Chuck Husick: Techno-Talk, January 2002 BoatUS Magazine -

Autopilots (Part II)
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.

We recently sailed our 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.

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 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 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. Next month we will look at a way to improve an autopilot's ability to hold northerly headings and to deal more effectively when steering the boat in a following sea.