A Seaman's Eye

It wasn’t a dark and stormy night. It was just plain dark. Heading home to our marina about 20 miles away, it was blacker than an undertaker’s overcoat. But, no worries, the pale green digits on the GPS showed that I was on track for my home waypoint. All was right with the world.

That is, until the midway point, when the GPS went blank. No amount of jiggling (or swearing) would bring it back to life.

Fog, dark of night, foul weather—these are the times that try a skipper’s soul. For those of us who didn’t achieve much success in high school mathematics, the thought of losing a GPS and having to work out complicated navigational formulas by hand is enough to raise the hackles on our necks and whiten our knuckles on the wheel.

But there are some tricks that seamen have used to navigate for centuries that don’t require higher mathematics. Some are good for emergencies like the one on that dark night, some are useful even in good weather, and a few are part of the “seaman’s eye,” a quick and simple art that is already your primary navigational tool in good weather.

Everyone says that our navigational black boxes are fail-safe, but, buyer beware. Think about how often your home computer, sitting in a warm, dry, vibration-free environment, suddenly goes on the fritz. Anyone who claims that his electronics have never had a glitch is either lying or is on his first voyage. For that reason, you must—must—carry paper charts. If you don’t, then shame on you, and you deserve to get lost.

Here are 15 more tips to make your boating safer, easier and even a bit more fun...

Know Your Horizon

You can navigate better if you know the distance to the horizon. Because the Earth curves, the higher your eyes are above the water, the farther away the horizon will be. There is a formula to figure it out (multiply the square root of the height in feet by 1.144), but the following table should be simpler:

Height - Distance to Horizon

5’ - 2.7 nm
6’ - 2.8 nm
7’ - 3.0 nm
8’ - 3.2 nm
9’ - 3.4 nm
10’ - 3.6 nm

If, for example, your eye level when standing at the helm is 6 feet, then the horizon is 2.8 nautical miles away. You can judge your distance from an object by watching to see when it first appears on the horizon.

Let’s say that you are heading for a channel entrance with a 10-foot marker on the jetty. When you first spot the top of that marker through your binoculars, you can figure your distance away by adding your distance to horizon (2.8 nm) and the distance to horizon for the marker (3.6 nm). You are 6.4 nautical miles from the marker.

Take Your Pet


It’s called “dog bark navigation,” and it relies on sound to find your way in foggy weather or on dark nights. If your coastline has sheer cliffs, you can time an echo to find out how far you are from those cliffs.

When you sound your horn, it might take eight seconds for the echo to bounce back. Divide that in two (the sound must go to shore and back) and then multiply it by 1,200, which is roughly the distance in feet that sound travels in a second. You’ll come up with 4,800 feet, or about three-quarters of a nautical mile off the cliffs.

Know Your Feet

If you multiply your speed in knots by 100, you’ll find your speed in feet per minute. For example, at 5 knots (5 x 100), you are moving at 500 feet per minute.

As an example of how to use this knowledge, imagine that you’re in a channel in dense fog and you know from your chart that a nearby buoy is 1,500 feet from the shore. You want to anchor a safe distance from the channel until the fog lifts, so you steer toward shore at 5 knots for two minutes and then anchor. Even though you can’t see a thing, you know that you are a safe 1,000 feet from the buoy and 500 feet from shore.

Walk a Mile

To determine a nautical mile on a chart that doesn’t have a mileage scale, use the latitude marks on the right and left borders. One minute of latitude equals (within a few feet, anyway) 1 nautical mile. Don’t ever use the longitude marks across the bottom or the top; they aren’t even close to a mile.

A nautical mile is 6,076 feet, while a statute mile (used on road maps and Great Lakes charts) is 5,280 feet. To convert nautical miles to statute, multiply the nautical miles by 1.1508. To convert statute to nautical, multiply by 0.8690.

Repeatable Headings


Many small boats (especially tenders) don’t have their compasses carefully corrected, nor do their skippers carry deviation cards showing the compass reading for all the various headings. So, in fog or dark of night, how can they trust their compasses?

The best way is by using what navigators call “repeatable headings.” In good weather, get out your chart while you’re using the channels in your normal boating areas. Check the compass when you’re on the proper course, and note the compass bearing on the chart. Do this for all the courses you need to find your way to your dock or mooring.

In poor visibility, you will then know that you’re on the proper course as long as your compass reads the bearing you wrote on the chart. Even if it is in error, your compass will always read the same bearing for each heading.

