ship biscuit vs hardtack

ship biscuit and hardtack both are nouns.

Word	Noun	Adjective	Verb	Adverb
ship biscuit	Yes	No	No	No
hardtack	Yes	No	No	No

As nouns, ship biscuit and hardtack are synonyms defined as:

ship biscuit and hardtack: very hard unsalted biscuit or bread; a former ship's staple

Other synonyms of ship biscuit include pilot biscuit, pilot bread, sea biscuit.

ship biscuit (noun)	hardtack (noun)
very hard unsalted biscuit or bread; a former ship's staple	very hard unsalted biscuit or bread; a former ship's staple
	a mountain mahogany