Weems & Plath Protractor 255

There are two types of charts: mercator and gnomonic.  

Mercator charts have parallel longitude and latitude lines, and they are used for coastal navigation.  

Gnomonic charts have curved longitude and latitude lines, and they are used for long range navigation.

  
NOAA Chart #11425

All charts designate how the depths are calculated.

Be aware that separate panels in a chart might be calibrated using a different standard.

Always double check the depth calibration in the panel and don't assume it will be the same as the general chart.

Chart Designations

Variation is the varying difference between true north and magnetic north.  It changes over time.  Locate the compass rose on the chart (or in the chart panel), and record the variation, year, and annual change. (e.g. variation 11 degrees 22 minutes W (1985) Annual change 7 minutes W).

Subtract the variation year from the current year, and multiply the difference by the annual change. Add the result of the calculation to the chart variation to determine the compass error.

Find the east and west compass error markings around the rotating disk on the chart protractor (Figure A). Place a pencil mark on the chart protractor to identify the compass error correction you calculated (Figure B). (Be sure to pay attention to whether the correction is East or West.) This pencil line will be the line you use to read your course headings or bearings, and you won't have to address variation again during your cruise if this is the only chart you will be using.

Figure A                     Figure B

Using a pencil, draw a line on the chart for the course (or segment of a course) you wish to travel. Mark the line with two arrows (i.e. >>) as this will become your track line (i.e. the actual course over which your boat will travel).

5. Check Water Depths.

Check the water depth along the track line to determine that there is sufficient water depths for your boat. If there is insufficient depth anywhere along the line, consider altering the track line.

 
Image by GarryKillian on Freepik

Lay the chart protractor along the track line. Be sure the chart protractor directional arrow points in the direction of travel.

7. Align the Chart Protractor Disk.

Rotate the protractor disk so it points north and the internal grid lines inside the disk align with a longitude or latitude line on the chart. (This may require repositioning the chart protractor along the track line.)

8. Determine the Course Heading.

Use the pencil mark on the compass error markings to read the course heading.

Record the course heading along the track line. (If in a sailboat, see #11 below.)  Place an M after the course heading (e.g. C = 18 degrees M). You have recorded the magnetic (M) course-to-steer. As long as you navigate using your compass, there are no further corrections needed.  (Note:  This makes the assumption that your compass does not need correction for deviation--see Basic Navigation.)  The track line becomes the course-to-steer line because there are no adjustments necessary due to leeway or current.

10. Determine and Record Distance.

Measure the track/course line with the dividers, and use the latitude minute markings on the side of the chart to determine the distance. (Remember: 1 minute latitude = 1 nautical mile.) Record the distance on the track/course line (e.g. D = 10 n.m.)

If in a sailboat, you must compensate for leeway (i.e. the sideways motion of the boat pushed by the wind as it makes headway.)  

12. Determine Wind Direction and Leeway. 

Determine the wind direction and how much leeway affects your boat. Most boats have 5 - 10 degrees leeway, and this can be affected by the amount of wind and the direction of travel in relation to the wind. You will have to make a determination for your own boat.  (You can determine the leeway of the boat by using a handheld compass at the stern of the boat to site the track line left by the boat's wake.  The difference in degrees between the track line and the compass course steered is the boat's leeway.  It can be affected by increased winds, angle of the wind, and sea conditions.)

13.  Record Corrected Course Heading.   

Do not enter leeway as a vector in your chart plotting, but manually add or subtract it from your final course heading calculation, and record the corrected course heading on the course line. An additional smaller vector arrow is added to the track line at the beginning point that indicates the recorded course heading has been corrected for leeway. You can record the degrees of leeway at the end of the small vector arrow. (E.g. if the wind is blowing out of the north, causing 5 degrees of leeway to the south, the corrected course-to-steer would be 85 degrees to compensate for the 5 degree of leeway.)

Variation is the varying difference between true north and magnetic north.  It changes over time.  Locate the compass rose on the chart (or in the chart panel), and record the variation, year, and annual change. (e.g. variation 11 degrees 22 minutes W (1985) Annual change 7 minutes W).

Subtract the variation year from the current year, and multiply the difference by the annual change. Add the result of the calculation to the chart variation to determine the compass error.

