Boat speed calibration mysteries

[mglonnro]

Originally Posted by Lodesman

There's always current, or wind or both. You should do sets of passes over a marked distance in both directions, then average the results. Typically in speed calibrations, you'd run a measured mile, do a Williamson turn then run the same track in the opposite direction.

Yes, absolutely, that's one common way of doing it, but

a) I can't find the explaining factors between SOG and STW by doing that, and
b) the end result would be just one linear correction factor when I really want a table and a curve.

If I want the table and the curve using your method, I could do it for a lot of different boat speeds, OR by getting a better understanding of the SOG reference point

[mglonnro]

Originally Posted by Orin

Sorry for double posting and such here.

If you choose to share those results I will help to identify the error types and remedies. I personally think being within 5% of span is reasonable. It would be interesting to see how close we can get. There will be limits of course, because I expect that even the salinity of the water will play a little role, density should affect how much friction a water wheel will overcome for instance.

I went to school for exactly this sort of thing.

Cool, I will try it again at some other point with absolutely no wind and better tagging of the data, and then I'll let you know! All the data is in the public domain as it is, but I think some more metadata (tagging) is needed to make it useful for anyone.

Just as a curiosity, here are the "calibration runs" I made:
https://charlotte.lc/boats/2zGrCQC2X...Z8upqSYSxUtDeY

(click on the graph button to the left to get the graphs, see attached pics)

And here is one snippet of raw data from the API:

https://community.nakedsailor.blog/a...!avg,speed!avg

[contrail]

Originally Posted by mglonnro

Throttle up and down, driving in straight lines without current (see attached picture where I am ).



Yes, I assume they will be off since those are the "raw" values from the different instruments. I'm just trying to figure out what the "off pattern" is. If it would be constantly X% (like the calibration in B&G Triton2 is entered), it would be easy, but it doesn't seem like that.

Might be that the internal damping of the GPS makes the comparison difficult.



Yes, I can add acceleration, heel, pitch etc to the equation, but for this first run I tried to drive just in very straight lines without wind, current, waves, etc.



Much appreciated!

And oh yes, the end product of this is a raspberry program that reads aws, awa, stw, sog, cog, heading, variation, pitch, roll from the N2K net and outputs adjusted STW back to the net. (Kind of like some of the B&G 5000 systems.)

How certain are you that there is no current? Did you take measurements going both ways, 180 degrees apart, as someone suggested? That is how it is usually done, to account for current. And then, the difference between the two readings is assumed to be the current. I am assuming a clean, freely spinning paddlewheel! I have always been able to calibrate to within 0.1 knots.

[Rohan]

As someone pointed out - your speed instrument and your GPS are so close in agreement, that I don't see a problem here.

The GPS is more precise, but it's based on a series of estimations. The speed instrument (which is probably a little paddlewheel underneath the boat) is getting data in real time, but it can be skewed, bent, turned slightly the wrong way and be affected by debris and current.

[mglonnro]

Originally Posted by contrail

How certain are you that there is no current? Did you take measurements going both ways, 180 degrees apart, as someone suggested? That is how it is usually done, to account for current. And then, the difference between the two readings is assumed to be the current. I am assuming a clean, freely spinning paddlewheel! I have always been able to calibrate to within 0.1 knots.

It's in a small bay, the water looks really still, and when anchored the boatspeed is at a constant 0.0 (of course there is some initial resistance, I guess, so it might not move at all at 0.3 knots).

But more importantly, the boat speed was above the SOG in both (opposite) directions.

[OBT]

Current.

[MJH]

Originally Posted by tkeithlu

If you can get two instruments measuring a phenomenon in different ways to agree to that degree, you have a career as a test engineer ahead of you.

Your GPS is comparing sequential locations. It does not actually measure speed. It derives speed by how much time separated readings at two different locations and how far apart they were. There is some damping in how often it makes this calculation and how many readings it includes in the calculation. Your water wheel does read speed, but as time between successive switch closures, or your pitot measures it as differential pressure translated into speed. There are bunches of opportunities for these to differ and to be in actual error by small amounts.

Be very happy with the readings you got.

"A man with two watches doesn't know what time it really is!"

"Vive la difference."

Enjoy the sailing; don't give yourself an ulcer.

~ ~ _/) ~ ~ MJH

[svfinlandia]

Originally Posted by mglonnro

With the end goal of creating a boat speed calibration table, I made some initial test runs today.

- GPS = B&G ZG100
- Speed as measured in water = DST800

I'm reading the raw values as they are sent by the sensors onto the N2K network, but I'm sure they internally incorporate some damping/filtering.

I'm scratching my head, though. The attached graph shows SOG in the background and speed in water in the foreground (generally a bit higher).

It's clear that during the top speed stretch, SOG is roughly 6.0 knots, and speed in water about 6.3 knots.

