Welcome to the United States Power Squadrons' Chart Smart Seminar The Mariner’s Compass (Section 2)

Class Agenda Section 2: Piloting - The Mariner's Compass How a Compass Works Selecting a Compass Installing a Compass Compass Care The Compass Course Calculating a Steering Course

What Does a Mariner's Compass Do ? Identifies direction Divides direction into degrees Shows direction the boat is headed Is the link between you and the chart

Basic Compass Idea

Magnetic Compass

Typical Compass Construction

Read a Compass at the Lubber’s Line

Compass Installation

Compass Care Cover your compass when not in use - sunlight may damage the card and fluid Remove your compass during winter storage - freezing may damage the card & bellows resulting in lose of fluid Remove your compass for trailering A quality compass, properly taken care of, will last for years and is an excellent investment.

Variation is the difference between true north and magnetic north True North Pole Magnetic North Pole N E S W Variation is the difference between true north and magnetic north

Variation Variation changes with geographic position True North Pole Magnetic North Pole N S W E Variation Variation changes with geographic position

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A Chart Compass Rose Variation is found in the center of the

A Chart Compass Rose One or more compass roses are found on each nautical chart

Calculating a Steering Course True is the “Language” of the chart Magnetic is the “Language” of the compass The differences between a true course and the steering course must be accounted for to ensure safer boating

True vs Steering (Compass) Course Cross Track Errors (XTE) Course Error (deg.) XTE 1 (course distance = 15 mi.) 2 distance = 30 3 distance = 60 5 1.31 2.62 5.25 10 2.64 5.29 10.58 15 4.02 8.04 16.08 tan. (course error) = XTE / (course distance) True vs Steering (Compass) Course Variation + Deviation (deg.) distance = 15 mi.) distance = 30 mi.) distance = 60 mi.) 13 (W or E) 3.5 7 14 60 30 course error The purpose of this page is to impress upon the student that if Variation (and Deviation) are not accounted for, then significant cross track errors will occur and the vessel could get into a dangerous situation. For example - - if there is a combined variation and deviation of 15 degress and a 60 mile track is followed, then the vessel could be as much as 16.08 miles away from its intended location. See XTE #3 in the table above.

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The TVMDC Formula ° True Course T Variation V Magnetic Course M Deviation D ° Compass Course C Add West Subtract East Subtract West Add East Deviation is the difference between magnetic and compass course headings due to the boat's magnetic field and is assumed to be 0 ° for demonstration

T _____ V _____ M _____ D _____ C _____ 206 Example #1 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 W Add West Subtract East 14 W 220 Compass course to steer is 220 degrees to make good a true course of 206 degrees ° 220

T _____ V _____ M _____ D _____ C _____ 214 Example #2 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 E 14 E 200 A compass course of 200 degrees on the vessel results in a true course of 214 degrees when plotted on the chart Subtract West Add East ° 200

End of Section 2 The Mariner’s Compass