What was the name of a merchant ship called?

It depends on their purpose. Some are Tankers, they hold liquid cargo. Some are Container ships, they hold cargo that is already in steel contaners that can also be transported by rail or truck. RO-RO is a ship that is designed with a large ramp that can be lowered so that the cargo ( things that are drivable ) can be driven on and off the vessel. Bulk Cargo ships carry grain or other granular materials that can be dumped into large holds.

Argosy