Signalman 1 & C - Historic Naval Ships Association

CHAPTER 9NAVIGATIONAL DUTIESNavigation is the art or science of determining theposition of a ship or aircraft and directing that ship oraircraft from one position to another. It can beregarded as an art because its application involves theexercise of special skills and fine techniques, whichcan be perfected only by experience and carefulpractice. On the other hand, navigation can beregarded as a science inasmuch as it is knowledgedealing with a body of facts and truths systematicallyarranged and showing the operation of general laws.Navigation has been practiced for thousands of years;however, modern methods date from the 18th centuryinvention of the chronometer, a precision timepiece.As a Signalman, you may be required to assist thenavigator by taking bearings, using the bearing circleor alidade. You may assist the officer of the deck(OOD) by sounding whistle signals and by being alertto aids to navigation.To better prepare you for your navigational duties,this chapter contains information on navigationalequipment, aids to navigation, and Rules of the Road.NAVIGATIONAL EQUIPMENTLEARNING OBJECTIVES: List and explainthe use of navigational equipment that you willcome in contact with as a Signalman.The equipment described in this section is theequipment most likely to be used by you in performingnavigational duties.COMPASSESThere are two types of compasses in generalshipboard use: the magnetic compass, which dependson Earth's magnetic field for its directive force, andthe gyrocompass, which operates on the gyroscopicprinciple of the spinning wheel.When you studied as a seaman, you learned thatthe magnetic compass points to the magnetic ratherthan the true North Pole, and that the magnetic pole islocated some distance away from the true pole. Youalso discovered how the Navy standard compass ismade, and how its needle is deflected by magneticmaterials either in a ship itself or by magneticmaterials brought near the compass.The gyrocompass, on the other hand, points to truenorth by operation of the gyroscopic principle. It may,however, have a slight mechanical error of a degree ortwo, which is known and for which due allowance ismade.Magnetic CompassThe ship’s magnetic compasses are named orclassed according to their use.The standard compass is the magnetic compassused by the navigator as a standard for checking othercompasses on the ship. It is so located that it is leastaffected by the internal magnetism of the ship.Courses or bearings given from it are designated perstandard compass (PSC).The steering compass is located near thehelmsman. Along with the gyro repeater, it is thecompass by which the ship is steered. Courses orbearings given from it are designated per steeringcompass (PSTCO).GyrocompassThe gyrocompass is not affected by variation anddeviation. Headings or bearings from it are designatedper gyrocompass (PGC).When in proper running order, the gyrocompasspoints constantly to true instead of magnetic north. Itmay have a slight mechanical error, called gyro error,which is computed easily and remains constant for anyheading.Despite the excellence of the gyro mechanism, itis the magnetic compass—not the gyro—that isstandard aboard ship. The reason is the magneticcompass operates through the attraction exerted byEarth. Consequently, the magnetic compass will nevergo out of commission because of power failure.The gyrocompass, on the other hand, is poweredby electricity. If the supply is cut off, the gyro isuseless. Being an extremely complicated and delicateinstrument, it is also subject to mechanical failure.9-1