Navigation
Navigation on the GMA OOW deck paper tests whether you can plan and monitor a passage, fix the ship's position with confidence, and apply the Collision Regulations as a decision-making framework rather than a list of rules.
Passage planning and monitoring
IMO Resolution A.893(21) sets four stages: appraisal, planning, execution and monitoring. Appraisal gathers charts, notices to mariners, sailing directions, tide tables, weather and vessel constraints. Planning lays the track with waypoints, no-go areas, abort points, under-keel clearance and contingency anchorages. Monitoring compares the actual position with the planned track at each fix and records cross-track error.
Position fixing: visual bearings should be spread, ideally 60–90° apart. A three-bearing fix gives a cocked hat; the centre is the position and the size indicates quality. Use transits and clearing bearings in pilotage. GPS positions must be checked against chart datum; on ECDIS, set safety depth and contour for the actual draught and check CATZOC.
COLREGs as a decision framework
Rules 5–8 (lookout, safe speed, risk of collision, action to avoid collision) are the foundations. Risk of collision exists if the compass bearing of an approaching vessel does not appreciably change. Rule 13: an overtaking vessel keeps clear until finally past and clear. Rule 14: head-on, both alter to starboard. Rule 15: crossing, the vessel with the other on her starboard side gives way. Rule 17: the stand-on vessel maintains course and speed but must act when collision cannot be avoided by the give-way vessel alone. Rule 19: in restricted visibility there is no stand-on or give-way vessel; safe speed and early action apply.
Worked example: course to steer
A vessel must make good 090° true at 12 knots. The current sets 180° at 2 knots. What course should she steer?
The current is 2 knots south, so the water track must be angled north. Course to steer = 090° + arcsin(2/12) = 099.6° true. Speed made good = √(12² − 2²) = 11.83 knots. Convert to compass: with variation 4° W and deviation 2° E, compass = true − variation − deviation = 99.6 − (−4) − 2 = 101.6°.
Traps
- Mixing variation and deviation; east is positive, west negative.
- Treating a cocked hat as a precise fix.
- On ECDIS, forgetting safety settings and CATZOC.
- Making a small alteration that is not readily apparent to the other vessel (Rule 8).
- Using a run between fixes without checking log error.