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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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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:
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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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Old 29 August 2017, 04:47   #11
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I'll give him a copy if he lets me loose diving in the bay!


Have you used the correct gearbox ratio in your calcs?
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Old 29 August 2017, 04:50   #12
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Quote:
Originally Posted by Gobuchul View Post
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?
The two speeds are for running up and then down the sound, sorry for any ambiguity, the two units were pretty much the same, although I was concentrating on the 50s which is in the line of sight!

It was low water slack or within minutes of it, hence the difference of only a couple of kn running up and down.
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Old 29 August 2017, 05:15   #13
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Be interesting to just post your calculations up so we can see how you got your max speed including the gear ratio.

Re: GPS doppler - depends a lot on satellite geometry as well as the unit in question. If all the satellites in view are lateral to your track then clearly there will be no doppler used in the speed calc.
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Old 29 August 2017, 05:39   #14
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Here we go.......


53.6kn = 60.9mph
Revs on tacho 5500rpm
Gearbox ratio = 2


Prop speed = 2750
Prop pitch 21


Distance travelled = 1604 yards per minute
Speed= 54.5mph


Put in a bit of 'slip'


Factor = 0.8


Speed = 43.6 mph


The slip factor is arguable but it ain't physically possible to be 1 or larger


Gearbox ratio, this appears correct although there are some who argue it is 1:1.83, again only a minor change in the end product.


I will get out the charts but that won't be until the weekend....


I accept that if satellites are lost due to topography 'things' might happen but the same kit was reading 48.8kn blasting down to the SS Breda in a relatively clear area.


Hey ho, much better to have a grown up discussion on the topic of WOT speeds, any comments on methodology appreciated.
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Old 29 August 2017, 06:09   #15
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Have a read of this. Explains about the speed measurement.

http://www.mikeg.me.uk/guides/GPS_De...Comparison.pdf

If the speed measurement you are using is from historical data and not the "live" read out then some units don't record the doppler shift measurements and use the position/distance/time which is subject to larger errors.
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Old 29 August 2017, 07:41   #16
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My SR5.4 with a 90hp mariner 2 Stroke would make a shade under 40 knots. 20" laser II propeller. cant remember what RPM as i sold it over 10 years ago!
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Old 29 August 2017, 07:48   #17
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My SR5.4 with a 90hp mariner 2 Stroke would make a shade under 40 knots. 20" laser II propeller. cant remember what RPM as i sold it over 10 years ago!
You see that ties up with what I would expect, it rattled along at about 37 knots with a 19" and not quite WOT, hence my disbelief at the indicated speed.
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Old 29 August 2017, 11:54   #18
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Quote:
Originally Posted by rnwright1969 View Post
Here we go.......


53.6kn = 60.9mph
Revs on tacho 5500rpm
Gearbox ratio = 2


Prop speed = 2750
Prop pitch 21


Distance travelled = 1604 yards per minute
Speed= 54.5mph


Put in a bit of 'slip'


Factor = 0.8


Speed = 43.6 mph


The slip factor is arguable but it ain't physically possible to be 1 or larger


Gearbox ratio, this appears correct although there are some who argue it is 1:1.83, again only a minor change in the end product.


I will get out the charts but that won't be until the weekend....


I accept that if satellites are lost due to topography 'things' might happen but the same kit was reading 48.8kn blasting down to the SS Breda in a relatively clear area.


Hey ho, much better to have a grown up discussion on the topic of WOT speeds, any comments on methodology appreciated.

Prop Slip Calculator | Mercury Racing


,,,, 1.83 v 2.0 gear ratio makes a 5mph difference at 5500 rpm with a 10% slip . At 5500, 21" , 1.83 ratio, 10% slip = speed of 54 mph
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Old 29 August 2017, 12:04   #19
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The maths looks fine. Given the updated info that both GPS units were providing the same reading, I share your confusion over the recorded speed!
Over what distance did you record this?

A certain way to prove things would be to time it over a known mile or so. My guess is prop pitch variation, with a caveat that it's just feels too damned fast for the rig you have!
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Old 29 August 2017, 12:57   #20
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I'd be double checking the units are set right on the gps.
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