What is a log used for in navigation?

• A. To measure depth
• B. To log weather
• C. To measure speed through water or distance traveled; used for speed and distance calculations.
• D. To communicate with port authorities

Answer: (C)

In navigation, the log is all about how fast the vessel is moving through the water. Knowing speed allows you to figure out how far you’ve traveled over a given period, which is essential for estimating your time of arrival, planning fuel use, and plotting your course. Historically, a log line and a drifting float were tossed overboard to measure speed by counting knots over a set time, giving rise to the unit of speed we still use: knots. Modern logs use electrical, electromagnetic, or Doppler methods, or rely on GPS, but the goal remains the same: quantify velocity to support speed and distance calculations. The other options relate to depth measurement, weather observation, or communications, which are handled by different instruments.