A more thorough explanation:

Definition: Tonnage duty is a charge imposed on a commercial vessel for entering, remaining in, or leaving a port, usually assessed on the basis of the ship's weight.

Example: A cargo ship weighing 10,000 tons enters a port and is charged a tonnage duty of $10 per ton. The total tonnage duty charged to the ship would be $100,000.

This example illustrates how tonnage duty is calculated based on the weight of the ship. It is a fee charged by the port authorities for using their facilities and services.