guard ship vs war vessel

guard ship and war vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
guard ship	Yes	No	No	No
war vessel	Yes	No	No	No

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

guard ship: a warship (at anchor or under way) required to maintain a higher degree of readiness than others in its squadron
war vessel: a government ship that is available for waging war

Other hypernyms of guard ship include combat ship, warship.

guard ship (noun)	war vessel (noun)
a warship (at anchor or under way) required to maintain a higher degree of readiness than others in its squadron	a government ship that is available for waging war