A Starfleet vessel that rendezvoused with the Enterprise in 2366 to take Ambassador Sarek and his party back to Vulcan after the Legaran conference.
The USS Merrimack is a Starfleet ship referred to in The Next Generation. In the episode Sarek, set in 2366, the Enterprise hosts the Vulcan ambassador's last great negotiation, a treaty with the Legarans, while he struggles with Bendii Syndrome.
After Picard shares a mind meld with him so that he can finish the talks, Sarek, his wife Perrin and his aides leave the Enterprise for the Merrimack, which is to carry them to Vulcan.
The ship is named again in The Game, where it is the vessel linked with Wesley Crusher's journey between the Enterprise and Starfleet Academy. It is never seen on screen.