branno, using udev rules could be an advantage as it allows for distinct naming of e.g. GPS or other devices connected to the USB
ports. It may otherwise happen that the 'ttyUSB'-number varies due to recognition order.
Look at this, all under Unix:
Connect the device and use the 'lsusb' command, it will display (as for my GPS) something like: Bus 005 Device 002: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port
The section ' 067b:2303' is the important one, it tells the vendor and product ID's.
Now do ''dmesg | grep usb', it will display that the device is connected to a 'ttyUSB'-port, this assuming there is a suitable kernel module loaded.
Disconnect the GPS and edit the udev rules, e.g. using 'sudo vi /etc/udev/rules.d/99-gps.rules' (or whatever editor you prefer) and add the line:
KERNEL=="ttyUSB*", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", GROUP="adm", SYMLINK+="ttyGPS", MODE="0666"
You recognize the GPS ID-numbers from the 'lsbusb' and the line should be self-explonatory.
Now connect the GPS again and do a 'ls -la /dev/tty*', you will find the GPS both as a symlink and under the autodetected name as the GPS will now (also) show up as 'ttyGPS'. You may now connect to '/dev/ttyGPS' in OpenCPN and that name will then never change.
/Lennart