PRINCIPLES OF NAVAL ENGINEERING 



stages of amplification, and drives the servo- 

 motor. 



DEGAUSSING INSTALLATIONS 



A ship is a magnet because of the presence 

 of magnetic material in its hull and machinery. 

 A ship is therefore surrounded by a magnetic 

 field which is strong near the ship and weak at a 

 considerable distance from the ship. As a ship 

 passes over a point on the surface of the earth, 

 the magnetic field of the ship is superimposed 

 upon the magnetic field of the earth, thus tending 

 to distort the earth's field around the ship. If 

 the ship is close to a magnetic mine or torpedo, 

 the distortion caused by the ship's field will 

 activate the firing mechanism to detonate the 

 mine or torpedo. 



Degaussing equipment is installed aboard 

 ship to neutralize the disturbance of the earth's 

 magnetic field caused by the ship, and thus to 

 reduce the possibility of detonating a magnetic 

 mine or torpedo. A shipboard degaussing instal- 

 lation consists of one or more coils of electric 

 cable in specific locations inside the ship's hull, 

 a d-c power source to energize these coils, and 

 a means of controlling the magnitude and polar- 

 ity of the current through the coils. Compass- 

 compensating equipment, consisting of compen- 

 sating coils and control boxes, is also installed 

 as a part of the degaussing system, to com- 

 pensate for the deviation effect of the degaussing 

 coils on the ship's magnetic compasses. 



Naval ships are tested periodically at mag- 

 netic range stations to determine the configu- 

 ration of the ship's magnetic field. Sensitive 



OUTPUT CONTROLLER 



OUTPUT 



INPUT CONTROLLER 



72.46 

 Figure 20-28.— Simplified block diagram of a 

 servomechanism. 



measuring coils, located at or near the bottom 

 of the channel, and recording equipment respond 

 to the signals induced in the coils as the ship 

 passes over them. These measurements indicate 

 the distortion of the earth's magnetic field 

 caused by the ship and are used to determine 

 the values of current needed in the ship's de- 

 gaussing coils to neutralize this distortion. 



GYROCOMPASSES 



The gyrocompass system provides a means 

 of indicating the ship's course at various sta- 

 tions throughout the ship. There are various 

 types of gyrocompasses in use; however, all 

 depend upon gyroscopic principles and the rota- 

 tion of the earth for their operation. 



The gyroscope is a heavy wheel, or rotor, 

 suspended so that it has the freedom to spin, 

 the freedom to turn, and the freedom to tilt. 

 These three degrees of freedom permit the 

 rotor to assume any position within the sup- 

 porting frame. 



When a gyroscope rotor is spinning rapidly, 

 the gyroscope develops two properties it does 

 not have when the rotor is at rest. These two 

 properties, which make it possible to develop 

 the gyroscope into a gyrocompass, are (1) 

 rigidity of plane, and (2) precession. 



Rigidity of plane results from the fact that 

 the rotating wheel of a gyroscope has high angu- 

 lar momentum and kinetic energy. When the 

 rotor is set spinning with its axle pointed in one 

 direction, it will continue to spin with its axle 

 pointing in the same direction, no matter how 

 the frame of the gyroscope is tilted or turned. 



Any force that attempts to change the angle 

 of the plane of rotation of a gyroscope with 

 respect to its earlier position produces a move- 

 ment known as precession. Precession takes 

 place whenever any torque tends to tilt the axle 

 of a spinning gyroscope rotor. 



A gyrocompass is simply a gyroscope with 

 a means of exerting a force at right angles to 

 the end of the axle whenever the axle tilts with 

 respect to the surface of the earth. Because of 

 the rotation of the earth, the axle tilts whenever 

 it is not on the meridian. The axle is precessed 

 automatically into a north-south direction. 



SAFETY PRECAUTIONS 



Because of the possibility of injury to per- 

 sonnel, the danger of fire, and the possibility of 

 damage to material, all repair and maintenance 



518 



