A ship which has no fixed schedule and travels wherever it can find cargoes to carry is a

A ship that does not have a fixed schedule and travels wherever it can find cargoes to carry is known as a tramp ship.

Here's why it is called a tramp ship:

Tramp ships operate differently from other types of vessels, such as liners or ferry boats. They do not follow a set route or schedule. Instead, they are flexible and can go to any port where there is cargo to be transported. This means that their operations are based on demand rather than a predetermined timetable.

To put it simply, a tramp ship is like a freelance vessel that looks for cargo opportunities and travels based on where the work is, rather than sticking to a specific route or timeline. This provides a degree of freedom and responsiveness to market demands that fixed-schedule ships, like liners, do not have.