How to Plot a Course on a Nautical Chart

Plotting a course is where all your navigation knowledge comes together. It is the process of deciding how to get from where you are to where you want to be, drawing that plan on the chart, and turning it into a direction to steer and a distance to run. Every skipper needs this skill, and it forms a central part of most practical boating exams.

The process is methodical rather than difficult. With a chart, a plotting instrument, a pair of dividers and a pencil, you can lay out a safe passage in a series of clear steps. This guide walks through those steps in order.

Step One: Choose Your Waypoints

Begin by deciding on your start point and your destination, then look for any hazards between them, such as shallow banks, rocks, wrecks, and areas of heavy traffic. Rather than drawing a single straight line that might cross danger, break the passage into legs joined by waypoints, safe positions where you change direction.

Choose waypoints that keep you in safe water with a comfortable margin, and place them where they are easy to identify, for example off a prominent buoy or headland. A good route balances directness against safety, never sacrificing clearance from hazards just to shave off a little distance.

Step Two: Draw the Track and Measure the Bearing

Draw a straight line between each pair of waypoints. This line is your intended track. Using your plotting instrument aligned with the meridians or parallels on the chart, measure the direction of the track in degrees. This gives you the true course from one waypoint to the next.

Remember that this bearing is a true bearing, measured relative to true north on the chart. Before it can be steered on a magnetic compass it will need to be converted, applying variation and any deviation, into a compass course. Note the true bearing of each leg clearly so you can carry out that conversion cleanly in the next stage.

Step Three: Measure the Distance

With the direction known, measure how far each leg runs. Open your dividers to span the leg, then carry that spacing to the latitude scale on the side of the chart, reading off the distance in nautical miles. Always use the latitude scale, never the longitude scale, because one minute of latitude equals one nautical mile.

If a leg is longer than your dividers can span in one go, step the dividers along the line a set distance at a time and count the steps. Add up all your legs to find the total distance of the passage, which you will need for estimating how long the trip will take.

Step Four: Correct for Tide and Leeway

On open water your boat rarely travels exactly along the line you drew, because moving water and wind push it off track. Tidal streams carry the boat sideways, and wind pushes it downwind, an effect called leeway. To arrive at your waypoint you must aim off to compensate.

Using tidal information for the area and time, you construct a simple vector triangle on the chart: your intended track, the direction and rate of the tidal stream, and your boat’s speed through the water. The result tells you the course to steer so that the combined effect of your heading and the tide carries you along your desired track. Allowing a few degrees extra into the wind accounts for leeway. These techniques are drilled in the boatingexamine course with worked chartwork examples.

Step Five: Estimate Time and Fuel

Once you know the distance and your expected boat speed, estimating the time for each leg is simple: time equals distance divided by speed. A boat making six knots covers six nautical miles in one hour, so a twelve mile leg takes two hours. Add up the legs for a total passage time.

Use this estimate to plan your departure so that you arrive in daylight, catch a favourable tide, and reach any harbour before it becomes difficult to enter. Combine your time estimate with fuel consumption figures to confirm you carry enough fuel, plus a sensible reserve for delays and diversions.

Keeping the Plan Alive Underway

A plotted course is a starting point, not a guarantee. Once underway, monitor your actual progress against the plan by fixing your position regularly and comparing it to where you expected to be. If you find yourself set off track, adjust your heading to regain the intended line.

Conditions change, so stay flexible. A rising wind, a shift in the tide, or reduced visibility may all call for a revised plan. The best navigators treat the chart as a living document, updating it throughout the passage rather than trusting a plan made hours earlier.

FAQ

What is the difference between a track and a heading? The track is the path over the ground you actually want to follow, drawn on the chart. The heading is the direction the boat points to achieve that track once tide and leeway are taken into account. In still water with no wind they are the same, but usually the heading differs.

Do I steer the true course I measured from the chart? No. The measured course is a true bearing. Before steering it you must convert it to a compass course by applying variation and deviation, because your compass reads relative to magnetic and compass north, not true north.

How much margin should I leave from hazards? Enough that a small error in position or a shift of tide will not put you in danger. The exact margin depends on conditions, visibility and the speed of your boat, but a cautious clearance always beats cutting corners near shallow water or rocks.