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I always thought it had to do with which way the vessel leaned while turning. Boats will lean into the turn, ships out. I guess that really only tells you if that's a displacement hull or planing hull, though.

https://en.wikipedia.org/wiki/Ship calls that "a US Navy rule of thumb".

A ship can carry a boat, but a boat cannot carry a ship.

And some ships can carry a ship.

https://en.wikipedia.org/wiki/MV_Blue_Marlin

Ahhh, the ship-shipping ship. There’s some great photos of it shipping ships.

Yeah, some of the ones mentioned are about 14' wide...