Thanks to a simple script written by someone else having the same GPS data exporting problems as I with their Lowrance HDS chartplotter, I was able to extract all the GPS data from the trip up to Langkawi. I have no idea how the guy got the USR4 file format info from Lowrance; I asked for the same thing multiple times and was ignored. But now that it’s documented in his code I may try my hand at adding support for USR4 to gpsbabel (probably read-only).
In the meantime I updated my map of Oia‘s travels with the Langkawi tracks. The trip was right around 540 nm over about eight and a half days, for an average SOG of about 2.6 kts. Not very fast.
There’s a bigger and easier to navigate version here.