Navigation
The Navigation paper checks whether you can fix and monitor a ship's position, plan a passage to a documented standard, and correct course and speed for set and drift. Expect chartwork questions on running fixes and tidal work, great-circle and rhumb-line sailing, and ECDIS safety settings.
Fixing and monitoring position
- A three-point fix is only as good as its cocked hat. Investigate any triangle larger than the scale suggests rather than splitting the difference.
- A running fix advances the earlier position line parallel to itself, through the point reached by the course and distance run. The time of the fix is the time of the second position line.
- Cross-check GNSS against a terrestrial or radar fix. ECDIS does not replace lookout by eye and radar.
Set, drift and course to steer
The current triangle is: course and distance through the water, current vector, and the resultant course and speed made good. To find the course to steer, plot the current from the start, then swing the boat's speed arc to cut the required ground track.
Passage planning
IMO Resolution A.893(21) sets four stages: appraisal, planning, execution and monitoring. Set no-go areas, abort points and a UKC policy (commonly about 10 per cent of draft in open water, more where the water is constrained or exposed). ECDIS safety contour and safety depth must reflect the charted depths; they are not lowered to silence alarms.
Worked example
Course steered 090°T at 15 kn; current sets 180°T at 3 kn. Speed made good = √(15² + 3²) = √234 = 15.3 kn, and the current carries the ship right by tan⁻¹(3/15) = 11.3°, so course made good is 101.3°T. To make good 090°T, steer about 11.5° north of it: 078.5°T, giving 14.7 kn along the ground track.
Traps
- Mixing true, magnetic and compass bearings in the same fix; deviation applies to the compass, variation to the magnetic bearing.
- Trusting a running fix built on an inaccurate course or distance run.
- Forgetting that in Region B buoyage the lateral colours are reversed compared with Region A, so confirm the region on the chart.
- Ignoring tidal stream when quoting ETA.