patrol ship vs vessel

patrol ship and vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
patrol ship	Yes	No	No	No
vessel	Yes	No	No	No

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

Other hypernyms of patrol ship include watercraft.

patrol ship (noun)	vessel (noun)
a vessel assigned to patrol an area	a craft designed for water transportation
	an object used as a container (especially for liquids)
	a tube in which a body fluid circulates