Navigation Part 2 1
Lesson Objectives Understand how a ships compass works and how it is used for navigation Measure distances on nautical charts Plot a course on a nautical chart Correct course headings for variation and deviation Understand and use the speed, time, distance formula 2
Lat. & Long. Review Latitude Angular distance N or S of equator Equator = 0º Must indicate N or S North pole = 90º N Longitude Angular distance E or W of prime meridian Prime meridian = 0º Must indicate E or W Naval Observatory, Washington, D.C. is 77º 3.9 W 3
Lat. & Long. Review Lat. 38º 0.0 N Long. 72º 0.0 W 4
Lat. & Long. Review 1 degree of arc = 60 minutes 1 minute of arc = 60 seconds Latitude / Longitude expressed as: dd-mm-ss or dd-mm.mm Example: 30 o -26-45 or 30 o -26.75 Do not forget! 5
The Ships Compass The magnetic compass was invented in China between the 2nd century BC and 1st century AD Evidence for use in Europe/China ~1100 Still in use today
The Ships Compass Points to North Magnetic Pole Magnetic north different than true north Magnetic north pole located in northern Canada True North Earth s axis Base for latitude and longitude 7
Compass Errors Variation and Deviation The ugly Truth Compasses don t point to True North! 8 8
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. 9
Relative Positions of Poles 10
Variation Variation is angular difference between the geographic and magnetic poles Geographic North Pole Observer s Position Magnetic North Pole 11 11
Variation Variation depends on observer s location Geographic North Pole Variation one thing at this location Magnetic North Pole 12 Variation different at this location 12
The Compass Rose Provides the chart s fundamental reference for magnetic directions Outer ring aligns with the chart grid of latitude and longitude or true north Inner ring aligns with magnetic north
The Compass Rose 14
Deviation Boat Specific Error Any Magnetic Field Aboard A Vessel Strong Enough To Effect The Compass Engines, electronic instruments, wires, steering wheel, especially the radio s speaker Deviation is specific to each individual vessel Deviation will vary depending upon vessel s heading» If more than one or two degrees, a DEVIATION TABLE should be constructed 15
Deviation Table 16
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. 17
Measuring Distance 360 degrees in a circle Each circle has 360 x 60 = 21,600 minutes Distance of 1 minute of arc is defined as 1 nautical mile 1 nautical mile = 1/21,600 th of distance around the earth s surface Therefore, the circumference of the earth = 21,600 nautical miles 18
Measuring Distance One degree of latitude equals 60 nautical miles One minute Of latitude equals one nautical mile Use Latitude scale at vertical side of chart, or Distance scale 19
Dividers & Compass Dividers Measure or mark off distances on a chart Pointed both arms Drafting compass Used for swinging an arc Dividers Compass 20 20
Measuring Distance Use dividers to transfer distances between objects and distance scales Use latitude scale or distance scale 21 21
Longer Distances Walk dividers to measure large distances 22 Read amount left-over on distance scale 22
Measuring Distance Remember: Most charts have distance scales in nautical miles, statute miles, and kilometers. On the water, use nautical miles because of the natural relationship with latitude 23
Distance and Speed 1 statute mile = 5,280 feet 1 nautical mile = 6,076 feet (=1.15 statute miles) 1 nautical mile = 2,000 yards A knot (kt) is a speed of 1 nautical mile (NM) per hour 24
Course Plotting Direction is measured clockwise from north and expressed in degrees East is 090 o South is 180 o West is 270 o North is 360 o or 000 o 25
Course Plotting 170 o Find course (direction) from buoy A to buoy B 170 o T 26
Parallel Rules Consists of a pair rules joined by two or more swinging hinges Serve as a straight edge for plotting Helps transfer a plotted bearing line across the chart to compass rose
Rolling Parallel Rules One straight edge Moves across the chart on a long roller 28
Protractor Plotting Tool Course Plotter Easier to use than parallel rules particularly in cramped spaces 29
Expressing Course or Direction True Heading Can be affected by variation Used to plot a course Magnetic Heading Difference from true caused by variation Compass Heading Deviation from magnetic heading caused by magnetic influences on boat Used to steer a course 30
Additive Effect of Compass Errors Error due to 015 W variation Additional error due to 005 W deviation Desired True heading Compass heading must be adjusted 020 to compensate. 31 31
Computing Compass Corrections T True Heading 000 V Variation on Chart 015 W +W M Magnetic Heading 015 D Deviation 005 W C Compass Course 020 TV Makes Dull Children, add Wisdom
Computing Compass Corrections +W T V M D C Memorize it! You re going to use it again and again and again! 33 33
Examples You measure a course heading on your chart and find it 47 o true What compass heading should you steer? Variation = 14 o W Deviation = 8 o E T V M D C 47 o 14 o W 61 o 8 o E 53 o 34
Examples Your compass heading to Fire Island light is 006 o What is your true heading? Variation = 14 o W Deviation = 3 o E T V M D C 355 o 14 o W 009 o 3 o E 006 o 35
Compass Review Variation Local difference, East or West, between the direction of true and magnetic north Deviation Error caused by magnetic influences within the boat based on its heading 36
Finding Your Position Fixes an accurate position determination Intersection of two or more lines of position
Computing Speed, Time and Distance Some Examples: Suppose you drive your boat at a speed of 10 Knots for 20 minutes. How far have you gone? How fast are you going if you travel 12 Nautical Miles in 40 minutes? How long does it take to travel 12 Nautical Miles at a speed of 15 Knots? 38
Speed Time Distance Sixty D Street 60D=ST Distance in nautical miles (NM) Speed in knots (kt) Time in minutes 39
Calculating Time, Speed & Distance Distance = Speed x Time divided by 60 D = ST/60 Speed = Distance x 60 divided by Time S = 60D/T Time = Distance x 60 divided by Speed T = 60D/S 40
Example Calculations Speed 10 kts, run time 20 min; compute distance: D = ST/60 = 10x20/60 = 3.3 NM Distance to destination 12 mi, time enroute is 40 min; compute speed: S = 60D/T = 60x12/40 = 18.0 kts Cruising at 15 kts, distance to destination is 12 mi; compute time to destination: T = 60D/T = 60x12/15 = 48 min
Speed Table Speed must be actual speed over the ground Timed runs made out & back on measured course at multiple RPMs Calculated speeds averaged at each RPM Eliminates effect of wind and current 42
Speed Curve Speed table data plotted in graphical form for easier use 43
Dead Reckoning Simplest, most basic technique of navigation One of the most important navigation techniques Likely term was originally deduced reckoning and shortened to dead (ded) reckoning 44
What is Dead Reckoning? Your intended or assumed path through the water Plotted as a line w/ symbols on a chart Based on: COURSE (direction traveled) SPEED & TIME (distance run) 45
Information Considered DR Plot Use: Course known to have been steered Speed over ground Do not consider: Effects of Current Effects of Wind 46 46
Purpose of a DR Plot Before sailing (voyage planning): Determine course and speed to make Identify checkpoints and waypoints Locate fuel and other needed stops Estimate: Fuel, time, and other parameters While underway to plot or follow actual progress, (tactical plot) 47
Typical DR Plot 0900 48
Three Key Words PRACTICE PRACTICE PRACTICE 49
Summary Compass errors Variation Deviation Measure distances on a nautical chart Plot a course on a nautical chart Convert between a true course and compass course TVMDC Speed/time/distance formula 60D=ST 50