# Distance Off by Echo Ranging

In fog, in inshore, cliffy waters you can try echo ranging for the distance off. This uses the time delay between the production of the sound and the arrival of its echo. This assumes that the speed of sound is a constant. It actually varies with the type of gas

12.10 Doubling Angle on the Bow

12.11 Four Point Fix

The sound travels from the boat, hits the cliff and the echo returns. Remember this sound is omnidirectional. If you are in a fiord you will hear an echo from both sides, probably at slightly different times.

The sound travels from the boat, hits the cliff and the echo returns. Remember this sound is omnidirectional. If you are in a fiord you will hear an echo from both sides, probably at slightly different times.

Distance off = speed of sound in feet per second x time / 2 = 1129 x 8.5/2 = 4798.3 ft = 0.79 nm

12.12 Echo Ranging and its temperature, and at 21 °C the answer is 669 knots (1129 feet per second or 344 metres per second). In water the speed of sound is 2877 knots (1480 metres per second).

You now know the speed at which your blast on the foghorn travels to the cliff, bounces off and returns, and the time this return journey took. Do your sums, halve the answer and you have your distance off (see Figure 12.12). A variation is finding out how far you are from an electrical storm. Time the interval between a flash of lightning and its thunder arriving. Do your sums, but this time do not divide the answer by two, and hope the storm is moving away from you.