A Step-By-Step Guide to Marine Chart Plotting
A simple way to navigate without a GPS or other electronics is to plot a course on a nautical chart, and for each leg of the course figure the bearing, speed, distance, and time you will travel. To follow the course on water, you simply use a stopwatch and your calculations.
- Difficulty: Average
- Time Required: 1 hour
What You Need for Nautical Navigation With a Chart
- Nautical chart for your area
- Parallel plotter (preferably with rollers)
- No. 2 Pencil
Step-by-Step Marine Chart Plotting
- Using a parallel plotter (preferably with rollers), draw a straight line from your departure point to your destination, or the first turn in your course. Draw as many course lines as you need to complete your trip.
- Lay one edge of the parallel rulers along the line you drew. Roll it to the nearest compass rose on the chart until the edge intersects the crossed lines at the center.
- Determine your magnetic bearing by reading where the course line intersects with the inside degree circle. Write this course on your chart above the plotted line in degrees magnetic (Example: C 345 M). Do this for each course line you drew on your chart.
- Determine the distance of each course in nautical miles using your dividers and the distance scale on the top or bottom of the chart. This is done by putting one end of the dividers on your start point, and the other end at your stop point or turn. Then, without moving the dividers, place them on the nautical mile scale and read the distance. Do this for each course line you drew, and write the distance on your chart below the course line (Example: 1.1 NM).
- Calculate the amount of time it will take to run each course by first determining your speed in knots based upon your normal cruising speed and current conditions. Write this on the top of your course line next to the bearing (Example: 10 KTS).
- Continue to calculate the amount of time it will take to run each course by multiplying the distance of the course times 60. Then divide that number by your predetermined speed in knots. The result is the amount of time in minutes and seconds it will take to complete the course line you plotted. Do this for each course you drew, and write this on the bottom on your course line (Example: 6 min 36 sec).
- The last step is to run the course using a stopwatch. At the start point of your course, come up to the determined speed and point your boat in the direction you plotted on your chart, ensuring that you continually keep the magnetic compass heading. Start the stopwatch and run a steady course and speed for the amount of time you calculated for your first course. When the time is up, if you plotted another course, turn and steady the boat on the next compass heading. Reset the stopwatch for this course. Either stop or continue on each course you drew on your chart.
Tips for Navigating with a Nautical Chart
- Be sure to plot the course in adequate water depth.
- Plot the course using buoys, lights, and other aids to navigation that show safe navigation areas.
- Always deviate from your plotted course to avoid unsafe conditions or a collision.