STW reading half what it should be

[Maughidhoo]

The STW readings on my opencpn (also open marine and mxtommy) show apron 50% lower value than they really are. The main nav system on the boat is a Raymarine system with an itc-5 for converting the log paddle wheel signals into the system. On the Raymarine network the STW reading agrees with the SOG and seems to display sensible values.

A vYacht router transfers the SeaTalk NG / NMEA 2000 data to the pc / RPi.

Maybe somebody has encountered a similar issue and knows the solution or a possibility to calibrate STW values in opencpn or similar.