boatswain's chair vs bosun's chair

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

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

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

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

boatswain's chair (noun)	bosun'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