Nautical Science and Navigation
Scope
The Nautical Science and Navigation section tests chartwork, position fixing, compass conversion, buoyage, tides, passage planning and basic collision avoidance. A GP Rating must read a chart, plot a fix and report a course or position without ambiguity.
Core rules
- Chart bearings are true. The compass reading includes deviation from the ship's magnetism and variation from Earth's magnetism.
- Compass to true: C + D = M; M + V = T. Add east and subtract west at each step.
- IALA Region A: red can buoys mark the port side of the channel on entry; green cone buoys mark starboard.
- A fix needs at least two position lines; three position lines show a cocked hat.
- Set is the direction a current flows toward; drift is its speed in knots.
Worked example
A vessel steers 046°C. Deviation is 2°E and variation is 3°W. Find the true course. First apply deviation: 046 + 2°E = 048°M. Then apply variation: 048 - 3°W = 045°T. The true course is 045°T. Reverse the steps to go from true to compass: subtract easterly error and add westerly error.
Traps
Applying variation before deviation, using a magnetic bearing as true, plotting a GPS fix without checking the chart datum, and confusing set with the direction a wind blows from all cost marks. A DR position is advanced by course and speed, not by guesswork.