Marine Charting Tools
- The twin points of navigational dividers resemble the compass you used in school. Used for plotting distances, the navigator would "walk" the points of the divider -- spread to a set distance often 10 nautical miles, based on the one nautical mile marks on the right and left sides of the navigational chart -- along the course line to determine the distance to a destination. A semi-circle was drawn on the course line at the place where the points of the divider landed; this indicated an estimated position or a planned position, allowing the navigator to quickly check the vessel's progress.
- Parallel rulers are two straight edges, usually 10 or 12 inches long, fastened together by two 2 1/2-inch, metal bars at 2 inches from each end. The metal bars are riveted at the ends to allow the rulers to move apart a distance of 1 inch, while remaining parallel to each other.
The compass rose on a navigational chart is two concentric circles, with all 360 degrees of a circle marked on their outside edge, one shows true north and true directions, the other, magnetic north and magnetic directions. When you lay the parallel rulers on a chart's compass rose, so that the top edge of the top ruler lays across the center of the compass rose and the planned course, you can slide the top ruler up, away from the bottom ruler a distance of about 1 inch. You may then slide the bottom ruler to the top ruler and it will stay parallel to the top edge of the top ruler. You then slide the top ruler again and let the bottom ruler catch up until you reach your departure point.
Then, you can draw the initial course line on the chart, certain that the line, drawn along the top edge of the top ruler, is the same angle found when you lay the top edge on the compass rose.
The Navigator's Blue Pencil
- The marks made by a blue navigator's pencil are highlighted in the red light used at night to preserve the crew's night vision. The marks the navigator makes on the charts include updates -- notes taken from Notices To Mariners, the warning bulletin issued as required by the U.S. Coast Guard to provide navigational information; information on obstructions, depths and changes in buoyage for each chart; the dead reckoning course, the course made good, times and locations of navigational observations.
- The rolling protractor, also called a "waypoint protactor" may be used to plot quick, approximate course changes. The circular protractor is centered on the course line and twisted so that the straight edge of the protractor card passes through the object or place the master wishes to avoid. A pencil line is then drawn along the straight edge and the protractor, which has a built in roller, is then rolled to the nearest compass rose to determine the "danger bearing."