Lordu, welcome to Cruisers Forum!
I don't know anything about the package you downloaded, so can't really comment there. I will say that, having used KAPs for 5 years, we now prefer mbTiles. KAPs would probably be OK for a short passage
, but both KAPs & mbTiles have to be made at sufficient resolution to show you everything you want to know about, & many that we see published don't have that resolution. It takes time to make high-quality charts, & there's lots of junk out there.
What many folks don't seem to realize is that it's quite easy to make your own high-quality, high resolution, perfectly geo-referenced charts yourself. I've published a How-To guide to Making mbTiles
that explains how. I've tried to write it so even novices can make their own high-quality charts, so I welcome comments & questions.
Unfortunately, it depends on the SAS Planet program, & SAS is Windows only, so you'll need a Windows environment
to make your mbTiles. Once made, the mbTiles should work
fine in your Ubuntu OpenCPN.
The page also focuses largely on automating the process, so you can make LOTS of charts relatively quickly. If you only want charts for a relatively small number of areas, then you can do so with SAS directly, without having to run the Sat2Chart automation engine
(which is also Windows only).