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Chuck Husick: Autopilots - Updated December 2008


Available immediately: competent helmsman for virtually any power or sail boat, will learn your boat’s behavior to improve existing skills, can stand watch endlessly without tiring, never need to go to the head and will live on only a few watts of electrical power per hour. Contact “Autopilot” in your marine equipment catalog.

An autopilot is a wonderful addition to almost any cruising boat. Properly installed and adjusted it will faithfully steer your boat in whatever direction you desire either following a chosen magnetic heading or, when connected to a GPS or Loran C navigator tracking precisely to a selected waypoint. I have sailed my boat down the Intracoastal Waterway from Norfolk, VA to Charleston, SC. As anyone familiar with the route is well aware there are long stretches where an autopilot that can hold a precise heading or track to a waypoint can make it possible to relax and enjoy the scenery. With the autopilot controlling the helm you will be free to survey the activities of other boats and the sea ahead, simultaneously improving your appreciation of the day and increasing your safety. An autopilot will improve the performance of your vessel by steering a course or holding a heading with more precision than a human helmsman.

The autopilot operates by repeatedly comparing the boat’s magnetic heading to the boat’s actual heading. The comparison is done many times each second with any deviation resulting in a correcting movement of the tiller or the wheel. When connected to a GPS or Loran C the boat’s course over the ground can in addition be compared with the COG to the waypoint, The autopilot’s “brain” accomplishes this task precisely while a human helmsman is easily distracted, becomes tired and often allows substantial steering errors to go unnoticed. The human
helmsman performs best when maintaining the general lookout necessary to ensure safe navigation and an enjoyable boating experience.

The addition of powerful microprocessors and gyro-stabilized heading sense systems have improved even modest cost autopilot performance to levels previously achieved by only the most expensive yacht autopilots. Sailboat owners benefit from new low electrical power drain rudder control servos. Many of today’s autopilots automatically recognize varying sea conditions, changing the way in which they move the helm in response to the changing impact of wind, waves and currents. Some of the latest pilots use a form of “fuzzy logic,” software programs that accumulate a history of the boat’s reaction to varying operating conditions and then use the information to improve the behavior of the steering system and thereby the performance of the boat.

Autopilot steering performance in a following sea has always presented a problem when the force of the sea throws the boat’s stern to one side or the other. A well trained human helmsman can sense the initial acceleration force that occurs a fraction of a second before the boat’s heading changes and can move the rudder the small amount needed to offset the turning force, preventing the boat from yawing to port or starboard. In the latest autopilots, a miniature solid-state gyro sensor in the heading sense system provides the autopilot with a “seat of the pants” sensitivity to the initial acceleration forces. The autopilot uses this acceleration information to emulate the best human helmsman, correcting for the yawing force before it can materially alter the boat’s heading.

To a casual observer, the precision with which the autopilot steers any selected magnetic heading may seem astounding, surpassing even the best helmsman. However the more astute mariner may have noticed that the autopilot's ability to precisely track a heading seems to depend on which way the boat is going. The boat seems to wander a bit more when heading toward north than when sailing south. This may seem perfectly reasonable during a cold northern winter when any self-respecting autopilot would want to take its boat south to a warmer climate, however, climate and time of year are not the source of this sometimes subtle, sometimes obvious difference in performance. The effect is caused by the Earth's magnetic field and affects both your autopilot and your mechanical magnetic compass.

The magnetic field on which our compasses and all other types of magnetic field sensors depend emanates from North and South magnetic poles which slowly drift about near their respective geographic poles. The invisible lines of magnetic force that run from pole to pole are parallel to the surface of the earth only near the equator. The magnetic poles are deep within the earth, not on its surface, therefore the lines of force must bend downward as they approach the poles, eventually becoming vertical at the poles. The degree to which the lines of magnetic force are not parallel to the Earth's surface creates a magnetic heading error, the dip error. The dip error causes all magnetic compasses, including the electronic flux gate that provides information to your autopilot to behave strangely on headings close to north or south.

A compass can provide accurate heading information only when the compass card is level. In the northern hemisphere the south pole of the magnetic beneath the compass card, the end that is attracted to the Earth's north pole, is pulled slightly downward as it tries to align itself with the downward tilt of the magnetic field. The compass maker attempts to offset this dip by intentionally unbalancing the compass card, making the South side of the card slightly heavier than the North side. In fact, some compass makers will tailor the unbalance to suite your usual boating area. This intentional unbalancing of the card works well so long as the compass is stationary. However, when on a moving boat, a turn to the east from a heading of north will result in the compass initially showing a turn to the west (and the opposite if the turn was to the west). The error is quickly corrected and is not likely to be noticed when hand steering. On headings close to south a turn to east or west will result in the compass showing an exaggerated turn in the correct direction. The flux gate that provides information to your autopilot is effected in a similar manner.

