Using a Hand-Bearing Compass
By Don Casey
Revised by BoatUS editors in April 2012
Perhaps you have wondered whether a hand-bearing compass is really useful or just another nautical toy that seems like a good idea when you buy it, but then spends its life inside a locker. To help you answer that question for yourself, here are some ways that a hand-bearing compass might be used.
In a crossing situation you can determine very early if there is any risk of collision by keeping track of the relative bearing of the other boat. If the bearing doesn't change, you are on a collision course, no matter what direction your bow is pointed in. This is an easy check with a hand-bearing compass. Let's say the other vessel is a ship. As soon as you see it, you get a bearing on it with your hand-bearing compass. Five minutes later you take another bearing. If the reading has changed appreciably, the ship will pass safely clear, either ahead or astern, assuming both vessels maintain current course and speed.
Fixing your position
When two or more charted features are visible and identifiable, a hand-bearing compass makes it very easy to determine your exact location. Suppose there is a water tower visible to the west of you and the stacks of a power plant visible to the south. First you take a bearing on the water tower, then plot that bearing on your chart, with the plotted line running through the charted location of the tower. Next you take a second bearing on the stacks and plot that line on your chart. Since you are on both lines--called lines of position--your exact location is where they cross.
You don't always need two bearings to determine your position. Let's assume you have plotted the bearing to the water tower, but haze obscures the power plant. Look at your depth sounder. If it reads 60 feet, for example, your location is where your bearing line crosses the 10-fathom contour line on the chart.
Perhaps you are running down the coast and you know there is a shallow area somewhere in front of you that extends out from the shoreline a mile according to the chart. How can you tell if you are far enough offshore to miss this hazard?
The traditional method is called bow and beam bearing. It is based on the geometric fact that two sides of a 45° right triangle are equal. When some feature at or near the shoreline is 45° off the bow, you begin keeping track of your distance through the water, either by log or by time and speed. Note that the bearing is relative, so the bearing compass reading will be 45° less or more than the steering compass reading, depending on whether the shoreside feature is to port or starboard. When the shoreside feature is abeam, bearing 90° relative to your heading, the distance you have run is equal to the distance off.
With a hand-bearing compass, an even easier method is available. As you pass a shoreline feature, the time in minutes it takes the bearing to change by same number of degrees as your boat speed gives you distance off in miles. Say what?
Here is how it works. You are motoring along at 12 knots. When the shoreline feature you are using (at night it would be a light) is about half your speed in degrees ahead of the beam, i.e., about 6° ahead of the beam, you start timing your run. Let's make this easier to understand by assuming that your heading is due south, 180°, so when the marker passes abeam on your starboard side, it will bear 270°. When it bears 264° you note the time, noting it again when the marker bears 276°. If it took two minutes and 15 seconds for the bearing to change 12° while you were running at 12 knots (or 6° while running 6 knots, or 30° while running 30 knots), your distance from the marker when it was abeam was 2 1/4 miles--the same as the time in minutes.
By the way, knowing you are 2 1/4 miles off gives you one line of position. A bearing on the shoreline feature gives you a second line of position. Where they cross is where you are.
Can't you do all of these using the boat's steering compass? The answer to that is not very well. The accuracy of these methods depends on the accuracy of the bearings, and a bearing compass easily provides bearing accuracy to about 1°. Accuracy is nowhere near that good when you are sighting over the globe of a bracket- or binnacle-mounted compass. Where the compass is bulkhead mounted, taking bearings without pointing the boat is virtually impossible.
For a more complete understanding of piloting and navigation, I recommend Boat Navigation for the Rest of Us by Captain Bill Brogdon.