Section 9: Gyrocompass Flashcards
What are the SOLAS carriage requirements for gyrocompasses?
As per SOLAS Chap V Reg 19 and enforced by M.O. 27 Safety of navigation and radio equipment.
The requirement for ships >/= 500 GT.
A gyrocompass, or other means, to determine and display their heading by shipborne non-magnetic means, being clearly readable by the helmsman at the main steering position. These means shall also transmit heading information for input to the X-Band RADAR, AIS and ARPA.
A gyrocompass heading repeater, or other means, to supply heading information visually at the emergency steering position if provided.
A gyrocompass bearing repeater, or other means, to take bearings over an arc of the horizon of 360° using the gyrocompass. However, ships of less than 1,600 gross tonnages shall be fitted with such means as far as possible.
Also, all ships > 50,000 GT are required to have a rate of turn indicator, which is not possible without having a gyro.
Basic principle of gyrocompass.
It is a navigational instrument which makes use of a continuously driven gyroscope to accurately seek the direction of true north. It operates by seeking an equilibrium direction under the combined effects of the properties of the gyroscpe and the daily rotation of Earth.
Define gyroscope.
A rapidly rotating mass free to move about one or both axes perpendicular to the axis of rotation and to each other. It is characterised by gyroscopic inertia and precession. Usually shortened to a ‘gyro’, the term also refers colloquially to the gyrocompass.
List some gyroscopic properties.
- Gyroscopic Inertia (Rigidity in Space).
- Gyroscopic Precession.
- Tilt
- Drift.
Define the gyroscopic inertia. (Rigidity in Space)
The axis of rotation of the gyro wheel tends to remain in a fixed direction in space if no force is applied to it.
Define gyroscopic precession.
The axis of rotation has a tendency to turn at a right angle to the direction of an applied force.
The horizontal component of precession is called drift, and the vertical component is called topple. Also called ‘induced precession’ or ‘real precession’.
Define apparent precession
The apparent change in the direction of the axis of rotation of a a gyroscope due to rotation of the earth. As a result of gyroscopic inertia, to an observer on the rotating earth, a gyroscope appears to turn or precess.
Define gyroscopic drift
The horizontal rotation of the spin axis of a gyroscope about the vertical axis.
Define true heading
The ‘‘true heading’ is the horizontal angle between the vertical plane passing through the true meridian and the vertical plane passing through the ship’s fore and aft datum line. It is measured from true north (000°) clockwise through 360°.
When is the compass considered to be settled? What is settle point heading?
The compass is said to be ‘settled’ if any three readings taken at intervals of thirty minutes when the compass is on a level and stationary base are within a band of 0.7°.
The ‘settle point heading’ is the mean value of ten readings taken at twenty-minute intervals after the compass has settled.
What is settle point error?
The ‘settle point error’ is the difference between settle point heading and true heading.
Explain why a gyrocompass has less directional force in higher latitudes than it does near the equator.
The rotational speed of the earth’s surface varies with increasing latitude from around 900 knots at the equator to zero at the poles – the higher the latitude, the slower the speed of the earth’s surface and the less directional force can be applied to the free gyroscope.
As the directional force reduces, the gyro finds it more and more difficult to settle on the meridian and the spin axis follows an elliptical path, swinging to either side of the meridian. The effects of alterations of vessel course and speed are more noticeable in higher latitudes.
What are the two categories of IMO performance standards for gyrocompass?
The two categories of IMO performance standards for gyrocompass are:
a. For Vessels of < 30 knots.
b. For Vessels of Speeds between 30 and 70 knots.
What is the allowed time period for settling of gyro?
When switched on in accordance with the manufacturer’s instructions, the compass should settle within six hours in latitudes of up to 60°.
What are the performance requirements regarding reliability and power for a gyro?
The equipment should be capable of operating continuously in accordance with the requirements of this recommendation in the presence of such variations of the power supply as are normally expected in a ship.
Means should be incorporated for the protection of the equipment from excessive currents and voltages, transients and accidental reversal of power supply polarity.
If the provision is made for operating the equipment from more than one source of electrical energy, arrangements for rapidly changing from one source of supply to the other should be incorporated.
