For centuries, sailors have relied on the magnetic compass as a primary navigation tool at sea. However, magnetic compasses are subject to errors caused by various factors. One such error is deviation, which refers to the discrepancy between the magnetic compass reading and the true magnetic direction. This article explores the concept of sailing deviation, its causes, and methods to account for it, ensuring accurate navigation while sailing.
Understanding Sailing Deviation:
Sailing deviation is the difference between the compass heading and the true magnetic heading due to magnetic disturbances caused by the ship’s own structure and equipment. Each vessel has its unique deviation characteristics, which can be determined through compass adjustment and observation.
Causes of Sailing Deviation:
Several factors contribute to sailing deviation:
- Magnetic Interference: The presence of ferromagnetic materials, such as iron and steel, on the ship can create localized magnetic fields that affect the compass reading.
- Electrical Equipment: The operation of electrical systems, such as generators, motors, and wiring, can induce magnetic fields that interfere with the compass.
- Compass Mounting: Improper compass installation or placement near magnetic objects can introduce additional errors.
- Magnetic Cargo: The presence of magnetic cargo, such as steel or ferromagnetic materials, can significantly affect compass readings.
Determining Sailing Deviation:
To determine the sailing deviation of a vessel, sailors can follow these steps:
- Swing Test: The ship is steered on various known compass courses, while deviations are recorded and compared to the true magnetic course.
- Comparing Bearings: By comparing compass bearings with known true bearings of distant objects, sailors can determine the deviation for specific headings.
- Tabulating Deviation: The deviations obtained from the swing test and bearing comparisons are recorded in a deviation table, which provides a reference for correcting compass readings during navigation.
Correcting for Sailing Deviation:
Once the deviation table is created, sailors can correct compass readings by applying the appropriate deviation values. The correction process involves:
- Compass Bearing Conversion: When plotting a course or determining a bearing, sailors convert the true magnetic bearing to the compass bearing using the deviation table.
- Course Adjustments: Sailors adjust the course steered based on the corrected compass bearing, ensuring they follow the intended route.
Periodic Compass Checks:
Sailing deviation is not a fixed value and can change over time due to factors such as equipment modifications, changes in magnetic cargo, or aging of the vessel. Therefore, regular compass checks and adjustments are necessary to maintain accurate navigation.
GPS and Electronic Navigation Systems:
While magnetic compasses and sailing deviation correction remain essential skills for sailors, modern navigation systems like GPS and electronic charting greatly enhance navigation accuracy. These systems provide reliable and precise positioning, reducing reliance on compass readings and deviation corrections.
By identifying the causes of deviation, determining accurate deviation values, and applying corrections, sailors can maintain course accuracy and safely navigate their vessels. While modern technology offers advanced navigation systems, the knowledge of sailing deviation remains essential as a backup and to ensure a comprehensive understanding of navigation principles.