vessel vs weather ship

vessel and weather ship both are nouns.

Word	Noun	Adjective	Verb	Adverb
vessel	Yes	No	No	No
weather ship	Yes	No	No	No

As nouns, weather ship is a hyponym of vessel; that is, weather ship is a word with a more specific, narrower meaning than vessel:

vessel: a craft designed for water transportation
weather ship: an oceangoing vessel equipped to make meteorological observations

vessel (noun)	weather ship (noun)
a craft designed for water transportation	an oceangoing vessel equipped to make meteorological observations
an object used as a container (especially for liquids)
a tube in which a body fluid circulates