One of the factors to navigate safely is to have a precise instrument for reliable direction finding. Two such instruments used for navigation are the Gyro compass and Magnetic compass. Let’s discuss the difference between these compasses onboard.
MAGNETIC COMPASS
A magnetic compass is a device that uses a magnetized needle to point towards the magnetic north pole of the Earth. The magnetic north pole is not exactly aligned with the true north pole (the geographic point where all meridians converge), but rather it varies depending on your location and time. This difference is called magnetic variation, and it has to be corrected for accurate navigation.
A magnetic compass also has another source of error called deviation. This is caused by the presence of ferromagnetic materials (such as iron or steel) on board the ship, which interfere with the Earth’s magnetic field and affect the needle’s alignment. Deviation can be reduced by placing compensating magnets near the compass or by using a deviation card that shows how much correction to apply for each heading.
GYRO COMPASS
A gyro compass is a device that uses a spinning wheel (called a gyroscope) to maintain its orientation towards the true north pole.
A gyroscope has two properties: gyroscopic inertia and precession.
- Gyroscopic inertia means that a spinning wheel tends to keep its plane of rotation constant unless an external force acts on it.
- Precession means that when an external force (such as gravity or torque) acts on a spinning wheel, it causes it to move at right angles to both the axis of rotation and the applied force.
A gyro compass uses these properties and two natural forces: gravity and Earth’s rotation, to align itself with true north. The gyroscope is mounted in such a way that it can tilt freely about three perpendicular axes: horizontal, vertical and spin axis. When power is applied, the gyroscope spins rapidly about its spin axis (usually several thousand revolutions per minute). As it spins, gravity pulls down on its horizontal axis, causing it to tilt slightly downwards.
At this point, Earth’s rotation comes into play. As Earth rotates from west to east at about 15 degrees per hour (360 degrees per day), it exerts a torque on the tilted gyroscope, causing it to precess clockwise about its vertical axis (when viewed from above). This precession makes the gyroscope gradually align its spin axis with Earth’s rotational axis (which points towards true north).
Once aligned, the gyro compass sends electrical signals to repeater compasses located throughout the ship, which display the true heading. The gyro compass also feeds data to other navigational instruments such as autopilot, radar, and GPS.
ADVANTAGES AND DISADVANTAGES
A gyro compass has several advantages over a magnetic compass. It is not affected by external magnetic fields, so it does not need any correction for variation or deviation. It can work reliably in any latitude, even near the poles where the magnetic field becomes weak and erratic. It can also withstand bad weather and night conditions, as it does not depend on any external reference.
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However, a gyro compass also has some drawbacks. It requires electric power to operate, and if power fails, it will lose its accuracy over time. It also needs regular maintenance to ensure proper functioning. It may also have some errors due to latitude, speed, and course changes of the ship, which need to be compensated by automatic or manual adjustments.
GYRO AND MAGNETIC COMPASS COMPARISON
- Principle of Operation: A magnetic compass operates based on the Earth’s magnetic field, while a gyro compass uses the principles of angular momentum to determine true north.
- Accuracy: A gyro compass is more accurate than a magnetic compass as it is not affected by magnetic disturbances, such as those caused by the ship’s own iron and steel structure. A magnetic compass, on the other hand, can be affected by these disturbances and its accuracy can be affected as a result.
- Maintenance: A gyro compass requires regular maintenance and recalibration, while a magnetic compass does not. However, a magnetic compass is simpler and less expensive to maintain.
- Cost: A gyro compass is typically more expensive than a magnetic compass, due to its complex mechanism and the need for regular maintenance.
- Power Requirements: A gyro compass requires a constant source of power to operate, while a magnetic compass does not.
The gyro compass and a magnetic compass are two different types of devices that help ships navigate across the oceans. They both have their pros and cons, and they complement each other in providing accurate information. Generally, ships are equipped with both types of compasses, but they rely more on the gyro compass for convenience and accuracy.
