Navigation study overview

Fixes, compass work, position quality and navigation equipment.

Practise observation and judgement using sailing scenarios. Interactive questions and your device-local progress load when JavaScript is available.

Study points

• A position line can be plotted through the charted light using the reciprocal of the observed bearing. The reciprocal of 090° is 270°, an addition of 180°. Do not use the yacht’s heading or confuse the direction towards the light with the direction from the light back to the observer.
• Bearing lines are geometrically undirected: 025° and 205° lie along the same orientation, so that pair gives nearly parallel or coincident position lines, not a robust 180° intersection. The 040°/130° pair crosses at approximately 90°, limiting the growth of positional error from small bearing errors. Identification and close observation timing remain necessary.
• DR advances a known position using steering direction and distance run; it does not automatically include all external effects.
• Electronic fixes can be wrong, stale or misunderstood. Compare independent observations and act conservatively until the position is resolved.
• The adverse stream reduces the aligned ground speed to 6 − 1.5 = 4.5 knots. Twenty minutes is one third of an hour, giving 4.5/3 = 1.5 NM east. The 2 NM log distance is through the water, not ground advance; adding this adverse stream would give an incorrectly optimistic 2.5 NM.
• Deviation is local error caused by the vessel and its equipment. Variation is the geographic difference between true and magnetic north.
• COG is the track over the ground; heading is the direction the vessel points. Current can make them differ.
• Independent cross-checks help identify instrument, plotting or interpretation errors. They do not replace current charts or a proper lookout.

Educational revision only. Check current official rules, charts and publications before navigating.