Overview #
A Garmin chart plotter vessel icon that appears to be “crabbing”—where the boat icon is moving in one direction while the bow appears pointed in another—is typically caused by differences between the vessel’s heading source, GPS source, and chart orientation settings.
Understanding the difference between Heading and Course Over Ground (COG) is important when diagnosing this condition:
- Heading = The direction the vessel’s bow is physically pointing. This requires a heading sensor or electronic compass.
- COG (Course Over Ground) = The actual direction the vessel is traveling based on GPS position movement.
Without a heading sensor, the chart plotter uses COG to estimate vessel direction. In conditions such as strong currents, wind, low-speed maneuvering, or drifting, the boat icon may appear to move sideways or point incorrectly. This behavior is expected when only GPS movement data is available.
This guide provides troubleshooting steps for verifying GPS sources, confirming heading sensor operation, and correcting vessel icon orientation on Garmin navigation systems installed on Ocean Alexander yachts.
Important System Considerations #
Multiple GPS Sources #
Ocean Alexander yachts may have multiple GPS inputs available, including:
- Internal GPS antenna.
- External GPS antenna.
- Network-connected NMEA 2000 GPS receivers.
- Integrated navigation system sources.
When multiple GPS sources are available, verify that all Garmin chart plotters and connected navigation equipment are using the intended GPS source.
For consistent navigation data:
- Select the same GPS source across all connected displays.
- Verify autopilot and navigation systems are receiving the same position data.
- Avoid having different displays use different GPS inputs.
NMEA 2000 GPS Antenna #
If using a GPS antenna connected through the NMEA 2000 network:
- Confirm the antenna is properly connected.
- Verify the device appears in the NMEA 2000 device list.
- Confirm the GPS source is correctly assigned within the Garmin system.
Heading Source #
The heading source is separate from the GPS source.
If the vessel is equipped with:
- Garmin SteadyCast™ Heading Sensor.
- Garmin Heading Sensor.
- Autopilot heading sensor.
- Integrated electronic compass.
Verify the heading source is properly configured and calibrated.
A properly calibrated heading sensor allows the chart plotter to display the vessel’s true bow direction rather than relying only on GPS movement.
Step-by-Step Troubleshooting #
1. Verify the Cause #
First, determine whether the issue is related to heading or GPS movement.
If the vessel:
- Points correctly while underway but appears incorrect at low speed → COG limitations are likely.
- Consistently points incorrectly at all speeds → Heading sensor calibration or source selection may be required.
2. Check for a Heading Sensor #
From the Garmin display:
Home → Settings → Communications → NMEA 2000 Device List
Look for:
- Heading Sensor
- Garmin SteadyCast
- Autopilot Compass
- Other electronic compass devices
If a Heading Sensor Is Installed: #
Confirm the sensor is calibrated.
Navigate to:
Settings → Installation → Compass Calibration
Follow the on-screen instructions. Calibration typically requires slowly turning the vessel through a controlled circle on the water.
If No Heading Sensor Is Installed: #
The vessel icon will display based on COG, meaning:
- The icon points in the direction of travel.
- Current, wind, or low speed can cause apparent sideways movement.
- This is normal operation without a heading reference.
3. Verify Boat Icon Orientation Settings #
From the Garmin chart display:
Navigate to:
Settings → My Vessel → Heading Line
Verify:
- Heading line is enabled.
- COG line is enabled (recommended for troubleshooting).
- Display settings are adjusted to clearly show vessel movement direction.
Comparing the heading line and COG line can help identify whether the issue is related to current, heading calibration, or GPS accuracy.
4. Verify GPS Source Selection #
To confirm the active GPS source:
- Open the Garmin main menu.
- Select Settings.
- Navigate to System.
- Locate GPS or satellite positioning settings.
- Review available GPS sources.
- Select the preferred GPS source.
- Confirm all connected Garmin chart plotters and autopilot systems use the same source.
Some Garmin systems include an Auto Detect function, which can automatically select the preferred GPS source.
5. Verify GPS Signal Quality #
Confirm:
- GPS antenna has a clear view of the sky.
- Satellite reception is strong.
- Position data is updating correctly.
- Speed data is accurate.
Low vessel speeds (typically below 1–2 knots) can result in unstable COG calculations, especially when affected by:
- Wind.
- Current.
- Tidal flow.
- Maneuvering conditions.
This typically corrects itself once the vessel reaches normal cruising speed.
Software Updates #
For optimal navigation performance:
- Verify Garmin chart plotters are running the latest software.
- Confirm connected network devices are updated.
- Check that GPS and heading sensors have compatible firmware.
Troubleshooting Summary Checklist #
| Step | Action |
|---|---|
| 1 | Confirm whether a heading sensor is installed |
| 2 | Calibrate heading sensor if equipped |
| 3 | Verify GPS source selection |
| 4 | Ensure all Garmin displays and autopilot use the same GPS source |
| 5 | Enable heading and COG lines for comparison |
| 6 | Verify GPS signal quality |
| 7 | Confirm software is current |
Service Support #
If GPS, heading, or navigation issues continue after completing these checks, contact the Ocean Alexander Service Team for assistance with system diagnostics and configuration.
East Coast Yacht Group
2955 West State Road 84
Fort Lauderdale, FL 33312
Service Department
954.530.2033
Our team is available to help ensure your vessel maintains safe, accurate, and reliable navigation performance.
