What instrument is commonly used to measure a ship's speed through the water?

• A. A log (pit log or log line).
• B. A radar speed sensor.
• C. Anemometer.
• D. Gyrocompass.

Answer: (A)

A log is the instrument commonly used to measure a ship's speed through the water. It determines how fast the water is moving past the hull, giving speed in knots. Traditionally, a log line with a weighted log is paid out for a measured time, and the number of knots that pass the line correlates to speed. Modern log devices may use an impeller or other sensors to sense water flow, but the principle remains: speed through the water is derived from the motion of the water relative to the vessel. The other instruments serve different purposes—radar speed sensors relate to targets or surface motion and can indicate speed over ground rather than through the water, an anemometer measures wind speed, and a gyrocompass provides heading and direction data.