"If a vehicle is less than 10% propellant, [c]hanges to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say."
This is why you don't let cabinet makers build ships.
The point is that they can actually. A cabinet maker could build a fairly large boat at least (maybe not a ship...) entirely by hand and mostly using very simple rules of thumb. It probably wouldn't perform very well but it would float and be able to sail around.
Amateurs build small boats that cross the Atlantic and even the Pacific all the time, precisely because ship building is easier than building rockets.
When I say it's easier, that doesn't mean that naval architects aren't as smart as rocket engineers, but the ratio of engineering effort to performance is much more favourable.
I read this story[1] once, although I don't remember where:
Someone somewhere needed a large number of cargo vessels built quickly (think WWII liberty ships, but those weren't made of wood), so they brought in a bunch of cabinet makers to bolster their shipwrights. It worked great, until the ships built by normal woodworkers saw significant wave action, at which time they broke up and sank. The punchline being that, on land, rigidity is the primary constraint; if you build it not to be floppy, it will be plenty strong. At sea (and especially in aerospace), strength is primary; if you build it to be rigid, it will be too heavy and if you build it to be not-heavy, it will probably fall apart.
An amateur can build a fairly large boat (although usually to plans by an actual naval architect), and a small boat can make it across an ocean, but if you're serious about schlepping things around, the design constraints for ships aren't much looser than those in aerospace.
This is why you don't let cabinet makers build ships.