I think the answer is, like many other such "on-topic" questions, is that it's not easy to draw a line. Of course questions that are more scientific than world-building are not on-topic. That's for another SE site. But this site is going to have a lot of overlaps with other sites.
I personally think most of these questions are fine. Take, for example, How would rudder protection against water elementals affect ship's performance? This would probably be marked as off topic very quickly on some other sites. But here it is fine. Why? Because it has to do with a specific situation in building a world. It does effect real life, water elementals don't exist, but in his world it is a very pertinent question. So it needs to be on topic here, the asker can't ask it anywhere else.
What's a basic guideline on where to draw the line? My guess is where it no longer is specific. Too broad is likely our biggest guideline here. If it isn't specfic, say something like, "How would I create a fireproof ship?" We can't answer it. It's too broad. He could ask, "How would I create a fireproof sail" and that would be fine, because we can give him a specific set of steps. Of course, if something is actually just history (say "How would medieval people fireproof their siege engines) or just science, (something like "What process of metabolism creates energy for plants") we don't answer it here. It belongs on another site.