What is the "rule of gross tonnage" used for?

The "rule of gross tonnage" is used primarily to determine a vessel's cargo capacity and the safety regulations that apply to it. Gross tonnage is a measurement that reflects the overall internal volume of a vessel, expressed in tons, which helps in assessing how much cargo it can safely carry. This measurement is essential for a variety of regulatory and safety reasons, as different classes of vessels are subject to different regulations based on their gross tonnage. These regulations might include requirements for life-saving equipment, crew training, stability, and other safety factors tailored to the capacity and structural implications of carrying cargo.

Other options, while related to different aspects of vessel operations, do not pertain to this specific measurement. For instance, measuring the length of a vessel or assessing fuel efficiency involve different parameters and metrics. Similarly, determining a vessel's speed is based on hull design and power output, not gross tonnage.