It falls under the common creative laws licensing for Australia, at least. It may vary from country to country... but this is exactly how things like a parody works where an artist can take a tune and rhythm directly from a popular artist, change the lyrics (there's your 20%+) and have zero commitment to the original creator or it's copyright holders.
Do some reading on "Creative Commons". While the 20% may or may not be accurate (I know it's roughly there-abouts), if you change something enough then it no longer considered being in it's original state (even though it may still be instantly recognisable) you can get away with it.
An example in this particular case might be adding 7 engines from a 747 jumbo, and making the main hull of the Enterprise slightly oval shaped instead of circular. Change a few colours, add some pin striping and a small wing on the top... and there you have it. While it will look nothing like the original Enterprise, certain familiarities people have with Star Trek may still make it instantly recognisable AS the Enterprise even though it looks vastly different.