Navigation and Seamanship

2.1.2.3. Precession

Precession is the movement of the spin axis when a force is applied to the gyroscope. When a force is applied about its horizontal axis the spin axis will turn at right angles to the applied force in the direction of the spin of the wheel similarly a couple applied about the vertical axis will make the spin axle turn about the horizontal axis in the direction of the spin of the wheel. This property is called ‘precession’.

If force is applied to the axis of the spinning gyroscope, the axis rotates not in the direction in the force is applied but 90^o from this. This reaction is known as “precession” which is defined as that property of a gyroscope which causes the spin axis to change direction when a torque or force is applied to the gyro. Precession is the property of a gyro by which it tends to reach to the external forces acting over it.

When the gyroscope maintain its axis of rotation with space, due to the earth rotation, an observer may feel that the gyro precession constantly. This gyroscope housed in a binnacle along with the compass card and lighting arrangement makes the compass

By rotating the wheel mechanically and at high speeds, the axis of the wheel will point to the true or geographical north pole. A compass card is fitted to the mounting of the gyroscope so that the north and south shown on the card is parallel to the line of the wheel are whatever way the ship turns the north on the card coil point to the true north.

The gyroscope is whenever possible, positioned on the centre line of the ship at the roll and pitch centre so that the movement of the vessel affects it as little as possible. The gyroscope is electrically powered and should be run up for a period of 5 hr. before use so that it will be settled and reliable.