Filter and convert incomming NMEA messages

[olewsaa]

I'm using a small Seatalk1 to NMEA/USB converter to convert Seatalk1 messages to NMEA sentences, most of it works fine. 
However, the wind seem to ba about 2x the expected values (as read out on the ST60 display). It looks quite like a knots - m/s error. 
The sentences received contains numbers postfixed with M, N and K for m/s, knots, km/h  and should be ok. When these 
sentences are fed into Openplotter (the 1.x version) the wind show up as m/s, but the value is closer knots. 

As the NMEA sentences are generated by the ST1 to NMEA converter the sentences might not be 100% compliant with the standard. 

Is there a config file whwer I can inpout a correction factor to get the numbers into a correct m/s value. As soon as the kplex and Openplotter 
get the correct value (ms/) it can display any unit (m/s, kn, km/h), in my case I prefer m/s.  

All other values from Seatalk1 are correct (as far as I have noticed), wind angle, depth, speed through water, water temperature and a range of
other values, bot from the ST60 instruments and from the E80.

If there is no config file to place a correction factor in or any other simple fix it should be possible to dig into the python scripts to introduce the factor 
there where the NMEA sentences are parsed. However, I don't know which file to start looking. 

A link to my setup.