sailor boy vs serviceman

sailor boy and serviceman both are nouns.

Word	Noun	Adjective	Verb	Adverb
sailor boy	Yes	No	No	No
serviceman	Yes	No	No	No

As nouns, serviceman is a hypernym of sailor boy; that is, serviceman is a word with a broader meaning than sailor boy:

sailor boy: a serviceman in the navy
serviceman: someone who serves in the armed forces; a member of a military force

Other hypernyms of sailor boy include man, military man, military personnel.

sailor boy (noun)	serviceman (noun)
a serviceman in the navy	someone who serves in the armed forces; a member of a military force