Navigation: Position, Passage Planning and Tides
The navigational core
A candidate for Officer of the Watch must be fluent in the four position-fixing families: visual bearings, terrestrial ranges, satellite fixes and dead reckoning. Understand chart datum, the significance of a drying height versus a charted sounding, and why soundings are reduced to Lowest Astronomical Tide. Know the difference between true, magnetic and compass bearing and the sequence of corrections: deviation applies to compass, variation applies to magnetic, and both must be applied with correct sign to reach true. Passage planning follows the four stages of appraisal, planning, execution and monitoring; questions often ask which stage a given activity belongs to.
Traps to expect
Mixed sign errors are the classic trap: easterly variation is subtracted from true to give magnetic. Another trap is confusing set and drift, where set is the direction the tidal stream flows towards and drift is its rate. Also watch for speed over ground versus speed through the water when a current is running, and for the difference between a position line and a line of position from a single object. Candidates also stumble on the meaning of 'safe water' when a charted depth is less than the vessel's draught plus under-keel clearance.
Worked rule of thumb
To convert compass to true: compass plus or minus deviation gives magnetic; magnetic plus easterly variation (or minus westerly) gives true. Example: compass 045, deviation 3E, variation 5W. Magnetic equals 045 minus 3 equals 042. True equals 042 minus 5 equals 037. Always sketch the relationship to check the sign. For under-keel clearance, use charted depth plus height of tide minus draught; if the result is less than your planned minimum, the route is unsafe.