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I'm not an expert, but I think that even yachts are also categorized as being either a "boat" or a "ship", depending on the length of the vessel. To be considered a yacht, I believe a boat has to be at least 30 feet in length, but don't quote me on that. :) I think that to be a yacht, as a ship, it would be around 200 feet in length.

I heard a hilarious answer that I think might be accurate too...

The difference is who washes it, if you do all the maintenance it's a boat, if you pay for it to be done by a crew it's a Yatch

That's probably more accurate than what I wrote, LOL!