He probably could design one. But engineering drawings without skilled laborers are fairly useless in this case. Might be that he just couldn't string together enough carpenters to have one made without getting too involved.
Beyond that, might just be a tough trip. If there are interactions like the gulf stream with the north sea, that would cause lots of issues. Right around where the cold waters of the North are meeting the warm coastal waters of Zeum, that would create some pretty bad storms and choppy water. Couple that with (speculating) limited natural ports akin to western Africa, and you've got yourself a huge problem.
As for chariots, I think the same reason that there are nonman tech that only the nonmen have an no one else. I dont know what that reason is, but there are several examples of this throughout the books. Kellhus is managing/manipulating things (people, gods, circumstances, etc.), rather than spending his time engineering technologies, magic or otherwise.