How to Re-site a Fluxgate on a Raymarine Autopilot

Fluxgate compasses feed directional data to autopilots on sail and powered boats.

The Raytheon Autohelm ST-70 fluxgate compass provides directional input to Raytheon Raymarine marine autopilots. The ST-70 is an electromagnetic device consisting of a remote compass sensing unit that utilizes coils to register the magnetic fields of the earth. The compass unit delivers directional data through a cable to a controller in a digital format used by autopilot computers. Over time, fluxgate compasses may be influenced by on-board electromagnetic sources, gradually increasing the magnetic deviation of the compass and degrading accuracy. This condition requires the user to re-site the compass sensing unit in the vessel. Deviation of the ST-70 fluxgate compass can be checked by a procedure known as "swinging the compass" in the ST-70 diagnostics.

Instructions

1. Navigate to an area of open water where you can safely steer the boat through 360-degree circles without encountering obstacles or endangering other craft.

2. Access the compass calibration diagnostic feature on the Raymarine autopilot controller screen. From the main menu select "Autopilot Calibration" and page forward to the "Swing Compass" option. Press "OK."

3. Turn the boat in slow circles, maintaining speed below 2 knots and completing each full, 360-degree circle in two minutes or more. Make a minimum of two circles.

4. Observe the Swing Compass screen. If a "Slow Down" message appears, adjust the helm to turn in a wider circle.

5. Stop circling when the autopilot beeps and the Swing Compass screen displays the calculated magnetic deviation figure. If the deviation figure exceeds 5 degrees, re-site the compass.

6. Re-site the compass sensing unit at least 3 feet from the boat's steering compass. Select a location farther from other electromagnetic sources such as a VHF radio transmitter or radio antenna cables, windshield wiper motors and other devices, while still remaining as close as possible to the boat's pitch and roll center.

7. Re-site the compass sensing unit farther from the engine if the magnetic deviation is 15 degrees or more. A high magnetic deviation indicates magnetic influence from ferrous materials, usually cast-iron engine components.

8. Repeat the Swing Compass procedure after re-siting to verify that the magnetic deviation of the fluxgate compass is now less than 5 degrees.

References

• Raytheon Inc: Raymarine ST-70 Plus Fluxgate Compass User's Guide PDF