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Old 16-04-2021, 14:49   #31
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Re: Boat speed calibration mysteries

Maybe this will help: https://maritime.org/doc/fleetsub/log/chap14.htm
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Old 16-04-2021, 15:00   #32
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Re: Boat speed calibration mysteries

Nice! Thanks.
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Old 16-04-2021, 20:12   #33
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Re: Boat speed calibration mysteries

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That's a great document!

And you get a table and a curve (or line) by doing it like it specifies for at least two different speeds

Still, if I could figure out the GPS reference model a bit better, it might be possible to do a bunch of "measured miles" in a much shorter time and covering a wide range of speeds. Hmm.
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Old 16-04-2021, 22:27   #34
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Re: Boat speed calibration mysteries

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Originally Posted by johnn33 View Post
Hallo mglonro You might use a measured mile. Possibly the institute https://www.aboamare.fi/Contact-us could help?
John

Or simply start and finish GPS co-ordinates.
If you've got those two, and a series of times and speeds, you can integrate both sets of readings and see which, if either is the more accurate speed.


You could even plot GPS coordinates at the time of each reading and do a full analyisis of distance/time between readings to determine a series of SOG values.


But, the graphed figures are so close to each other, I wouldn't bother. They are the closest I've ever seen for a continuous match between STW and SOG readings. LIke other posters, I'm very surprised at the consistency.


Is it the paddlewheel or ultrasonic DST800 ?
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Old 17-04-2021, 00:33   #35
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Re: Boat speed calibration mysteries

There is one answer and it's due to latency in the GPS. Nearly all GPS have significant latency in their position data, often 0.5 secs or more, and then to calculate speed, they will use data from several previous positions which is of course even staler. So on acceleration the GPS speed will be lower and during deceleration, higher. Found this out by doing land tests on a GPS setup to measure the contractual stopping distance of a warship.
Latency should not be confused with update rate, the GPS may update at 10 Hz, but have considerable latency.
If your data is in a spreadsheet, you can artificially delay the water speed data by a varying amount until you get a consistent calibration across the sample. Mathematically, when the standard deviation of the two parameters is minimised, at that point, the slope is the speed calibration factor.
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Old 17-04-2021, 00:35   #36
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Re: Boat speed calibration mysteries

Of course, the water speed will also have some latency, but generally lower than the GPS
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Old 17-04-2021, 01:14   #37
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Re: Boat speed calibration mysteries

Don't forget to go sailing.
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Old 17-04-2021, 04:31   #38
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Re: Boat speed calibration mysteries

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Originally Posted by Rapanui View Post
There is one answer and it's due to latency in the GPS. Nearly all GPS have significant latency in their position data, often 0.5 secs or more, and then to calculate speed, they will use data from several previous positions which is of course even staler. So on acceleration the GPS speed will be lower and during deceleration, higher. Found this out by doing land tests on a GPS setup to measure the contractual stopping distance of a warship.
Latency should not be confused with update rate, the GPS may update at 10 Hz, but have considerable latency.
If your data is in a spreadsheet, you can artificially delay the water speed data by a varying amount until you get a consistent calibration across the sample. Mathematically, when the standard deviation of the two parameters is minimised, at that point, the slope is the speed calibration factor.
Yes, this sounds intuitive and logical! The problem with this is that the graph is showing the opposite! When accelerating, the GPS is matching the paddlewheel, and when decelerating, it's lower.

Maybe there is some unknown latency in the paddlewheel, but it's not symmetric. For some reason
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Old 17-04-2021, 05:09   #39
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Re: Boat speed calibration mysteries

So,

Today, before starting the sail towards our home harbor, I did some calibration runs again by motoring in opposite directions at a few different GPS speeds: about 2 knots, about 4 knots, and about 6 knots.

Here's the whole picture of the different runs (same as before, SOG in the background, slightly slower most of the time):



Here are the 2 knots runs:





4 knots.





6 knots





Now if I put these figures into an excel and average the opposite runs, I get:



If I then draw the correction percentages into a curve, I can get an estimated polynomial function for that:



This function I can then use as a "correction table" when feeding the N2K system with corrected boat speed values. I GUESS

Here are some sample corrections:

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Old 17-04-2021, 06:19   #40
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Re: Boat speed calibration mysteries

Those are big extrapolations down to 1kt (50% of lowest tested speed) and up to 9kts(near 50% above highest tested speed).
Since there is no proposed reason for the dip in STW at 4kts, it is hard to justify the extrapolations outside the tested range,

The data and graphs are very interesting.
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Old 17-04-2021, 06:42   #41
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Re: Boat speed calibration mysteries

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Those are big extrapolations down to 1kt (50% of lowest tested speed) and up to 9kts(near 50% above highest tested speed).
Since there is no proposed reason for the dip in STW at 4kts, it is hard to justify the extrapolations outside the tested range,

The data and graphs are very interesting.
Yes, big extrapolations, true. But how would you do it? What would be the most reasonable choice for 9 knots, for instance, given the data?
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Old 17-04-2021, 06:42   #42
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Re: Boat speed calibration mysteries

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Is it the paddlewheel or ultrasonic DST800 ?
Paddlewheel!
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Old 17-04-2021, 07:22   #43
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Re: Boat speed calibration mysteries

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Yes, big extrapolations, true. But how would you do it? What would be the most reasonable choice for 9 knots, for instance, given the data?
I can't come up with a justification for the dip at 4 kts, so it is hard to extrapolate that far out. Based on the data you have, I'd stick with the gain adjustment you have at 6 kts and use it above that.
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Old 17-04-2021, 07:29   #44
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Re: Boat speed calibration mysteries

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I can't come up with a justification for the dip at 4 kts, so it is hard to extrapolate that far out. Based on the data you have, I'd stick with the gain adjustment you have at 6 kts and use it above that.
sounds smart to do it like that
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Old 17-04-2021, 07:43   #45
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Re: Boat speed calibration mysteries

Quote:
Originally Posted by mglonnro;3389400r.

Maybe there is some unknown latency in the paddlewheel, but it's not symmetric. For some reason
Inertia when accelerating/decellerating? I'd be surprised if there were none.
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