Although a helmsman may not notice or be bothered by the momentary heading confusion caused by the dip error the autopilot is alert to even the smallest deviations. The result is that your autopilot will usually hold headings close to south noticeably more accurately than when heading north. At very large dip angles near the magnetic pole both your mechanical compass your autopilot will be quite useless. As you have already figured out, the effects are reversed in the Southern Hemisphere, when headings of north are followed more accurately than those to the south.

Although knowing about magnetic dip error may be interesting and useful in explaining the phenomenon you have observed the knowledge won't improve the steering accuracy of your autopilot. However there are ways to partially compensate for this effect and some other heading sense errors that can degrade autopilot performance. Some of the more complex electronics installations incorporate GPS compasses that can provide precise true north heading information with none of the errors created by the characteristics and anomalies in the earth’s magnetic field. Although still quite costly (typically at least $2,000) the GPS compass is gaining in popularity due to the superior results it delivers when used with a chart plotter in radar and image overlay mode. The convenience in being able to navigate without reference to the changes in magnetic variation are also greatly appreciated by many navigators.

The most common autopilot magnetic heading sensor is a flux gate, a type of electronic compass. It may be built into the pilot, located in the system control box or a separate module intended for remote mounting. Regardless of where the sensor is located it must be protected from local magnetic fields such as those emanating from wires carrying DC current. Pilots using built-in flux gates may be have to be quite close to the loud speakers for the boat's entertainment system or the speaker in the VHF radio. Loudspeakers contain strong permanent magnets and are common sources of strong magnetic interference. Microphones and some cellphones may also contain permanent magnets and can interfere with the heading sensor. If you install an autopilot and find that it steers the boat in small circles it may be because it has fallen in love with the starboard hi-fi speaker. Magnetically shielded marine grade loud speakers should be used if the speaker and the autopilot's sensor must be less than about three feet apart. Remote mounted flux gates are best installed near the center of the boat, at the place of least vessel motion in a seaway. Check the area to be sure there are no magnetic materials stored nearby and that crew or visitors don't decide to put their portable radio or a pile of canned food next to the flux gate.

The flux gate systems include an automatic means for correcting for deviation, the errors created by local magnetic effects. The precise procedure will vary but generally involves nothing more complicated than making a 540° or 720° circle in smooth water. Precise compensation for deviation may not be an issue where the vessel's main steering compass is used as the heading reference.

Sailboat systems offer steer to wind options, using information from an apparent wind direction sensor to maintain the desired relative wind angle. Many autopilots provide a pre programmed 100 degree course change mode for tacking a sailboat. The power consumption of small boat autopilots is usually quite modest. Tillerpilots on small sailboats can be powered for many hours from a modest size deep cycle battery. The power required to steer even a fairly large boat, 40-50 feet in length, usually averages less than 60 watts, 5 amperes on a 12 volt system.

One of the distinguishing points in autopilot design is the use of a rudder position sensor. A human helmsman makes constant use of his knowledge of the position of the vessel's rudder while holding a heading. Many autopilots are equipped with a rudder position sensor, connected directly to the boat's rudder post. The autopilot is programmed to use this information to ensure that the rudder is deflected only as far as necessary to make a timely correction from an off heading condition. Use of rudder position information greatly improves overall autopilot performance, especially in a seaway. Autopilots that function without knowledge of rudder position will generally steer less efficiently and with less precision than those supplied with rudder position feedback. Some autopilots, particularly those specifically designed for use with outboard motors compensate quite effectively for lack of a rudder heading sensor by using internal logic to simulate for the heading sensor

Owner installation of an autopilot need not be difficult and provides the advantage that the boat owner will learn how to make the adjustments required to match the system to the boat. Installation of autopilots that connect directly to the boat's existing hydraulic steering system may benefit from professional help with the plumbing and initial set up of the hydraulic portion of the system. A mistake or a poor installation could deprive the boat of all steering command. Installation of autopilots that are interfaced with other on board equipment may require the assistance of an electronics technician to ensure that all systems are "talking" the same electronic language usually NMEA 0183 or in the newest systems NMEA 2000.

If at any time a steering system problem arises on an autopilot equipped boat immediately turn the pilot off and try hand steering. If the problem persists, open the circuit breaker that serves the autopilot or remove the fuse. A defect in an autopilot that is supposedly off but actually partially operating can
disable a boat's steering system.


By Chuck Husick





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