bosun's chair vs boatswain's chair

bosun's chair and boatswain's chair both are nouns.

Word	Noun	Adjective	Verb	Adverb
bosun's chair	Yes	No	No	No
boatswain's chair	Yes	No	No	No

As nouns, bosun's chair and boatswain's chair are synonyms defined as:

bosun's chair and boatswain's chair: a seat consisting of a board and a rope; used while working aloft or over the side of a ship

bosun's chair (noun)	boatswain's chair (noun)
a seat consisting of a board and a rope; used while working aloft or over the side of a ship	a seat consisting of a board and a rope; used while working aloft or over the side of a ship