flattop vs war vessel

flattop and war vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
flattop	Yes	No	No	No
war vessel	Yes	No	No	No

As nouns, war vessel is a hypernym of flattop; that is, war vessel is a word with a broader meaning than flattop:

flattop: a large warship that carries planes and has a long flat deck for takeoffs and landings
war vessel: a government ship that is available for waging war

Other hypernyms of flattop include combat ship, warship.

flattop (noun)	war vessel (noun)
a large warship that carries planes and has a long flat deck for takeoffs and landings	a government ship that is available for waging war
a closely cropped haircut; usually for men or boys