Navigation and Position Fixing
Core concepts
A coastal skipper must be able to fix position by at least two independent methods and know the accuracy limits of each. Understand: latitude/longitude and the minute-of-arc relationship (1 minute of latitude = 1 nautical mile); true, magnetic and compass bearings and the CADET rule (Compass + Deviation = Magnetic; Magnetic + Variation = True, with signs handled East-add/West-subtract); the three-bearing fix and cocked hat; running fix using a transferred position line; dead reckoning (course and speed through water, no allowance for current) versus estimated position (DR plus leeway and set/drift); transit and clearing bearings; and the use of depth contours as a position line.
Traps
Questions frequently mix up deviation (boat-specific, changes with heading) with variation (location-specific, from the chart/rose). Conversion direction is the classic error: always work from the value you have toward the value you need, applying East as positive. Another trap is confusing speed through the water with speed over the ground when a current is running, and forgetting that a running fix assumes the vessel held one course and speed.
Worked example
Compass bearing 090°, deviation 4°E, variation 22°W. Compass + Deviation = Magnetic: 090° + 4° = 094°M. Magnetic + Variation = True: 094° + (−22°) = 072°T. Check the answer is sensible: a large westerly variation should pull the true bearing well below the compass reading.
Rule of thumb
'Can Dead Men Vote Twice At Elections' - Compass, Deviation, Magnetic, Variation, True, Add East. Reverse it going the other way.