Getting Your Bearing Straight
Years ago, I learned a fast lesson in the importance of an accurate compass. The trip was a short night passage on a 50-footer we were delivering across Lake Michigan from Chicago to the Michigan side of the lake. A bit of breeze was blowing, so it wouldnt take too long. As the miles slipped under the keel, a stereo in the cockpit kept the on watch entertained.
|Lubber lines at 90 and 45 degrees and a 360-degree dome on the Ritchie Globemaster help the helmsman to make tacking decisions and take quick bearings in any direction.
After six hours of faithfully following our compass course, we were shocked to discover we were far off of our dead-reckoning (DR) position. It didnt take us long to find the culprit: Speakers from the portable stereo had been placed too close to the compass, creating a magnetic field that fooled the compass.
Despite the advances in electronic navigation over the last few decades, one of the most important pieces of onboard equipment is the compass, a relatively simple device thats based on technology that traces its origins back 2,200 years. The modern magnetic compass has undergone a number of changes, and its accuracy far surpasses earlier designs. Meanwhile, new types of compasses have been developed to serve specialized applications, support other navigational equipment, or back up the magnetic versions.
Fluxgate compasses and gyrocompasses (or otherwise stabilized compasses) are making more frequent appearances in cruising boats navigation stations. Each type of compass has advantages and disadvantages. And each can be prone to specific errors, of which we need to be aware. For all its simplicity, the ships compass isnt foolproof.
To understand compasses, you first must have a basic understanding of Earths magnetic field. When thinking of the magnetic field surrounding Earth, think of a bar magnet thrust through the center of the planet and inclined at an angle of about 11 degrees to Earths axis (see the illustration on this page). One end of the "bar magnet" extends to a point near the North Pole, at about 78.9 degrees north and 103.8 degrees west. The other end of the "bar magnet" is somewhat off the South Pole—near 65.4 degrees south and 139.5 degrees east. The magnetic field, which is constantly changing, splays out from these poles around the world in all directions. Only at the magnetic equator is the magnetic field horizontal—that is, concentric—with Earths surface. At all other points, the magnetic field is inclined toward the magnetic pole. This magnetic inclination is also referred to as "dip."
Dip is most pronounced at the poles, where the dip angle is 90 degrees. The magnetic field aims straight down toward the surface of the planet; theres virtually no horizontal component within the field. Because the magnetic field is vertical in this spot, compasses have difficulty finding magnetic north or south near the magnetic pole. Most compasses purchased in North America need to be professionally balanced if you intend to venture into polar regions or into the Southern Hemisphere. More manufacturers are introducing globally balanced compasses, which dont need to be adjusted for dip.
In strictest terms, the needle of a magnetic compass doesnt point toward the magnetic pole. Rather, the needle aligns with the horizontal component of the magnetic field at the spot where the compass is located. The magnetic declination, or variation (how the magnetic field varies from true north), is the difference between the direction of the fields horizontal component and the direction for true north. Since Earths magnetic field is a swirl of directions, the compass needle cant point to a single pole or point on the planet.
Knowing the direction of the magnetic field is only part of the problem. Metal objects and electrical currents have their own magnetic fields, and these can further interfere with a magnetic compass. In some cases, you can adjust the compass to compensate for such things as the magnetic field of a metal hull or an engine block by adding or shifting small magnets near the compass. In other instances, youll need a deviation card, a table indicating how many degrees to add or subtract in order to obtain the correct magnetic reading. Because local magnetic fields will affect the compass needle differently on different headings, the deviation card lists separate corrections for different headings. Most basic navigation books give instructions for making a deviation card.
Magnetic compasses have improved greatly in the last decade. Weems & Plath, Danforth, and Suunto offer magnetic compasses that can be used globally, even in the high latitudes of the Northern and Southern hemispheres. Magnetic compasses have several advantages over other types. They dont require electricity to work, so if theres an electrical failure, the compass still points the way. And depending on how the conventional magnetic compass is mounted, its analog display may be used to take bearings as well as to indicate the vessels heading, something thats impossible with a digital display.
Now for the disadvantages. A magnetic compass doesnt provide digital output that you can interface with such electronic navigational tools as a chart plotter, an autopilot, and so forth. You cant mount a magnetic compass remotely from its display.
As already mentioned, onboard magnetic disturbances from large metal objects or electrical wires near the compass can also cause compass error. If theres wiring near the magnetic compass, you may need to twist the positive and negative leads or use shielded cable to minimize the magnetic disturbance.
