I have found and tested this: Nautical Chart Reprojector from the NOAA to change polyconic and utm etc to Mercator which OpenCPN
will quilt. It doesn't change the "Fathoms" to metres in my old chs kaps despite there being no Fathoms option! Nautical Chart Reprojector - Tools - Digital Coast
versions for windows and linux
. a bit clunky in the GUI for windows but it does the job.There's also a cmd line option and instructions for geeks. The originals may need changing from "read only". It also extracts tiffs and apparently can do much else in datums and projections.. I advise plopping some waypoints in with openCPN and comparing where these are in the old and new.So far,it's worked perfectly.Please advise if you find it's screwy...