How is the depth beneath the keel recorded?

The depth beneath the keel is accurately recorded using a fathometer, which is a device specifically designed for measuring the depth of water. It does this by sending sound waves through the water column to the seafloor. When these sound waves hit the bottom, they bounce back to the device. The fathometer calculates the depth by measuring the time it takes for the sound waves to return, using the known speed of sound in water.

Although other devices like sonar and echo sounders also utilize the principles of sound waves to measure distances underwater, they may not be specifically calibrated or designated for measuring depth in the same way a fathometer is. The terms "sonar" and "echo sounder" can refer to broader categories or different functionalities that might not focus solely on depth measurement. Therefore, the specificity and purpose of a fathometer make it the most accurate choice for recording the depth beneath the keel.