Mismatch between BG Boat speed and OpenCPN STW

[SVvroom]

Hey Everyone!

I'm finally using the boat after a really big refit.

One annoying issue im having is that there seems to be a big mismatch between the Boat Speed as registered on the new BG instruments, and the STW as registered in Dashboard in opencpn.Like right now the instruments are reading about 1kt faster than the dashboard, while the SOG in both opencpn and the instruments seems well aligned.

Any ideas?
Thanks!!!
greg

[Corsair63]

More or less always will be a difference between STW and SOG.
as you say STW is given by one system based in movement of the water in the transceiver.
while SOG is given by the GNSS based in latest positions of the satellite systems.

that you have two different STW, it sounds that either you have two systems giving the STW at the same time and for unknown reason O is getting preference of one of them, or really, I don't know what to say.
check in the connection Tab in the button "setup priority of comms" if you have more than one system to give priority to the new system e.g. "new BG".
or just switch off the new and check the O display and after that switch off the old one and on the new one to do a value display check in O.
in case that you cannot find the root search in the nmea windows for the STW input and just in case make a filter rejecting the error sentences, maybe they came in nmea string from different sources, the two letters just after the $ in the nmea sentence.

[stevead]

Another problem to ponder over my morning coffee....(aka too much free time)

'm finally using the boat after a really big refit.

Do we assume that the refit includes new B&G instruments and transducers?

If so, then do we subsequently assume that you have calibrated the instruments as per the B&G instructions.

It probably requires confirmation but I'm reasonably certain that STW calibration is stored in the B&G displays rather than in the paddlewheel sensor.

If all of the above assumptions are correct, that would explain why you observe a discrepancy between STW displayed on the B&G instruments and on the OpenCPN Dashboard.

To confirm the above, we would need a log of the raw NMEA 2000 data which could either be obtained from the B&G displays or if you are using a NMEA 2000 Gateway to connect to OpenCPN, via that device.

Unfortunately the Dashboard has no mechanism for calibrating the STW display. There are possible kludgy workarounds, but it depends on how OpenCPN is connected to the B&G network. If by NMEA 0183 (perhaps you are using WiFi), then some custom script using the NMEA Converter plugin, if by NMEA 2000, then possibly a custom script using the Java script plugin.

[barnakiel]

Both SOG and STW may/will contain some form of data smoothing prior to display.


How the data is smoothed will bear on what you display.


But if you do not smooth the data, the log will be way off.


b.

[SVvroom]

Hey everyone!

Thanks so much for the responses.

A few more details.....

Yes, the refit include a new airmar DST810 transducer, and a digital yachts nmea2k-0183 gateway.

I looked at the nmea window and was getting the following sentence:

$YDVHW,280.3,T,272.9,M,4.9,N,9.1,K,*66

Based on other posts it seems to me that opencpn is expecting the STW data to be in m/s, and the Digital Yachts is providing Kts. The numbers dont totally add up (bc if you assume Opencpn is converting kts thinking its m/s than id get a really fast speed), but it seems like this might be part of the problem.

In terms of calibration.....calibration is done in the BG system, and would also affect the reading on the BG displays, so thus wouldnt address the problem (which is that there is a difference between the opencpn displayed speed and the BG display speed).

Based on, admittedly rough, observations, it seems like the Opencpn speed is about 1.25 slow. Ie: it reads 4.8 kts when the BG reads 6kts.

Thoughts?

Thanks!!!!
greg

[stevead]

OpenCPN Dashboard uses knots internally.

Your sentence

$YDVHW,280.3,T,272.9,M,4.9,N,9.1,K,*66

reads as 4.9 Knots and 9.1 km/h.

For the above VHW sentence the Dashboard will parse both, converting the km/h value to knots and will display the value correctly.

When you performed the B&G speed calibration, what was the adjusted % value?

If you could log the raw NMEA 2000 that would be helpful. There are instructions for Digital Yacht here

If you have a B&G MFD, goto Settings->Network->Diagnostics->Save CAN data. There is a cheat sheet in this post that describes how to retrieve files from the B&G MFD over the WiFi network.

