cross thwart vs crosspiece

cross thwart and crosspiece both are nouns.

Word	Noun	Adjective	Verb	Adverb
cross thwart	Yes	No	No	No
crosspiece	Yes	No	No	No

As nouns, crosspiece is a hypernym of cross thwart; that is, crosspiece is a word with a broader meaning than cross thwart:

cross thwart: a crosspiece spreading the gunnels of a boat; used as a seat in a rowboat
crosspiece: a transverse brace

cross thwart (noun)	crosspiece (noun)
a crosspiece spreading the gunnels of a boat; used as a seat in a rowboat	a transverse brace
	a horizontal beam that extends across something