Some things that affect magnetic compasses are beyond our control. Volcanoes and solar storms both have the potential for altering Earths magnetic field. In some regions, Earths magnetic field shifts on a diurnal, a yearly, or an even longer basis. Some of these shifts are predictable and will be indicated on marine charts. The annual change in variation indicated on a charts compass rose is an example of such a predictable magnetic shift.
Although magnetic compasses are fairly simple and robust, they can develop a variety of problems. The glass dome can craze over the years. Oil can leak out and create an air pocket in the compass. If left too long, this bubble can cause pitting of the compass card or other internal parts.
There are many other potential problems, and a qualified compass adjuster may be able to assist you with your own problems. A more comprehensive list of what can go wrong and how to fix it can be found on professional adjustor Ray Andrews website (www.andrewscompass.com/index.html).
Like the traditional magnetic compass, the more modern fluxgate compass also detects Earths magnetic field, but it measures it electronically rather than merely pointing in the direction of the field. On todays cruising boats, most digital heading data, whether from the autopilot or the masthead wind indicator, is from a fluxgate compass.
Originally developed for aircraft during World War II, the fluxgate compass depends upon electrical power, usually the vessels batteries. It differs significantly from the standard magnetic compass in that it uses a magnetometer, which is a doughnut-shaped core wrapped with wire that converts Earths magnetism into electricity. Magnetometers float in liquid and are sometimes encased to cancel out deviation. The magnetometer orients itself to Earths magnetic field and creates a measurable current that can be translated into a digital readout. A fluxgate compass has two sensors, and it determines magnetic direction by comparing the fore-and-aft component of Earths magnetic field to the athwartships component. These compasses must be calibrated, a relatively simple process in which internal microprocessors compensate for deviation. Sometimes they can also be set, again using the microprocessors, to factor in local variation so they always read true.
Because they measure electrical current rather than merely pointing, fluxgate compasses can translate the current into digital signals. Other electronic instruments can read this digital output, allowing you to interface fluxgate compasses with autopilots, radar, and computers to provide an array of information, ranging from true-wind direction to sophisticated performance data. You can mount the compass sensor far away from any magnetic disturbances and still have displays in one or several convenient locations. Fluxgate compasses are extremely light—robotics versions weigh only a few ounces—and very inexpensive.
Fluxgates do have some disadvantages, however. They rely on electrical power to operate. They can have a bad electrical component, although theyre generally quite reliable. And some fluxgates can be less stable than a good magnetic compass. When stability is important, some sailors choose fluxgates designed for powerboats and built to take more slamming.
As with traditional magnetic compasses, you cant place magnetically charged items near fluxgates. I once was aboard a boat that had its fluxgate compass in a locker, and when someone placed the medicine kit in the same locker, our course reading was thrown off by about 70 degrees.
Autopilots have been known to veer off course when someone powers up or transmits on another electrical device, such as an SSB. If this happens, the cables to the SSB should be shielded and a radio-frequency choke might be required. Occasionally, the remedy is as simple as installing ferrite "beads" on the cable to reduce the magnetic field associated with the SSB or other electrical device. Grounding problems can also occur, such as when the SSB is grounded near the ground for the instruments or fluxgate compass. Separately grounding the systems may eliminate that problem.
With all of the increasingly sophisticated electronic equipment aboard the modern sailing vessel, accuracy and speed become an issue. In the past, gyrocompasses have been not only expensive but also energy hogs. In a gyrocompass, the same principles that keep a gyroscope perfectly balanced are applied to a compass so that it will always seek and maintain true north. To accomplish this, the gyros spin axis must always seek the meridian (north-south) plane. This axis must be oriented horizontally and needs to stay that way, no matter how much the boat pitches, heels, or rolls.
Until fairly recently, such compasses were almost exclusively for megayachts and ships. Now, however, KVH makes a magnetically stabilized gyro-like compass thats practical for small sailboats that might need an extremely accurate compass. This is the type of compass youll find aboard many high-tech ocean racers, like the Volvo 60s.
By the 1998 Whitbread Round the World Race, several of the boats used KVH magnetically stabilized compasses. Gyros at that time consumed too much power to make them practical aboard a racing yacht. Keeping that spin axis moving, after all, required constant energy. A model later introduced by Brookes & Gatehouse, however, uses only about 1.5 amps to keep the gyro spinning, and the compass is accurate in all conditions. Boats in the 2001-2002 Volvo Ocean Race put this type of compass to the test.
In the 1998 Whitbread, the stabilized compasses were used primarily to keep the Inmarsat B satellite-communications antenna pointed toward the satellite, even in a heavy seaway. But since the gyro requires relatively little power and the speed and accuracy is enhanced, it inevitably becomes the primary source of digital heading information for some of the boats. Today, manufacturers like Navman are bringing gyro accuracy well within reach of the average cruising sailor.
