In 1988 Florida tried to make a law protecting boat hull designs and was shot down by the US Supreme Court. Legislators got involved and so part of the 1998 Digital Millennium Copyright Act includes protection of hull designs.
The rationale is: As a boat maker you have experts do a lot of very expensive work with thinking, simulation, real world modeling, building and testing full sized prototypes sweating all the little fiddly details. You finally stumble upon a good hull for some market. Then you spend dump truck loads of marketing money to convince people that it is a good hull for their boating needs. Success! So some clown then borrows one of the boats, makes a mold, and starts selling them cheaper with no R&D expenses and customers flock to him because they can tell its "just like" your boat.
Fun legal/programmer note: The law had to be fixed in 2009 because some of aforesaid clowns figured out that "hull" was defined to be the hull and the deck, so they were making molds, copying the hull and changing an irrelevant detail on the deck and counting as a different design. The 2009 law splits hull and deck, fixes a bunch of AND and OR logic, and closes that bug report on the law.