the "gadgetpool.de " is ok ( and isolated!) for the hardware
The "idea" is to realize the Stk-nmea converter in software and include it in Opencpn.
As the Stk is using a 9bit serial, classically it is possible to read it on an UART at 8bit and using the parity bit. On my implementation, I use a particularity of POSIX driver in LINUX ,wich leave intact the data with parity error and include two special characters before the data with "faulty" parity. Unfortunatly,this behaviour is not present in the windows driver ( The "faulty" data is zeroed !!...).
As far as I know, it's necessary to write a "special" serial device driver for Windows (not so easy task...).
It's possible that this solution should work with MacOs, but I can't test it.
I will test it on Prolific driver ( serial to usb) asap.
@Muttnik thanks for your comments