with submersible navigation primarily dis- 

 cuss what could be and not what is: Hence, 

 the magnetic compass is simply specified and 

 then ignored. Specific guidelines for the com- 

 pass are difficult to provide owing to the 

 diversity of influences in individual vehicles. 

 One might benefit, nevertheless, by checking 

 its accuracy once the vehicle is completely 

 outfitted and ready to dive. Subsequent rear- 

 rangement of components or outfitting of 

 new instruments should be followed by fur- 

 ther rechecking. Furthermore, geographic lo- 

 cation, magnetic storms and local magnetic 

 anomalies within the earth call for frequent 

 rechecking. 



A typical local magnetic anomaly might be 

 an offshore oil rig. John Newman of Perry 

 Submarine Builders relates that magnetic 

 compass readings were all but useless in a 

 North Sea diving operation because the vehi- 

 cle was operating in close proximity to a 

 drilling platform. At one point the submers- 

 ible had inadvertently collided with the plat- 

 form; wishing to clear the area, the vehicle 

 turned 180 degrees from its collision course 

 and proceeded only 10 feet before it collided 

 with the same rig once again. Obviously, 

 navigation by some means other than mag- 

 netic compasses should be considered in un- 

 dersea pipeline or hardware inspections. 



Gyrocompass — Since the gyrocompass is not 

 affected by a magnetic field, it is subject to 

 none of the magnetic errors of the magnetic 

 compass, and it is not useless near the 

 earth's magnetic poles. Errors which are 

 present are the same on all headings. On the 

 other hand, a gyrocompass requires a suita- 

 ble source of electrical (AC) power and if 

 power is interrupted, a period of time (up to 4 

 hr in some cases) is required for it to stabi- 

 lize. It is complex and requires more mainte- 

 nance than a magnetic compass. The gyro- 

 compass is also subject to several systematic 

 errors (e.g., precession) which can be elimi- 

 nated or offset in the design or can be man- 

 ually adjusted to correct. According to Bow- 

 ditch, the gyro error of modern compasses is 

 generally so small that it can be ignored, but 

 errors can be introduced which make fre- 

 quent checking a good practice. 



Several commercially available gyroscopes 

 are used in submersibles. Figure 10.20 shows 

 the Sperry MK 27 and its repeater used 



Fig 10.20 A Sperry MK27 Gyrocompass aboard DS-2000 located aft beneath the 

 observers seat. The repeater for this unit (inset) is installed forward on the instrument 



panel. 



Fig. 1020 (inset) 



506 