Find the east and west compass error markings around the rotating disk on the chart protractor (Figure A). Place a pencil mark on the chart protractor to identify the compass error correction you calculated (Figure B). (Be sure to pay attention to whether the correction is East or West.) This pencil line will be the line you use to read your course headings or bearings, and you won't have to address variation again during your cruise if this is the only chart you will be using.

Figure A                     Figure B

Using a pencil, draw a line on the chart for the course (or segment of a course) you wish to travel. Mark the line with two arrows (i.e. >>) as this will become your track line (i.e. the actual course over which your boat will travel).
 
 

5. Check Water Depths.

Check the water depths along the track line to determine that there is sufficient water depth for your boat. If there is insufficient depth anywhere along the line, consider altering the track line.

 

Identify the current from a current marking on the chart or from a cruising guide. Record its set (direction) and drift (speed in knots) (e.g. current = 0 degrees due north at 1 knot). Currents on charts will be identified with arrows; exact directional degrees will not be given. The current's speed in knots will be listed beside the arrow.  (See Tides & Currents for determining set and drift of current while underway.)

 
Image by GarryKillian on Freepik

Draw a current vector in the direction of the current from the starting point on the track line. The length of the vector should match the distance an object pushed by the current would travel in one hour using minutes of latitude (e.g. a 1 knot current travels 1 nautical mile in one hour or 1 minute of latitude). Use the dividers to determine the length of the current vector based on the latitude markings on the side of the chart and mark the current vector on the chart with three arrows ( i. e. >>>).

8. Draw the Course Line. 

Determine your probable boat speed over that hour of travel, and set your dividers to match that speed where one knot of speed corresponds to one minute of latitude. Place one end of the dividers on the end of the current vector, and swing the dividers until the other end intersects the track line. Draw a line from the end of the current vector to where the dividers intersect the track line. This is your course-to-steer line, and it should be marked with one arrow (i.e. >).

NOTE: In the illustration to the right, the course line happens to intersect the track line at the destination (by chance), but it could intersect anywhere on the track line before or beyond the destination. Never draw the course line to the destination unless it naturally intersects there. Draw the course line to the intersection point even if falls before or beyond the destination.

Lay the chart protractor along the course line. Be sure the chart protractor directional arrow points in the direction of travel.

10. Align the Chart Protractor Disk. 

Rotate the protractor disk so it points north and the internal grid lines inside the disk align with a longitude or latitude line on the chart. (This may require repositioning the chart protractor along the course line.)

11. Determine the Course Heading. 

Use the pencil mark on the compass error markings to read the course heading.

Record your course-to-steer above the course line (e.g. C = 36 degrees M). This is the magnetic course you will steer to end up at your destination having accounted for the current.  (Note:  This makes the assumption that your compass does not need to be corrected for deviation--see Basic Navigation.)

13. Record the Distance. 

As you did in Step 4, measure the track line with the dividers, and use the latitude minute markings on the side of the chart to determine the distance. (Remember 1 minute latitude = 1 nautical mile). Record the distance on the track line (e.g. D = 10 n.m.). Use the distance and speed to compute your anticipated time in route.

If in a sailboat, you must compensate for leeway (i.e. the sideways motion of the boat pushed by the wind as it makes headway.)  

15. Determine Wind Direction and Leeway. 

Determine the wind direction and how much leeway affects your boat. Most boats have 5 - 10 degrees leeway, and this can be affected by the amount of wind and the direction of travel in relation to the wind. You will have to make a determination for your own boat.  (You can determine the leeway of the boat by using a handheld compass at the stern of the boat to site the track line left by the boat's wake.  The difference in degrees between the track line and the compass course steered is the boat's leeway.  It can be affected by increased winds, angle of the wind, and sea conditions.)

16.  Record Corrected Course Heading.   

Do not enter leeway as a vector in your chart plotting, but manually add or subtract it from your final course heading calculation, and record the corrected course heading on the course line. An additional smaller vector arrow is added to the track line at the beginning point that indicates the recorded course heading has been corrected for leeway. You can record the degrees of leeway at the end of the small vector arrow. (E.g. if the wind is blowing out of the north, causing 5 degrees of leeway to the south, the corrected course-to-steer would be 85 degrees to compensate for the 5 degrees of leeway.)