In terms of calibration.....calibration is done in the BG system, and would also affect the reading on the BG displays, so thus wouldnt address the problem

That is why the raw NMEA 2000 data from the Airmar transducer would be helpful. It is difficult to determine whether the calibration adjustment is stored in the transducer or in the B&G displays. If the latter, it would explain the discrepancy between the B&G Displays and OpenCPN.

[SVvroom]

Wow steve thanks for the amazing reply.

I'll provide the data streams you mentioned in a little bit. Right now i need to focus on tacking up this channel jajaja

But one quick reply.....I understand that opencpn reads that sentence as 4.9kts.....i guess im wondering if the digital yachts device is properly converting? Bc when it was reading 4.9kts it should have been about 6kts.

I guess we can better answer that once i get you the requested data! Will do, and thanks in advance!
greg

[AedanC]

To check the YD is converting correctly connect to the DST810 with Bluetooth from your phone. It will show you what speed it’s putting out onto the network.

[barnakiel]

For older Ray the % was held in the instrument.


Possibly still the same story for B&G instruments.



probably easy to find this info in B&G manuals


sensors are very unlikely to store as they are most of the time plain analog to digital converters - without memory chips


b.

[SVvroom]

Hey Everyone!

Thanks so much for all your help, I feel like im getting closer.

I recalibrated the paddlewheel using three runs of 1/2 mile each. The % adjustment was about 110%

I connected via bluetooth to the DST810 and confirmed that opencpn is displaying the same uncorrected value that the Transducer is putting out.

So i guess the solution is that i need to use nmeaconverter_pi to make the adjustment in the nmea sentence?

If the original sentence was:
$YDVHW,280.3,T,272.9,M,4.9,N,9.1,K,*66

Would the new sentence be

$MIVHW,$YDVHW5*1.1,$YDVHW6

That assumes a correction of 1.1 times the tranducer reported speed. Or am i way off base? Sorry Ive never played with adjusting the bmea sentences and im under way right now so dont want to mess things up until im at anchor! jajaja

Thanks!!!
g

[Corsair63]

If the original sentence was:
$YDVHW,280.3,T,272.9,M,4.9,N,9.1,K,*66

Would the new sentence be

$MIVHW,$YDVHW5*1.1,$YDVHW6

I have to do sometimes some adjustment with NMEAConverter and I guess that your conversion is not complete, but double check the manual:
https://opencpn-manuals.github.io/ma...ter/index.html

if I'm right, the conversion should be like this:

$YDVHW,280.3,T,272.9,M,4.9,N,9.1,K,*66
$YDVHW,1,2,3,4,5,6,7,8,*66 (only to know the field number)

Converted sentence:

$MIVHW,$YDHW1,$YDHW2,$YDHW3,$YDHW4,$YDHW5*1.1,$YDHW6,$YDHW7*1.1,$YDHW8

or if you're not interested in KM/H

$MIVHW,$YDHW1,$YDHW2,$YDHW3,$YDHW4,$YDHW5*1.1,$YDHW6,,

The first conversion is the proper one, on such way you give the same correction to both fields, just in case.

[AedanC]

Have you tried to apply the calibration in the DST810 using the Airmar App? Perhaps then the B&G and OpenCpn would get the calibrated value.

[SVvroom]

Aeden!

Wow, until this thread i didnt know about that app, and until your suggestions i didnt poke deeper to see that i can indeed calibrate it at the transducer! great idea, ill give that a shot and report back!

[SVvroom]

Hi Everyone!

Just wanted to follow up because with your help i solved the problem. As suggested, i was able to calibrate the tranducer itself, rather than at the BG instruments. So now the transducer sends a corrected value out and the instruments all read the same thing wihto0ut having to make any adjustments!

Thanks so much for your help!
Sail fast!
g

[AedanC]

Good to hear you have it all working now. Thanks for letting us know what worked.