Does chartwork seem like an indecipherable puzzle? Don’t worry — many sailing enthusiasts, even experienced ones, find plotting a course to be a major hurdle to overcome.
What is a nautical course, and why should you learn how to calculate it?
A nautical course is the path you intend to follow during your voyage. It may sound simple, but knowing how to calculate it correctly is essential to reaching your destination without trouble, avoiding shoals, rocks, and other unpleasant surprises.
The first step is learning how to orient yourself: the four cardinal points — N (North), S (South), E (East), W (West) — and the compass rose, which shows both the cardinal directions and their corresponding degree values, form the basis of navigation.
Each time we work on a chart, we should picture ourselves at the center of a giant compass rose, expanded to the very limits of the visible horizon. Courses can be great circle routes (orthodromic) or rhumb line routes (loxodromic), so let’s clarify this first difference.
On great circle routes, all course lines are curved because the Earth is spherical. These are used for ocean crossings or very long passages. For voyages within about 300 nautical miles, rhumb line routes are used, where all those curved lines become straight. On these routes, the angles measured on the chart are equal to the actual angles on Earth (isogony).
Essential tools for plotting a course: what they are and how to use them
To calculate a nautical course, you’ll need a few fundamental tools:
Nautical chart: a detailed map of the sea showing depths, hazards, lighthouses, buoys, and other navigational references.
Divider/compass: used to measure distances and angles on the chart and to plot your course.
Parallel rulers or navigation triangles: similar to the set squares we used at school, but marked with two graduated scales. The graduations run from 0° to 180° and from 180° to 360° counterclockwise along both short sides, along with cardinal and intercardinal directions.
Magnetic compass: indicates magnetic north and allows you to follow the course you’ve calculated.
GPS: provides your exact position and helps you stay on your plotted course with precision.
Navigation apps: can simplify course calculation and provide additional useful information for safe navigation.
Calculating a nautical course: comparing two methods
There are two main ways to calculate a nautical course: the traditional method, known as chartwork, which uses a nautical chart and dividers, and the modern method, based on GPS and navigation apps. One doesn’t exclude the other, of course, but knowing how to calculate your course without instruments allows you to sail more safely under any conditions. Let’s take a closer look at both methods.
Traditional method: chartwork with a nautical chart, dividers, and a bit of math
After identifying your current position and destination on the chart, draw a straight line between the two points: this represents your True Course (TC) — the actual path relative to the seabed. Next, measure the angle between the true course and true north using dividers or navigation triangles. This gives you the True Heading (TH).
However, the course you must actually steer will also depend on wind and current conditions at the time of departure. These elements affect navigation and can push you off track: the effect of current is called drift, while the effect of wind is called leeway. To compensate, you’ll need to correct your course by taking into account their direction and intensity.
Modern method: GPS and navigation apps (even for the less tech-savvy)
The modern approach involves using GPS or navigation apps that you can install directly on your smartphone. Simply enter your destination, and the device will provide step-by-step instructions. It will display the course to steer, distance, estimated time of arrival, and other useful details.
You won’t need to perform any calculations: these devices automatically compensate for all factors, including magnetic variation, deviation, drift, and leeway. The modern method is certainly quicker and easier, but it’s still important to know chartwork — in case your GPS or app fails.
