patrol boat vs vessel

patrol boat and vessel both are nouns.

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

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

Other hypernyms of patrol boat include watercraft.

patrol boat (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