propeller vs mechanical device

propeller and mechanical device both are nouns.

Word	Noun	Adjective	Verb	Adverb
propeller	Yes	No	No	No
mechanical device	Yes	No	No	No

As nouns, mechanical device is a hypernym of propeller; that is, mechanical device is a word with a broader meaning than propeller:

propeller: a mechanical device that rotates to push against air or water
mechanical device: mechanism consisting of a device that works on mechanical principles

propeller (noun)	mechanical device (noun)
a mechanical device that rotates to push against air or water	mechanism consisting of a device that works on mechanical principles