The OP confirmed that the same setup worked fine on another computer, so it's not necessarily the hardware
Here's the latest official driver for Mac
, if you don't already have it
Since there's data, but it's not correct, I believe the driver's working ok but speed setting may be wrong. First thing to try - change the expected speed in OpenCpn
to 9600 and see what happens. [and 38400 too as per Jean]
What is supplying the GPS serial
signal? Check its settings for speed and format.
I'm not a Mac guy, so I have nothing further to suggest. Maybe hit the Mac forums
to find out what they recommend as the best USB-serial cable for Mac.