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United States Coast Guard U. S. Coast Guard Auxiliary Basic Coastal Navigation Chapter 2 United States Coast Guard U. S. Coast Guard Auxiliary Basic Coastal Navigation Chapter 2 The Marine Magnetic Compass

What We’ll Cover Brief history Compass design Principles of operation Deviation & swinging ship What We’ll Cover Brief history Compass design Principles of operation Deviation & swinging ship Compass calculations 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 2

Brief History Phoenicians used magnetic direction finder. Evidence for use in Europe/China ~1100. Compass Brief History Phoenicians used magnetic direction finder. Evidence for use in Europe/China ~1100. Compass card invented ~ 1300. Variation understood by 1492. Charts showed variation by 1700. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 3

Brief History Flinders bar to compensate for deviation developed 1801. Ritchie liquid filled compass Brief History Flinders bar to compensate for deviation developed 1801. Ritchie liquid filled compass - 1859. Modern gyrocompass developed by Speery and Anschutz-Kanpfe ~1900. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 4

Parts of a Compass Constructed of nonmagnetic materials. 12 DEC 04 Basic Coastal Navigation Parts of a Compass Constructed of nonmagnetic materials. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 5

Parts of a Compass Indexed card to read direction. 12 DEC 04 Basic Coastal Parts of a Compass Indexed card to read direction. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 6

Parts of a Compass Lubber line pins to align compass with boat. 12 DEC Parts of a Compass Lubber line pins to align compass with boat. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 7

Parts of a Compass Gimbal system to keep card level when boat heels and Parts of a Compass Gimbal system to keep card level when boat heels and pitches. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 8

Parts of a Compass Spherical bowl with expansion diaphragm. 12 DEC 04 Basic Coastal Parts of a Compass Spherical bowl with expansion diaphragm. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 9

Parts of a Compass Fluid filled damping system to impede rapid card movement. 12 Parts of a Compass Fluid filled damping system to impede rapid card movement. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 10

Parts of a Compass Compensation system to reduce deviation error. 12 DEC 04 Basic Parts of a Compass Compensation system to reduce deviation error. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 11

Steering Compass Styles Front Reading Card Used when compass mounted on bulkhead. Top Reading Steering Compass Styles Front Reading Card Used when compass mounted on bulkhead. Top Reading Card Usually used in binnacle steering stations. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 12

Hand Bearing Compass Styles Hockey Puck Front/Top Reading Digital/Scope Style 12 DEC 04 Basic Hand Bearing Compass Styles Hockey Puck Front/Top Reading Digital/Scope Style 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 13

Compass Card Graduated in degrees from 000 to 359. Graduated in 1, 2, 5 Compass Card Graduated in degrees from 000 to 359. Graduated in 1, 2, 5 degree increments. Numbers spaced every 10 -30 degrees. Usually show cardinal points: N, S, E & W. May show intercardinal points: NE, SW & NW. Navigation - Chapter 2 Basic Coastal 12 DEC 04 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 14

Compass Errors Deviation and Variation The ugly Truth … Compasses don’t point to True Compass Errors Deviation and Variation The ugly Truth … Compasses don’t point to True North! 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 15

Compass Errors Variation: Geographic (true) vs. magnetic north. Common to all parts of the Compass Errors Variation: Geographic (true) vs. magnetic north. Common to all parts of the globe. Identified on every chart’s compass rose(s). Deviation: Caused by shipboard magnetic influences. Exists on all vessels; different for each vessel. Depends on vessel's heading. Can vary widely. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 16

Variation: Earth - A Magnet Locating exact source of magnetism difficult. Alignment of magnetism Variation: Earth - A Magnet Locating exact source of magnetism difficult. Alignment of magnetism changes. Strength of pull varies over earth. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 17

World-Wide Variation Isogonic Chart Illustrates magnetic variation (1995) 12 DEC 04 Basic Coastal Navigation World-Wide Variation Isogonic Chart Illustrates magnetic variation (1995) 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 18

