A ship that travels without a fixed schedule and carries cargo from one port to another as per requirement is called a "tramp" or "tramp steamer". Tramp ships do not have a fixed route or a set timetable, but instead operate on a voyage basis, carrying cargo wherever it is needed. They are commonly used for transporting bulk commodities such as coal, grain, and ore, and are often chartered by companies that need to move cargo between different ports. The term "tramp" is believed to have originated from the fact that these ships "tramp" from one port to another without any fixed schedule or route.