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capnJack

I have a 2015 Raymarine C95 MFD connected to a 2005 Raymarine C70 MFD via NMEA0183 at 4800 baud.
The C70 has a Seatalk (1) network connected to a Raymarine Wind Speed instrument ST60+.
The wind speed shows correct apparent wind direction and speed on its display as does the C70.
The C95 shows direction accurately but the wind speed shows about 3 knots when the instrument shows about 5.6 knots. The ratio between the speeds is about the same over different wind speeds.
The C95 is getting the wind speed and direction via the same NMEA0183 sentence MWV.
I disabled all NMEA0183 sentences from the C70 apart from the MWV output and observed the sentence received on the C95. It showed the correct speed and angle and measurement in knots, while the C95 display showed an incorrect speed also in knots.
To me this showed that the C95 is using the MWV and that it is receiving the correct value but displaying an incorrect value.
My assumption is that the C95 has a bug in its display software.
The C95 uses the Lighthouse II operating system. The version in the C95 was old so I downloaded the latest version (R19) and updated the C95. I searched the revision notes for mention of wind speed and found nothing.
Same problem of incorrect apparent wind speed with version 19.
I searched the Raymarine Forum and found nothing relevant.
I find it hard to believe that no one has reported the same problem so I checked the following:
The C95 is set to knots and the MWV message shows knots.
Perhaps the C95 is very intelligent and is translating mast head speed to cockpit (ground) speed? So I checked the boat setup to see if there is a setting for mast height. There is none, so perhaps there is a factor that its using?
Next I searched the C95 users manual and found a setting for wind damping. This setting affects both angle and wind speed. 1 is the slowest and 15 is the fastest, the default is 12. My C95 is set to 12 for both speed and angle.Changing it to 15 made no difference.
Any suggestions?