot down the bay, Rudy our engine builder/ rigger and Billy from Diamond performance have this boat dialed in and really ripping, I believe we should see 95mph out of her in the rough.
This GPS thing has me a little perplexed as all summer a few people have questioned it's accuracy after 85mph @ approx 5000rpms vrs another 500 trimmed up rpms for only 3 mph or so GPS
Thank you all for your input and the truth is that other than for 5 seconds of bragging rights that know one cares about anyway, weather she can turn 88 or 95mph she is a true blast to run and with these new mufflers is also a real sleeper we can actually talk and here what people are saying now @ cruise speeds , I love it!

DiamondPerformance

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