donkey boiler vs steam boiler

donkey boiler and steam boiler both are nouns.

Word	Noun	Adjective	Verb	Adverb
donkey boiler	Yes	No	No	No
steam boiler	Yes	No	No	No

As nouns, steam boiler is a hypernym of donkey boiler; that is, steam boiler is a word with a broader meaning than donkey boiler:

donkey boiler: (nautical) an extra boiler (as a ship's boiler that is used while the ship is in port)
steam boiler: sealed vessel where water is converted to steam

Other hypernyms of donkey boiler include boiler.

donkey boiler (noun)	steam boiler (noun)
(nautical) an extra boiler (as a ship's boiler that is used while the ship is in port)	sealed vessel where water is converted to steam