What is the primary use of a rodmeter?

The primary use of a rodmeter is to measure the speed of a ship through the water. This instrument works by utilizing a rotating mechanism with submerged components that interact with the water flow, allowing it to calculate the vessel's speed based on the distance traveled over time. Understanding a ship’s speed is critical for navigation, planning and managing routes, and ensuring timely arrivals.

While other options relate to important aspects of maritime operations, they do not represent the primary function of a rodmeter. For instance, measuring water depth is typically carried out by echo sounders, and determining a ship's heading is conducted with compasses and gyroscopic devices. Fuel efficiency monitoring involves more complex calculations based on speed, engine performance, and other factors, rather than direct measurements made by a rodmeter. Thus, the focus on speed measurement distinctly classifies the rodmeter's primary utility in maritime navigation.