The USS Merrimac (NCC-1715) was a Federation starship in service during the 2270s. (Star Trek: The Motion Picture)
Background
Although not considered as canon, Franz Joseph's Star Fleet Technical Manual lists the Merrimack as a Constitution-class starship. However, because the Technical Manual was used for the references in Star Trek: The Motion Picture, it is plausible to assume that this ship was indeed of the Constitution-class.
This starship is not mentioned in the director's cut.
The name Merrimack has a confused history in the Star Trek universe for several reasons. First, several computer displays in the Enterprise-D conference room misspelled as Merrimac (without the k).
The second confusion is an historical inaccuracy. The United States frigate USS Merrimack was captured by Confederate forces during the American Civil War, who later rebuilt the ship as one of the first ironclad warships on Earth. The ship was then re-christened as CSS Virginia. The Virginia fought in Earth's first naval battle between ironclad warships against the Union warship USS Monitor. Therefore, it is erroneous to refer to the battle as between the Monitor and the Merrimack.
Confusing matters further, there was even another Civil War-era US warship named USS Merrimac (spelled without the k)!