After: The Atlas Caravan is played using the Worlds Without Number system. A number of the system's resources are available for free, including the core rules you need to make a character.
If you have access to the paid content in the same system- for example "The Atlas of Latter Earth", you're free to use that content when making your characters, but please don't post the paid content anywhere, as we're not out to pirate any books in our community. Links to buy these books can be found here:
The Atlas of the Latter Earth: https://preview.drivethrurpg.com/en/product/416284/the-atlas-of-the-latter-earth
Worlds Without Number Core Rulebook Deluxe Edition:
https://preview.drivethrurpg.com/en/product/348791/worlds-without-number
Quick sea rules and the full naval section. This knowledge is needed to play the game. Please consider getting the Full Atlas of Latter Earth as your funds permit.
The SRD from Drive thru. This is not much use on the player side of the game, but if you are trying to figure out mechanics this helps. Naval combat rules are not in here because they were too tedious to strip out.