the author really underestimates the complexity of modern aircraft carriers, i very much doubt MC has blueprint knowledge about battleships
That's precisely why the MC makes it a huge deal to Castor that he went out of his way to look for people with the skills and talents he lacks to work on his side for the good of their small Kingdom. On top of that, magic exists, so there are things that would be an issue in our world (construction time here in particular) that can now be reduced.
Everything else that needed to be said, Shinigami_Death already mentioned.
erm yeah I totally built this massive aircraft carrier in secret and never told anyone (or the reader) about it until we conveniently needed it
I was just thinking that I must have forgotten the scene were this was foreshadowed. But I guess it was a retcon line, then, to say he launched the project back then.
iirc it was foreshadowed in a throwaway line during the arc involving conversations with their friendly neighbourhood empire, that Souma had a plan b if the talks went to hell. It bothers me that I can't remember which chapter it was and the characters involved.
It could also be that I'm flipping the manga and LN over and that such a line wasn't said in the manga and only in the LN.