Question 954Navigation General

The correct answer is B) actual depth of water below keel. Echo-sounders, also known as depth finders, measure the actual depth of water below the vessel's keel. They do this by transmitting a sound pulse and measuring the time it takes for the pulse to reflect off the seafloor and return to the transducer. This time measurement is then converted into a depth reading, which represents the true water depth beneath the keel. The other answer choices are incorrect because they do not accurately describe the primary function of an echo-sounder. Option A refers to the average depth to a soft bottom, which an echo-sounder cannot measure directly. Option C is too broad, as echo-sounders measure the actual depth, not just the average. Option D refers to the average depth from the waterline, which is not what an echo-sounder is designed to measure.