Page 341 equipment and instruments 4423 



(c) The bearing repeaters are similar to the steering repeater. There is usually one on each 

 wing of the bridge, one on the flying bridge, and one installed in the radio direction finder. The re- 

 peaters on the wings of the bridge are mounted on stands and supported in gimbals. They are really 

 gyro peloruses, and have the advantage that bearings taken with them are true bearings and do not 

 have to be corrected for momentary deviations in course. 



The repeater on the flying bridge is usually installed where it can be used for steering as well as 

 for taking bearings. 



The repeater installed in the radio direction finder is called a radio repeater. It is not mounted 

 in gimbals. 



(d) The course recorder is a repeater of a different form. It is usually mounted in a convenient 

 location in the pilothouse. It preserves a graphic record of the actual courses steered and the times 

 of all changes in course. It is synchronized with the master compass so that courses steered are 

 graphically recorded in ink on a paper chart with a speed of about lli inches per hour. In addition 

 to changes in course, yawing, poor steering, and the effect of weather on steering are automatically 

 recorded. 



(e) The gyro pilot is described in 4431. 



4423. Advantages and Disadvantages of the Gyrocompass 



Since the master compass alines itself in the true meridian, all courses and bear- 

 ings registered by the various gyro repeaters are automatically true. Because the 

 master compass is unaffected by the earth's magnetic field and the local magnetic 

 forces within the ship itself, corrections for variation and deviation are eliminated. 



There is little lag, as compared with a magnetic compass. More accurate steer- 

 ing is possible, even by hand, and less rudder needs to be used. 



Disadvantages are that the gyrocompass needs a constant source of electric power 

 to function. Storage batteries should be provided to supply automatically the defi- 

 ciency for a time, if the ship's power fails. The mechanism must be put in operation 

 several hours before it is to be used. 



The mechanism is complicated and somewhat delicate, requiring repairs by experts 

 should anything go wrong. Because of this, magnetic compasses must always be 

 available for instant use if needed. 



There is an error, of varying magnitude in different models, due to acceleration 

 accompanying a change in course or speed. The acceleration exerts a force on the 

 axis of rotation of the gyroscope and the compass starts to precess. This is a disad- 

 vantage in hydrographic surveying because the frequent 90° or 180° turns may start 

 an oscillation of as much as 2° or 3°. The magnitude and duration of the precession 

 depend i3n the period of oscillation and the amount of damping effect in each com- 

 pass. vSome gyrocompasses have a switch that can be turned just before such a 

 maneuver to eliminate this difficulty more or less. 



4424. Starting the Gyrocompass 



The master compass should be started about 4 hours before the ship leaves port, 

 depending on its period of oscillation and how nearly in the true meridian it can be 

 started. It should be set by hand as nearly as possible in the true meridian before 

 the starting switch is pressed. This can be done by comparison with the magnetic 

 compass or by the following method if the vessel is alongside a wharf or pier: After 

 the vessel is tied up, switch off the repeaters first. This will leave them synchronized 

 and they will also indicate the true heading, assuming the vessel does not move. Then 

 in starting the master compass, it can be set with reference to the repeaters. After 

 the master compass has run long enough to level itself, the repeaters can be switched 

 in when the readings agree. 



