Navigation: Passage Planning, Position Fixing and Collision Avoidance
The planning chain
A candidate must be able to move from berth to berth through appraisal, planning, execution and monitoring. Know the difference between a safe water line, a no-go area and a contingency anchorage, and be able to justify each with charted depths, under-keel clearance and squat.
Position fixing and its errors
Understand the geometry of position lines: a three-point fix with a cocked hat, the effect of a small intersection angle, and why a transit is superior to a bearing. Know the limitations of GNSS, including multipath, ionospheric delay, datum mismatch and the need for independent cross-checks. Be ready to explain radar parallel indexing and the use of ARPA trial manoeuvres.
Collision avoidance
Questions test whether you apply the Rules as a system rather than reciting them. Know which vessel gives way, when a stand-on vessel must act, and the danger of a series of small alterations. Understand relative versus true motion, aspect changes, and the value of a bold alteration that is readily apparent by radar and visually.
Traps
- Confusing course over ground with heading in a cross-current.
- Treating a plotted target's closest point of approach as fixed when own ship alters.
- Forgetting that a vessel restricted in ability to manoeuvre is not automatically the stand-on vessel.
- Assuming a chart's datum matches the GNSS datum.
Worked example
A vessel making 12 knots must pass a shoal with 2 m charted depth. With a draught of 8 m and a required under-keel clearance of 10 per cent of draught, the minimum available depth is 8.8 m. Add allowance for squat, tide and chart accuracy before drawing the safe water line; the line must reflect the worst credible combination, not the average.