Question 56 FCP01 - First Class Pilot

The Correct Answer is A. ### Why Option A is Correct Option A states that the speed of sound through ocean water is nearly always **faster than the speed of calibration for the fathometer**. 1. **Fathometer Calibration Standard:** Fathometers (echo sounders) are devices used to measure water depth. Historically and conventionally, they are often manufactured and calibrated based on a standard speed of sound in water. The most common standard used for calibration is **4,800 feet per second (ft/s)**, or approximately 1,463 meters per second (m/s). This standard value is representative of sound speed in freshwater or slightly brackish water. 2. **Speed of Sound in Ocean Water:** The actual speed of sound in the ocean depends on temperature, pressure (depth), and salinity. Typical sound speeds in the ocean range from about 1,480 m/s to over 1,540 m/s (or about 4,850 ft/s to over 5,050 ft/s). Because ocean water has high salinity and is under pressure, the sound speed is generally significantly higher than the conventional 4,800 ft/s calibration standard. 3. **Conclusion:** Since the actual speed of sound in the ocean (typically > 4,850 ft/s) is higher than the standard calibration speed (typically 4,800 ft/s), the speed of sound through ocean water is nearly always faster than the speed of calibration for the fathometer. *** ### Why Other Options Are Incorrect **B) the same speed as the speed of calibration for the fathometer** This is incorrect. As explained above, the actual speed of sound in saline ocean water is almost always higher than the conventional calibration standard (4,800 ft/s). If the speed were the same, the depth reading would be accurate; however, since the actual speed is higher, the fathometer usually reads a depth slightly shallower than the true depth unless adjusted. **C) faster than the speed of calibration for the fathometer, unless the water is very warm** This is incorrect because it includes a false qualifier. Sound speed *increases* significantly with temperature. In fact, if the water is very warm (especially in surface layers), the speed of sound would be even *faster* than the standard calibration speed, making the initial statement even more true. Warm water raises the sound speed, it does not lower it below the calibration standard. **D) slower than the speed of calibration for the fathometer** This is incorrect. While it is possible for sound speed to drop slightly below the calibration standard in extremely cold, deep freshwater lakes or certain low-salinity, high-pressure environments, this is not typical for the broad range of oceanic conditions. The presence of salinity and the general range of oceanic temperatures ensure that sound speed is nearly always above the conventional 4,800 ft/s calibration standard.