Navigation: Passage Planning, Position Fixing and Compass Error
What this domain tests
Navigation at chief mate level covers passage planning to IMO Resolution A.893(21), position fixing by visual, terrestrial and electronic means, compass error, tidal height and under-keel clearance, and chart and projection knowledge. Expect short computational items, not essay answers.
Core rules
- Passage plan stages: appraisal, planning, execution, monitoring. Mark no-go areas, abort points, wheel-over positions, contingency anchorages, and state the UKC policy.
- Mercator: rhumb lines plot straight, scale varies as sec lat. Gnomonic: great circles plot straight and are used for ocean planning. Never navigate on an overscale ENC.
- Compass chain: true = compass + deviation + variation (easterly positive). Deviation comes from a transit, a distant object with a known true bearing, or an amplitude/azimuth.
- UKC = charted depth + height of tide - draught - allowances (squat, heel, wave, survey and chart accuracy).
- Position fixing intervals must be set in the plan and tightened in restricted waters, at landfall and in poor visibility.
Worked example
Latitude 30°N, Sun declination 10°N at rising. sin A = sin 10° / cos 30° = 0.1736 / 0.8660 = 0.2005, so amplitude A = 11.6°. Rising with north declination gives 078.4°T. A gyro bearing of 081.0° means a gyro error of 2.6° high; a compass bearing of 075.0° means a compass error of 3.4°E (078.4 - 075.0).
Traps
- The amplitude formula uses the observer's latitude and the declination at the moment of observation, not at meridian passage.
- Advancing a running fix means advancing the first line of position by course and distance run, not by the bearing of the second object.
- Applying variation directly to a compass bearing, forgetting deviation.
- Quoting UKC without subtracting squat or without the tidal height at the actual time of transit.