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Old 29 August 2017, 02:32   #1
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Whats wrong GPS or physics?

I was going to add this to the 'how fast' forum, but the problem is I would like sensible comments rather than 'willy waving'......

I recently went on a few days RIBing around Oban and what with perfect conditions in the Sound of Kerrera I gave it rock all both up and down the Sound at around slack water. The current Frau Wright took a photo of the Chart-plotter which is where everything went a bit existential!

The chart-plotter is a Garmin 50s, it was backed up with a 45s
The RIB is my 5.4 Searider
The speed was shown as 53.6kn and 51.3kn
This equates to 60.9mph

Now the engine was given a full rebuild just under 20 running hours (May 2017) so does run very well and I hope according to manufacturers spec.

The prop is a stainless steel 21 pitch

The engine was running (howling?) at an indicated 5500rpm

When I do the math the absolute maximum theoretical speed is....

ta-dah 54.5mph

Factoring in a bit of slip 43.6mph

The boat was lightly loaded with me, her and 20l of fuel for these runs so where is the problem, the math or the GPS? I have tended to think that GPS would be accurate within given constraints but to be so far out?

Thoughts greatly appreciated, oh and I will add a photo of the GPS to the RIB base details sometime tomorrow I hope.
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Old 29 August 2017, 03:00   #2
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Sure it was reading knots and not kmh?
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Old 29 August 2017, 03:18   #3
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Is your tach accurate?
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Old 29 August 2017, 03:28   #4
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Oban HM will love your photo.

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Old 29 August 2017, 03:29   #5
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Dear Gents,
The units are not configured for metric, I still work in imperial for most things.....
The tacho, well it's a possibility but to get anywhere near the indicated speed it would need to be going at closer to 7500rpm, from the literature it revs to 5500 to 6000rpm which is about 'as indicated'
That said thanks for the good comments/questions.
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Old 29 August 2017, 03:31   #6
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Quote:
Originally Posted by willk View Post
Oban HM will love your photo.

I'll give him a copy if he lets me loose diving in the bay!
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Old 29 August 2017, 03:47   #7
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Stating the obvious, but people sometime forget the theory..
Speed = distance/time. The maths will be correct.
Time = Pretty accurate in a GPS. Once locked to satellite, 100% perfect. And I mean 100%.

The distance is just measuring a snapshot of position and this is where the errors come in.


1. How long did you maintain that speed for?
2. How long did the two GPS units maintain that discrepancy for?
3. Clear sky or obscured by high cliffs?
4. Did you check position between the two? What distance do they show between each other?
5. Were you near a military base?
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Old 29 August 2017, 03:51   #8
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Fine to do your sums down to 0.1 mph, but when did you last get your prop calibrated?
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Old 29 August 2017, 04:23   #9
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Did you check the settings on the 2 GPS units? Did one have GNSS enabled?

You wouldn't normally expect such a difference.

Also, GPS don't just measure speed by distance and time, they also use a doppler shift measurement from the received signals. That's why are normally very accurate even over a short period.

Are you sure it was slack water?
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Old 29 August 2017, 04:44   #10
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Quote:
Originally Posted by Gobuchul View Post

Also, GPS don't just measure speed by distance and time, they also use a doppler shift measurement from the received signals. That's why are normally very accurate even over a short period.
I thought most consumer GPS receivers, although incorporating some phase change techniques, were biased within the filter towards position based speed measurement?
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