vessel vs patrol ship

vessel and patrol ship both are nouns.

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

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

vessel (noun)	patrol ship (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