Indiaman vs sailing ship

Indiaman and sailing ship both are nouns.

Word	Noun	Adjective	Verb	Adverb
Indiaman	Yes	No	No	No
sailing ship	Yes	No	No	No

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

Indiaman: a large sailing ship that was engaged in the British trade with India
sailing ship: a vessel that is powered by the wind; often having several masts

Other hypernyms of Indiaman include sailing vessel.

Indiaman (noun)	sailing ship (noun)
a large sailing ship that was engaged in the British trade with India	a vessel that is powered by the wind; often having several masts