Determining Distance from the Horizon - 01/13/2010

You’re offshore, well out of sight of land. You are anchored on an old wreck and the fish are biting and the good times are rolling. All of a sudden, one of your buddies sees a speck on the horizon heading your way. He asks if he should up anchor or does he have time to catch a few more. How long can he continue fishing? It’s a reasonable question and having an idea of how far away your horizon is will help with the answer. That’s one of those little things that you should always have in the back of your mind. Now it takes a little figuring, but it’s not hard to do. With a cheap calculator you can tell him how long he has to fill the fish box.

We see a speck on the horizon but how far away is it? A) 3.3 nm, B) 6.2 nm, C) Unknown.

Capt. Steve Says...

The formula we need is the one for determining the distance to the horizon. Since it stands to reason that the higher you are, the farther you can see, the formula will be dependent on the height of your eye above the water’s surface. It goes like this.

D=1.17 x (square root of your eye height in feet)

With this formula, “D” is the distance in nautical miles. For this example, we’ll figure that in your 24’ center console, your height of eye is 8’ above the surface. Break out your calculator and find the square root of 8 (unless you’re one of those captains that can figure it out in your head). It comes to 2.8 (yes, I used my calc.) I then multiply 2.8 x 1.17, and every time I do that I come up with 3.3 nautical miles for a distance to my horizon. Ta Dah...

“I see you... But from how far away will you see me???” A) 3.3 nm, B) 6.5 nm, C) The same distance.

Now, There Are Two Horizons

Let’s take it one step further. At the entrance to my harbor, there’s a 35’ (as indicated on my chart) lighthouse attracting the seagulls and interested boaters. I figure how far that lighthouse can “see” with the same formula. So... the square root of 35 is 5.9, 5.9 x 1.17 is 6.9 nautical miles. The lighthouse can “see” 6.9 nm.

By adding the 3.3 nm that I can see to the 6.9 nm that the lighthouse can “see”, I know that I will be able to see the lighthouse when I am exactly 10.2 nautical miles away from it. At this point the savvy captain will be able to tell exactly when he’ll be 10.2 miles away.

That speck on the horizon will be up on you faster than you might think. Use our simple calculation to predict its ETA.