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The Mariner’s Compass(Section 2)

Welcome to the

United States Power Squadrons'

Chart Smart Seminar



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

Lubber’s line

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

SW



True North Pole

Magnetic North Pole

N

S

W

E

Variation

Variation changes with geographic

position





A Chart Compass Rose

Variation is found in

the center of the

compass rose



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



Cross Track Errors (XTE)

Course Error (deg.)

XTE 1 (course distance = 15

mi.)


mi.)


mi.)

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) CourseVariation + Deviation(deg.)

XTE 1 (course distance = 15 mi.)



13 (W or E) 3.5 7 14

60

30

15

XTE 3

XTE 2

XTE 1

course

error





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

The TVMDC Formula

True Course T

Variation V

Magnetic CourseM

Deviation D 0

Compass CourseC

Add West

Subtract East

Subtract West

Add East



Example #1 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 W

T _____

V _____

M _____

D _____

C _____

206

14 W

220

0

220

Add West

Subtract East

Compass course to steer is 220 degrees to make good a true course of 206 degrees



Example #2 notes - deviation is assumed to be zero for demonstration only; local variation is assumed to be 14 E

T _____

V _____

M _____

D _____

C _____

214

14 E

200

0

200

Subtract West

Add East

A compass course of 200 degrees on the vessel results in a true course of 214 degrees when plotted on the chart



The Mariner’s Compass

End of Section 2