Autopilot Magnetic Interference: How to Avoid It

The heading sensor takes a magnetic reading

An autopilot steering in automatic mode needs to know which way the boat is pointing. That job belongs to the heading sensor, sometimes called a compass. The sensor reads the Earth's magnetic field and reports a heading to the autopilot processor, which then commands the drive unit to hold that heading.

Some autopilots build the heading sensor into the processor unit. Raymarine's Evolution line calls its sensor "the autopilot central processor and 3D heading sensor at the heart of every Evolution autopilot system." Other autopilots, including Furuno's NavPilot 300, require a separate heading source on the NMEA 2000 network, such as a PG700, SC33, or SCX20. The word "magnetic" is the useful clue: the sensor works by detecting magnetic fields, so any other magnetic field near it can distort the reading. See heading sensor types for the options a listing may name.

Sources of magnetic interference aboard

A boat is full of things that create magnetic fields. The heading sensor does not care whether a field comes from the Earth or from your equipment: it reads the total field at its location. The practical question is what sits close to the sensor.

• Speakers: the permanent magnets in stereo speakers are a common source, especially in a cockpit or helm area.
• Electric motors: windlasses, wiper motors, trim tab motors, refrigerator compressors, and engine starter motors contain magnets and draw current.
• Batteries and battery cables: current flowing in a wire creates a magnetic field, and starter and charging cables carry a lot of current.
• Ferrous metal: steel bulkheads, fuel tanks, rudder posts, and engine blocks concentrate and distort the field.
• Magnetized tools: a screwdriver or knife kept near the sensor can leave a small permanent field of its own.
• Other electronics: radar, radio antennas, and inverter wiring in close proximity add their own fields.

Checking sensor placement before you buy

Listings on this site state the parts in the box, the model line, the steering type, and the drive. They do not state where to mount the heading sensor. That information lives in the maker's manual, which covers siting in detail. Before you order, decide whether you will read the manual yourself or hand it to a qualified installer. The sensor requirements article explains what a heading sensor needs in general terms.

The general rule in those manuals is to put the sensor away from large metal masses and away from wires that carry high current. A spot away from the engine block and the battery bank is the usual goal. The exact distance from any source is stated by the maker, not by this site.

If you are buying a complete pack or a core pack, you can see whether the heading sensor is in the box. Raymarine's Evolution packs build the sensor into the processor. Furuno states that its NavPilot 300 requires a PG700, SC33, SCX20, or another compatible NMEA 2000 heading source. Ask the seller or the dealer which heading sources the pack expects. The core packs guide compares packs by what the box contains.

If you are not sure what steering system your boat has, Raymarine advises consulting an authorized dealer. The same advice applies to sensor placement: a dealer or qualified installer who knows your boat can tell you where the sensor can sit and what equipment is already near that spot.

What a listing will and won't tell you

A listing title might read "EV-200 Sail Pilot" or "NavPilot 300 with pump." These names tell you the model line, the steering, and the parts. They never tell you the sensor's mounting location, and they never promise a course-holding result.

Use the listing to confirm that the pack includes a control head, a drive, and a heading sensor. Then move to the maker's page and the manual for siting. The NMEA 2000 standard describes a network that lets marine electronics share data, including commands and status, so the heading sensor can talk to a chartplotter over that network. See NMEA 2000 connectivity for how that works.

If a listing is missing a detail about the sensor, treat that as a question for the seller, not as an assumption. The guides on this site compare stated details, and placement is not one of those details. For a full view of what a pack should include, the complete autopilot packs guide is the place to start.

Why calibration comes after placement

Once the sensor is mounted, the installer runs the autopilot's calibration routine, which usually includes a compass calibration. The manual and the installer handle that step. You will read more in the calibration article.

Calibration corrects for the boat's own magnetic signature, but it cannot fix a sensor sitting next to a speaker magnet. Placement comes first, calibration second. Compass offset, another calibration step, is covered in the compass offset article.

Planning a sensor spot on your boat

For a sailboat with a tiller, the autopilot is typically a self-contained unit. Tiller pilots carry their own sensor in the cockpit, so the placement question applies to the whole unit rather than a separate sensor.

For a powerboat with hydraulic steering, the sensor may be a separate unit in the console or under a berth. The same rule applies: keep it away from current-carrying cables and ferrous metal. If your pack does not include a sensor, plan for a compatible NMEA 2000 heading source.

The best moment to think about magnetic interference is before you buy, when you can still choose a pack that fits your boat and read the manual for siting.

What to pick for your situation

Questions

What causes magnetic interference in an autopilot system?

Anything magnetic or current-carrying near the heading sensor: speakers, electric motors, batteries, battery cables, ferrous metal, and magnetized tools. The sensor reads the total magnetic field at its location, so a nearby source distorts the reading.

Can a listing tell me whether the heading sensor will work on my boat?

No. A listing states the parts and the model line. The maker's manual and a qualified installer decide where the sensor can sit.

Does calibration fix magnetic interference?

Calibration happens after mounting and corrects for the boat's own magnetic signature. It is not a substitute for a mounting spot away from magnets and high-current wires.

Some packs do not include a heading sensor. What does that mean?

You supply a compatible heading source, usually on the NMEA 2000 network. The maker's page names the compatible sources, such as Furuno's PG700, SC33, or SCX20 for the NavPilot 300.

Should I mention my boat's layout when buying?

Yes. Tell the dealer or installer where the sensor might go and what equipment sits nearby. They can recommend a pack with or without an integrated sensor.