USS Merrimack vs Merrimac

USS Merrimack and Merrimac both are nouns.

Word	Noun	Adjective	Verb	Adverb
USS Merrimack	Yes	No	No	No
Merrimac	Yes	No	No	No

As nouns, USS Merrimack and Merrimac are synonyms defined as:

USS Merrimack and Merrimac: an ironclad vessel built by the Confederate forces in the hope of breaking the blockade imposed by the North

USS Merrimack (noun)	Merrimac (noun)
an ironclad vessel built by the Confederate forces in the hope of breaking the blockade imposed by the North	an ironclad vessel built by the Confederate forces in the hope of breaking the blockade imposed by the North