vessel vs patrol boat

vessel and patrol boat both are nouns.

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

As nouns, patrol boat is a hyponym of vessel; that is, patrol boat is a word with a more specific, narrower meaning than vessel:

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