Be a Dip


At night, you can use the same distance-to-horizon concept to “dip” a light. On your chart, a lighthouse or beacon will have its visible range shown. That range is the result of the height of the light above water. A 100-foot-tall lighthouse, for example, is visible (from water level) for 11.4 nautical miles. But, for a skipper standing at a height of 6 feet above the water, you need to add an additional 2.8 nm. So, you’ll first see the light at a distance of 14.2 nm away.

To check that distance, dip the light by bending your knees to lower your head a few inches. If the light disappears when your eye level is slightly lower, then you’ll know that you’re right at the edge of visibility.


The seaman’s thumb rule is that you should never come closer to a potential danger than the width of your thumb on the chart. This amount of space keeps you well away from the hazards on a small-scale (and less detailed) chart, and allows you to pass closer on a large-scale chart that shows all the necessary details for safe navigation.

Known Error


In reduced visibility or if you don’t know exactly where you are, build a known error into your course. For example, when heading for a distant harbor that you can’t see, steer a few degrees to the right of the proper course (assuming there are no hazards in that area, of course).

In that way, when you reach the shore, you’ll know that you need to turn left along the shoreline to find your objective.

The Stars are Bright


You don’t have to be an astronomer to navigate by the stars. Locate the Big Dipper (if you don’t know what the Big Dipper is, you’re hopeless). Imagine a line between the two stars that form the front edge of the Big Dipper’s cup. If you extend that line about five lengths (use your fingers to measure) above the bowl, you’ll see a not-so-bright star that doesn’t move like other stars. That is Polaris, the pole star, which is almost directly above the North Pole. You now know which way is north, and can use that information to set a course.

Rock Around the Clock

If a commercial radio station has its transmitting antenna marked on your chart and you have a portable radio on board, then you can use it to find the direction of the tower to help locate your position. Turn the radio slowly (you may have to lay the radio on its side) until the radio station sounds the weakest. At that point, the antenna is pointing either directly toward or away from the tower, and that angle will give you an idea of your position.

Be Prepared


Make sure your compass is working properly, and carry a spare compass if you can. Don’t put all your faith in your electronics, because they can go zap in an instant. And, always have basic navigational equipment on board. The basics are a couple of pencils, a pair of dividers for measuring distances, a course protractor or parallel rules and, of course, the charts for your area.

With the right tools and the knowledge to use them, you may find basic navigation to be easier than high school math.

Know Your Speed


Knowing how fast your boat is going at various engine settings is invaluable in finding your way home, particularly when the electronic gizmos fail you. With an average load of passengers and fuel, time your boat at 500 rpm settings over a measured distance, and make a speed table. Using that table, you can easily determine how far you’ve traveled, as long as your speed is constant. Let’s say you know that the entrance buoy to your destination is 10 nautical miles away and, at 1200 rpm, you are doing 5 knots. If you steer along the proper course, you will be at the buoy in two hours, barring current.

Give it Three Fingers

It’s often difficult to judge distance, particularly over water where you have large, empty expanses. One of the oldest seaman’s eye tricks is the three-finger rule, which is an easy way to find your distance from an object of known height.

If you hold three fingers out sideways at arm’s length, you can use them as a height indicator. Three fingers as a measure of height equal an angle of roughly 6 degrees, and any object that makes a 6-degree angle is 10 times as far away as it is high (you’ll have to trust me on the mathematics for this one).

So, let’s say that you can see a radio tower on shore. Your chart says it is 100 feet high. When it appears to be three fingers high, you are about 1,000 feet away from it.

Ah, you say, but it’s only about a finger and a half high. Now what? In that case, it’s 20 times the height, or 2,000 feet away. If it’s twice the height of three fingers, then it’s five times the height, or 500 feet away.

Use this method to keep a comfortable distance offshore when passing lighthouses or other objects.

Due South

When all the gremlins combine forces so that neither your compass nor your electronics are working, you can easily find south if you have a wristwatch (sorry, digital watches don’t count). Hold the watch horizontally and point the hour hand at the sun. As long as you’re in the northern hemisphere, the point halfway between the hour hand and 12 will indicate roughly south. Use that to set a course for home, where you can buy a spare compass.

Use Your Depthsounder


To help find your position, don’t just rely on the objects above the water like landmarks or buoys. Use the water itself.

Your chart is marked with depths and, in most areas, these deepen gradually as you go farther from shore. By checking the depth of the water using your fishfinder or depthsounder, you can tell your distance from shore and, in some cases, even pinpoint your location in a particular area, such as an offshore seamount or canyon.