What needs explaining, however, is that during the acceleration they show almost the same speed, but during deceleration, speed in water stays above SOG during the descent.

I am totally amazed that they are that close. I assume that your test was on a small lake with no wind and no current. My SOG and SOW are never anywhere near that close.

Al

[Cadence]

Originally Posted by mglonnro

With the end goal of creating a boat speed calibration table, I made some initial test runs today.

- GPS = B&G ZG100
- Speed as measured in water = DST800

I'm reading the raw values as they are sent by the sensors onto the N2K network, but I'm sure they internally incorporate some damping/filtering.

I'm scratching my head, though. The attached graph shows SOG in the background and speed in water in the foreground (generally a bit higher).

It's clear that during the top speed stretch, SOG is roughly 6.0 knots, and speed in water about 6.3 knots.

What needs explaining, however, is that during the acceleration they show almost the same speed, but during deceleration, speed in water stays above SOG during the descent.

Not to be crass. Does it really matter? If you are racing maybe for pleasure who cares.
I'd guess your running against the current. Are all taken under the identical situations?

[mglonnro]

Originally Posted by Cadence

Not to be crass. Does it really matter? If you are racing maybe for pleasure who cares.
I'd guess your running against the current. Are all taken under the identical situations?

To your last question, yes and going in opposite directions.

To the first comment: I think it's a well-recognized fact that the paddlewheel differs from SOG (for a lot of understandable reasons), and the simple solution is to use a linear correction factor that is "good enough". My though was to go one step further and make a non-linear correction table instead of just one constant correction factor.

With the data I recorded, however, I noticed a non-linear two-dimensional table isn't enough

And "does it matter"? Well, I guess that is really subjective. Speed through water is a main ingredient for calculating true wind angle + speed, and they are the main ingredients for making boat polars, and boat polars are a well used ways to compare one boat to another, and I want to get them as right as possible

[Orin]

Originally Posted by mglonnro

To your last question, yes and going in opposite directions.

To the first comment: I think it's a well-recognized fact that the paddlewheel differs from SOG (for a lot of understandable reasons), and the simple solution is to use a linear correction factor that is "good enough". My though was to go one step further and make a non-linear correction table instead of just one constant correction factor.

With the data I recorded, however, I noticed a non-linear two-dimensional table isn't enough

And "does it matter"? Well, I guess that is really subjective. Speed through water is a main ingredient for calculating true wind angle + speed, and they are the main ingredients for making boat polars, and boat polars are a well used ways to compare one boat to another, and I want to get them as right as possible

Mikael this is really interesting. I think it would be very worthwhile to do a bench calibration on this unit. It is the surest way to remove all other factors. Can you write a speed integration program on your raspberry pi?

[Lodesman]

Originally Posted by mglonnro

Yes, absolutely, that's one common way of doing it, but

a) I can't find the explaining factors between SOG and STW by doing that, and
b) the end result would be just one linear correction factor when I really want a table and a curve.

If I want the table and the curve using your method, I could do it for a lot of different boat speeds, OR by getting a better understanding of the SOG reference point

a) If there is a current component, going reciprocal courses over the same measured range zeroes that out. You're only worried about the for and against vectors - any component that would shift you laterally will not factor in the speed figure - that's why you steer the set courses (180º from each other) rather than courses that correct for set. Conversely with wind, you'll get a for and against component with the opposing runs, which should be eliminated by averaging the speeds.
b) Ultimately in a calibration, your aim is to eliminate or at least know the instrument error. By eliminating the current and wind, you should be able to test the instrument reading versus actual (measured by time) speed at a given throttle setting. Of course you would need to check at different "speeds" as the error might not be a linear function. If you're able to adjust the log, then zero the error for a typical cruising speed and tabulate the errors for other speed values.

[johnn33]

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

[Orin]

If anyone is the slightest bit interested this is what I was talking about with bench measurements. Could even use the cockpit as the bucket if you’re pressed for space.

[Dr. D]

Originally Posted by tkeithlu

Your GPS is comparing sequential locations. It does not actually measure speed. It derives speed by how much time separated readings at two different locations and how far apart they were.

No, the GPS receivers use Doppler processing to determine the receiver speed and direction. Here is the clearest explanation I could find (in a few minutes):

https://www.vboxautomotive.co.uk/ind...k-gps-accuracy

And a more technical one:

How a GPS Receiver Locks

Why does the GPS receiver bother with Doppler? As noted in the second link above, it must so it can keep locked onto the signal transmitted from each satellite. A satellite directly overhead will have zero Doppler shift. A satellite rising or setting has maximum Doppler shift. As the satellites are orbiting at 14,000 km/hr (7,550 nm/hr) there is a good amount of shift. Hence, the receiver has the data to determine speed via Doppler shift.