Chartwork Tips and Techniques for Day Skipper

Good chartwork starts with the right tools. You need a Portland plotter or parallel rulers, a pair of dividers, a soft pencil (2B), an eraser, and a calculator. For the exam, you will be given a practice chart (usually the 5055 training chart), almanac extracts, and tidal stream data.

Keep your pencil sharp and your lines thin. Accuracy matters — a line plotted 1° off course translates to roughly 1 nautical mile of error for every 60 miles sailed. Label all positions, times, and bearings clearly on your chart.

Positions are given as latitude and longitude. To plot a position, find the latitude on the side scale and the longitude on the top or bottom scale. Use dividers to measure from the nearest graduation mark. Mark the position with a small circle or dot and label it with the time.

Remember: latitude lines run east–west (but measure north–south position), and longitude lines run north–south (but measure east–west position). One minute of latitude equals one nautical mile. Longitude minutes vary with latitude but are close to one nautical mile in UK waters.

Course to steer (CTS) is the compass heading you must follow to reach your destination, accounting for tidal stream and leeway. Start with the ground track (the line from your position to the destination). Then apply the tidal stream vector to find the water track. Finally, apply variation and deviation to convert from true to compass.

Draw the tidal stream vector from your starting position. Its length represents one hour of tidal drift at the given rate. From the end of the tidal vector, swing an arc with radius equal to your boat speed. Where this arc crosses the ground track line gives you the water track direction — your course to steer (true).

A Dead Reckoning (DR) position is calculated from your last known position using only your course steered and distance logged. It does not account for tidal stream or leeway. Mark it with a cross and label it 'DR'.

An Estimated Position (EP) starts with the DR but then applies the effect of tidal stream and any estimated leeway. This gives a more realistic estimate of where you actually are. Mark it with a triangle and label it 'EP'. An EP is always more accurate than a DR because it accounts for more factors.

A fix is your actual position, determined from observations. The most common methods are: a three-point fix using compass bearings to charted objects, a two-bearing fix with a running fix technique, transit bearings, and GPS positions.

For a three-point fix, take bearings to three identifiable charted features spread roughly 60° apart. Plot the bearings from the objects on the chart. The triangle formed where they intersect (the 'cocked hat') contains your position. Your fix is at the centre of the cocked hat, or on the side nearest danger.