Which device measures speed through water?

• A. Echo sounder
• B. Gyrocompass
• C. Speed log
• D. Barometer

Answer: (C)

Speed through water is the vessel’s velocity relative to the water it’s moving in. The device that directly measures this is the speed log. It works by sensing the flow of water past the hull—older types use a paddle wheel that turns with the water, newer ones may use electromagnetic sensing or Doppler techniques to generate a signal proportional to water speed. This gives you the speed through the water (STW), which is different from speed over ground (SOG) that you get from GPS and current, because SOG combines STW with water current.

Echo sounder, by contrast, sends sound pulses to the seabed to determine depth, not speed. Gyrocompass measures heading using a gyroscope, not speed. Barometer measures atmospheric pressure to help with weather forecasting and altitude references, not speed.