ship of the line vs combat ship

ship of the line and combat ship both are nouns.

Word	Noun	Adjective	Verb	Adverb
ship of the line	Yes	No	No	No
combat ship	Yes	No	No	No

As nouns, combat ship is a hypernym of ship of the line; that is, combat ship is a word with a broader meaning than ship of the line:

Other hypernyms of ship of the line include war vessel, warship.

ship of the line (noun)	combat ship (noun)
a warship intended for combat	a government ship that is available for waging war