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Old 04-01-2015, 12:17   #1
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Wind transducer: True vs. apparent wind?

I have ordered this new wireless wind transducer (delivery coming march or April), and I have a question about using it with OpenCPN:

The transducer transmits apparent wind speed and direction via Bluetooth Smart protocol (BT4.0 low energy mode). It comes with no display console - I plan to use OpenCPN exclusively to display the data.

The transducer is totally free-standing, so it has no connection to GPS or paddlewheel for SOG or SOW. So there is no way it can calculate actual wind speed/direction when the boat is moving - it will only transmit apparent speed/direction.

I was hoping to display both apparent and true wind speed, but after reading some issues with the latest release candidate it appears that OpenCPN expects to receive true wind speed though NMEA in order to display it. Is this true?

And if it is true, would it be possible to modify the Dashboard plugin to provide the capability to calculate the true speed/direction from the apparent speed that the wind transducer provides and from the SOG/COG that the GPS provides? The vector math is simple, we just need to do it.

What I would envision is a few new "gauges" added to the Dashboard. Whereas there are now dials for "Apparent" and "True" speed/direction, there would be a new ones for "Calculated True" speed/direction. Or, perhaps the dashboard cal look for NMEA sentences for true speed/direction, and if it does not find them, it will automatically calculate them. Or maybe it already does this?

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Old 04-01-2015, 12:42   #2
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Re: Wind transducer: True vs. apparent wind?

The Dashboard currently does not do any calculations, please file a feature request in flyspray for it.


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Old 04-01-2015, 13:09   #3
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Re: Wind transducer: True vs. apparent wind?

While waiting for additional features in the opencpn dashboard, try NavMonPc (windows only). It will do the needed calculations, with a choice of SOG or SOW and can send true wind values to OpenCPN via virtual com port or TCP.

For years now, I have routed all NMEA stuff thru NavMonPc, it has extensive NMEA routing/filtering/calculation capabilities, including AIS caching. When my speed wheel clogs up and the B&G system sends wrong True Wind values, I tell NavMonPc to ignore the True Wind sentences and calculate True Wind based on SOG (GPS).


I would be very interested to hear if this works for you on the windows 8 tablet. Maybe one of these tablets is in my future.

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Old 04-01-2015, 14:28   #4
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Re: Wind transducer: True vs. apparent wind?

I added the new plugin feature in Tracker.

Thanks for the tip on NavMonPC. Sound like an acceptable interim solution, especially if it will share the calculations to allow OpenCPN to display it.

Here's what NavMonPC says it does. It's exactly what i'm looking for:

NavMonPc will display true wind data whenever possible (it also displays apparent wind data). If true wind data (MWD, MWV, VWT) is received from the attached navsystem gear, NavMonPc displays this data. If true wind information is not available, NavMonPc attempts to calculate it from the available apparent wind data (and from vessel speed and heading). MWD and MWV NMEA sentences that contain this true wind data can be generated and sent to other programs and systems, under the control of the options on the NMEA Options, and the NMEA Routing windows.
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