Deviation: Boat-Specific Error Due to on-board magnetic influences: Magnetic items Items to which magnets Deviation: Boat-Specific Error Due to on-board magnetic influences: Magnetic items Items to which magnets are attracted. Wires carrying DC electrical current. Reduce effects as much as possible: Keep compass away from influences Twist nearby DC wire pairs. Usually can’t eliminate them all. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 19

Compensation Deviation often greater than 10º. Special magnets (compensators) installed in compasses to reduce Compensation Deviation often greater than 10º. Special magnets (compensators) installed in compasses to reduce error. Most compasses come with instructions. Or, engage services of professional compass adjuster. Can still have significant deviation error after compensation. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 20

“Swinging Ship” Process of measuring residual deviation error after compensation. Usually determined in 15º “Swinging Ship” Process of measuring residual deviation error after compensation. Usually determined in 15º - 30º heading increments. Recorded in form of deviation table. Compass adjuster can provide service. Or, you can do it yourself. . . 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 21

Measuring Deviation Yourself We’ll get to that later. First you have to know how Measuring Deviation Yourself We’ll get to that later. First you have to know how to do some compass calculations. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 22

Additive Effect of Compass Error due to 015° W variation Additional error due to Additive Effect of Compass Error due to 015° W variation Additional error due to 005° W deviation 12 DEC 04 Desired True heading Compass heading must be adjusted 020° to compensate. Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 23

Computing Compass Corrections +W 12 DEC 04 T V M D C True Heading Computing Compass Corrections +W 12 DEC 04 T V M D C True Heading 000 Variation on Chart 015 W Magnetic Heading 015 Deviation 005 W Compass Course 020 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 24

Computing Compass Corrections +W 12 DEC 04 T V M D C Memorize it! Computing Compass Corrections +W 12 DEC 04 T V M D C Memorize it! You’re going to use it again and again! Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 25

Back to Measuring Deviation For each 15º - 30º, you need to: Know exactly Back to Measuring Deviation For each 15º - 30º, you need to: Know exactly what direction (magnetic) your boat is pointed. Compare with compass reading. Compute deviation. Key to puzzle is knowing exactly what direction you’re headed. There are several ways. . . 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 26

Example Way to Know Your Heading In calm water with minimal current: Remaining in Example Way to Know Your Heading In calm water with minimal current: Remaining in vicinity of ATON … Record both compass heading & relative bearing Means to Take Relative every 15° -- 30°. Bearing Using TVMDC, compute deviation for each heading. 12 DEC 04 Charted Object 6+ Miles Distant Known Direction Close to Reliable ATON Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 27

Measuring Relative Bearings: The Pelorus Mounted parallel to vessel’s keel with all-around view. 0º Measuring Relative Bearings: The Pelorus Mounted parallel to vessel’s keel with all-around view. 0º aligned with keel. Align targets in sights. Read relative bearing on scale. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 28

Compasses: Selection & Mounting Selection Best you can afford. Dial large enough for easy Compasses: Selection & Mounting Selection Best you can afford. Dial large enough for easy reading. Style suitable for mounting location. Handheld compass recommended for taking bearings. 12 DEC 04 Mounting Easily visible to helm. Away from magnetic influences. Align lubber line. Determine deviation. Recheck when new equipment installed. Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 29

Electronic Steering Compasses Use fluxgate transducers and digital displays. Transducer can be mounted remotely, Electronic Steering Compasses Use fluxgate transducers and digital displays. Transducer can be mounted remotely, away from magnetic influences. Feature automated deviation elimination! 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 30

Local Magnetic Disturbances Compass still seems wrong? In some locations there are local magnetic Local Magnetic Disturbances Compass still seems wrong? In some locations there are local magnetic disturbances that can affect you compass. Often noted on charts. 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 31

All this sound complex? Perhaps, but the best solution is: Practice. . . Practice! All this sound complex? Perhaps, but the best solution is: Practice. . . Practice! 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 32

End Chapter 2 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © End Chapter 2 12 DEC 04 Basic Coastal Navigation - Chapter 2 Copyright © 2004 Coast Guard Auxiliary Association, Inc. 33