Keep in mind that the jacht was
the very first User Content ship made. It was literally made with no plans to add it to the game; things just worked out that way, to the delight of all

On most points the packet-boat will be the better exemplar, though we were still figuring out what works best when she was completed.
A rough guide to what can be merged is if parts:
A) use the same texture file;
B) would be the same material in an actual ship;
C) function as a unit and/or are connected to the same mast or spar;
...then it's usually safe to merge.
When in doubt, leave things unmerged. Name your meshes clearly and systematically though, that will help in processing them for the game.