Merchant vessels (MV) are ships that are built for the purpose of transporting cargo or goods. In battleship craft they usually have no guns mounted and are purely aesthetic.
Most merchant vessels in game have the base of a battleship but have cargo containers and decorations instead of guns. Merchant vessels usually cost around 0-100,000 resources depending on what engines/decorations you use. To design one just purchase a basic base and add decorations/objects that match the type of merchant vessel you want to build.
Ideas for Merchant VesselCrude carriers
Hospital Ship (not technically a merchant vessel.)
Transport for nuclear waste
Grain Container Ships