Gyros, even with their enhanced accuracy, do have some disadvantages, of course. With a conventional gyrocompass, if power is lost, it can take hours for a knowledgeable technician to get the compass working again after power is restored. Even the magnetically stabilized gyros require an outside power source to operate.
Gyros also require checking by the prudent navigator, of course. A case is on record of a ship with a gyro that had an error that went undetected for 12 hours. The ship steamed 110 degrees off course and went aground some 200 miles off its DR position. The incident serves as a reminder that the traditional magnetic binnacle compass is far from obsolete. If theres a power failure, the magnetic compass can get you most places around the world, or at least safely to the next port. But with advances in electronics and their heightened accuracy, fluxgates and stabilized compasses are making significant strides forward.
Last year, Simrad introduced a new alternative to high-accuracy, fast-reacting compasses, its new HS50 GPS Compass. Using phase-change technology rather than positioning information from satellites, the HS50 reportedly offers accuracy to .01 degrees as it maintains true north. Even while the boat is motionless, the phase-change technology, along with two separate GPS antennae in an elongated housing, allow the unit to determine which way the boat is pointed. The rate of response is so fast that it can track a 90-degree turn in 1 second—much faster than youd ever change course in a sailboat. Output is in RS-232, NMEA 183, and other digital protocols to allow connection with virtually any other equipment you might have in your nav station, including autopilots, radar, and computers. Aside from its electrical-power requirement and its relatively large size, perhaps the only disadvantage of the HS50 is its price. At slightly under $8,000, accuracy must be highly prized.
Normally, GPS offers heading information based on the track of a vessel over time. The system provides us with reliable position data a high percentage of the time. As a secondary set of information, it can provide us with an average course steered over ground. As we vary the damping on the system, we can narrow that course to tell us the course weve been able to achieve over a relatively short period of time.
What GPS—aside from the Simrad HS50—typically doesnt tell us is the direction that were pointing the boat. GPS collects data about our very recent track or course over the ground. That includes all of the other forces working on the boat in addition to where were pointing the vessel. These other forces include tides, currents, leeway, and any other anomalies that we may not even think about.
A GPS also serves us well by giving us the bearing and distances to waypoints from our current position. But we should be aware that a GPS often gives us default bearings along a great-circle route. Although its true that the great-circle route is shorter than the rhumb-line route over great distances, the initial great-circle heading toward a distant object might differ by 10 degrees or more from the rhumb-line heading, depending on how far away the waypoint is. Both routes will clearly differ when plotted on a Mercator Projection, and the difference can be critical. One route may be more advantageous than the other with respect to weather patterns, current, or even safe water. The prudent navigator will have a thorough understanding of exactly what route defaults are set in his or her machine and the consequences of following the GPS bearing.
Check all of your compasses periodically. In some cases, it may just be a quick check to insure that youre not 110 degrees off course, like the ship mentioned above. In other cases, as you prepare for a long passage or your annual relaunch, you may want a qualified compass adjuster to check your compass. The adjuster will work to ensure that the compass is properly compensated and that an accurate deviation card is created for each compass on board. At sea, you can check your own compasses by noting the bearings of the setting sun and comparing that bearing to the published amplitude in a nautical almanac. This information appears in the section entitled "Astronomical Information" in Reeds Nautical Almanac.
For a quick check, I often note the location and bearing of Polaris as I start a night watch. Other celestial bodies closer to the horizon may offer more accuracy because you can directly sight them over the compass lubber line. But since Polaris is always located near true north, you dont have to consult any tables. I use this method primarily to make sure that were not off course and that the compass heading makes sense relative to the stars.
Near shore, a precise way to check your compass is to use a range and compare the ranges bearing indicated on your chart to the reading your binnacle compass gives you. With a fluxgate compass, you can travel down the range and compare the compass reading to published data. Because compass error varies with the vessels heading, you must make any corrections—even when using shore-based land references—on a variety of headings to be accurate. This process of "swinging" your compass is an important skill to possess when long-range cruising.
Compasses can and should be instruments of precision. Their accuracy has increased in step with their growing sophistication, and today theyre capable of accuracy to within less than 1 degree of error—if we calibrate them appropriately. Contemporary compasses have certainly come a long way from the days when a needle piercing a cork was set floating in a bowl of water. But the principles remain the same, and your safety can hinge on knowing that your humble compass is working properly.
Bill Biewenga is a weather consultant and frequent contributor to Cruising World.