If you want to know the distance to the horizon, first calculate your height of eye. That is the distance that your eyes are off the surface of the water. If you were sitting in a canoe, that would probably be about three feet. If you were standing on the beach, your height of eye is the distance from your eye to your feet. In the cockpit of your boat, add the height of the cockpit above the waterline to the equation.
Once you know your height of eye, you simply plug that into the following formula:
1.17 X the square root of height of eye = distance to the horizon in nautical miles
For example, let’s say you are on the water in a friend’s sport-fishing boat and your height of eye is 9 feet above the surface of the water. The formula to calculate distance to the horizon is:
1.17 X the square root of 9 = distance to the horizon in nautical miles
1.17 X 3 = 3.51 nautical miles