High Quality Content by WIKIPEDIA articles! The Type C6 ship is a United States Maritime Administration (MARAD) designation for a container ship developed during the transition years from moving goods by breakbulk cargo to containerization. The Type C4 ships Mariner class, arranged with its house/engine-room in the center vessel with cargo hatches at 4 forward and 2 aft, was very successful. Eleven Type C4 ships were converted into Type C6 container ships, and eight vessels were new built. As of December 2011, three converted C4s are extant as crane ships in the National Defense Reserve Fleet, and three of the new-built C6s remain in active service.