Deviation: Compass Error

Deviation is a compass error caused by local magnetic influences on a boat that interfere with the accuracy of a magnetic compass. Unlike variation, which is the difference between magnetic north and true north based on geographical location, deviation is specific to each vessel and results from the presence of metal objects, electronic devices, and wiring that create magnetic fields and distort the compass reading. Deviation can vary depending on the heading of the boat, meaning it may be different when facing east compared to when facing west or other directions.

Understanding and accounting for deviation is essential for accurate navigation, as an uncorrected deviation can lead to significant navigational errors. Sailors use a deviation card, specific to their boat, to adjust for this error and achieve accurate compass readings, ensuring they can plot and follow a precise course.

Causes of Deviation

Deviation arises from magnetic fields generated by objects and equipment on board that affect the compass needle. Here are common sources of deviation:

1. Metal Objects: Iron or steel objects, such as anchor chains, tools, and engine parts, can interfere with the compass. The compass needle is sensitive to any magnetic influences, so even small metallic items near the compass can alter its accuracy.
2. Electronic Devices: Electrical systems and devices on board, such as radios, GPS units, and batteries, create magnetic fields that can distort the compass reading. Modern boats are equipped with numerous electronic systems, making deviation a common issue.
3. Wiring and Circuitry: Electrical wiring throughout the vessel produces a magnetic field, especially if the wiring is close to the compass. This interference may fluctuate depending on the electrical load, such as turning on lights or other equipment.
4. Tools and Equipment Storage: Storing ferromagnetic tools or equipment near the compass can introduce additional magnetic interference. Sailors are advised to keep tools and heavy metallic objects at a safe distance from the compass to minimize deviation.
5. Steel Hulls: On boats with steel hulls, deviation can be particularly challenging, as the entire structure generates a magnetic field that affects the compass. Correcting deviation on steel-hulled vessels requires careful placement and calibration of the compass.

Measuring and Correcting Deviation

To measure and correct deviation, navigators create a deviation card for their vessel. This process involves taking compass readings while aligning the vessel with known magnetic headings, allowing for comparison and correction. Here’s how it typically works:

1. Take Bearings on Known Headings: The navigator steers the boat on known headings (using ranges or transit marks) and compares the boat’s compass reading with a known magnetic or true bearing, identifying any difference between the two readings.
2. Record Deviation for Each Heading: For each heading, the difference between the actual magnetic bearing and the compass reading is recorded as the deviation for that heading. This process is repeated at several points, typically every 15 or 30 degrees.
3. Create a Deviation Card: The results are compiled into a deviation card, showing the deviation for each heading. This card becomes a reference tool, allowing navigators to adjust their compass course based on the heading and the known deviation.
4. Adjust the Compass: Some vessels have adjustable compensators near the compass that allow for minor adjustments. These compensators are small magnets or iron bars placed around the compass to neutralize specific deviations. While this can reduce deviation, a deviation card is still used to account for any remaining error.
5. Regular Updates: Because deviation can change over time due to new equipment, hull repairs, or shifts in stored items, regular checks and updates to the deviation card are necessary.

Deviation vs. Variation

While both deviation and variation affect compass readings, they are distinct errors:

• Variation: Variation is the difference between magnetic north and true north, based on the Earth’s magnetic field. It varies by geographic location and is shown on nautical charts.
• Deviation: Deviation is the vessel-specific compass error caused by local magnetic influences on board. It varies depending on the boat’s heading and must be accounted for in addition to variation.

Navigators account for both variation and deviation when calculating a true heading from a compass heading. The formula for this is as follows:

1. Start with the compass heading.
2. Apply deviation (from the deviation card) to find the magnetic heading.
3. Apply variation (from the chart) to find the true heading.

Practical Importance of Correcting Deviation

Ignoring deviation can lead to significant errors in navigation, especially on longer passages where small compass errors accumulate over time and distance. Properly correcting for deviation ensures that the boat stays on its intended course, reducing the risk of veering off track or encountering unexpected obstacles.

For example, a one-degree deviation may seem minor, but over 60 nautical miles, this results in a one-mile error from the intended position. When compounded with variation, currents, and wind, failing to account for deviation can have serious consequences, particularly in challenging weather or poor visibility.

Frequently Asked Questions about Deviation

Can deviation change over time?

Yes, deviation can change due to new equipment installations, repairs, or changes in the boat’s magnetic environment. Regularly updating the deviation card is essential, as even small adjustments in equipment placement can impact the compass.

How often should I check my deviation card?

Most navigators recommend checking the deviation card annually, or any time significant changes are made to the boat’s layout, such as installing new electronics or adding heavy equipment. Regular checks help maintain accurate navigation and avoid unexpected compass errors.

Can deviation be eliminated entirely?

While it’s difficult to eliminate deviation entirely, especially on boats with metal hulls or extensive electronics, it can be minimized. Placing the compass in an optimal location and using compensators can reduce deviation. However, a deviation card is still necessary to correct for remaining errors.

How does deviation affect autopilot systems?

Autopilot systems that rely on magnetic compasses are subject to deviation errors if not calibrated correctly. Many autopilots have built-in compensation for deviation, but a properly calibrated compass and an accurate deviation card improve autopilot accuracy.

Managing Deviation on Different Types of Boats

1. Fiberglass Boats: Fiberglass boats generally have minimal deviation, as fiberglass is non-magnetic. However, electronics, metal fittings, and other equipment can still create localized deviation. Regular checks and mindful placement of electronics help minimize deviation on fiberglass vessels.
2. Steel-Hulled Boats: Steel-hulled boats present a greater challenge for managing deviation due to the hull’s magnetic influence. On these boats, compass placement is especially critical, and extensive calibration with compensators may be needed. Regularly updating the deviation card is also essential.
3. Wooden Boats: Wooden boats, like fiberglass boats, typically have low deviation, although metal fittings, tools, and electronics still pose potential sources of interference. Deviation is often minor but should still be checked periodically.

Using Deviation as Part of Compass Navigation

Deviation is a key consideration in compass-based navigation, as it is an unavoidable factor on most boats. When planning a course, navigators use the deviation card along with known variation to convert a compass heading into a true heading, which aligns with nautical charts. This ensures that the plotted course is accurate, allowing the navigator to reach the intended destination without errors caused by uncorrected compass deviations.

Monitoring deviation also builds navigational awareness, helping sailors recognize how different factors onboard can affect compass readings. Many experienced sailors develop an intuition for compass corrections, incorporating both variation and deviation seamlessly into their navigational practices.

Conclusion

Deviation, though often minor, is a critical factor in compass navigation, influencing the accuracy of a vessel’s course and overall safety. By understanding and correcting for deviation through a deviation card and careful compass placement, sailors can minimize navigational errors and maintain precision in their heading. Regularly checking for deviation, especially after equipment changes or repairs, is essential for ensuring that the compass remains a reliable tool for charting a safe and accurate course across open water.

In modern navigation, where GPS and electronic aids are available, the practice of correcting for deviation may seem less essential. However, understanding and accounting for deviation enhances navigational skills and keeps sailors prepared for situations when electronic systems may fail, embodying the timeless art and science of traditional seamanship.