sailing boat vs sailing vessel

sailing boat and sailing vessel both are nouns.

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

As nouns, sailing vessel is a hypernym of sailing boat; that is, sailing vessel is a word with a broader meaning than sailing boat:

sailing boat: a small sailing vessel; usually with a single mast
sailing vessel: a vessel that is powered by the wind; often having several masts

Other hypernyms of sailing boat include sailing ship.

sailing boat (noun)	sailing vessel (noun)
a small sailing vessel; usually with a single mast	a vessel that is powered by the wind; often having several masts