Nautical Science and Navigation
What the exam tests
Nautical Science and Navigation tests terrestrial coordinates, chartwork, position fixing, tidal and current calculations, passage planning, and bridge equipment used to keep a vessel on a safe track. Questions often combine a definition with a short calculation or a practical decision.
Core rules
- Latitude is angular distance north or south of the equator; longitude is angular distance east or west of Greenwich. One minute of latitude equals one nautical mile, 1,852 m.
- Speed, distance and time use S = D/T; keep units consistent in knots, nautical miles and hours.
- For a course along a parallel, departure = change of longitude × cos(latitude).
- A position fix should be plotted from position lines crossing at a good angle; a single range or bearing is not a fix.
- Set is the direction of current; drift is its speed. Apply current as a vector to course and speed through the water.
- Chart soundings are reduced to chart datum; predicted tide must be added to obtain actual depth.
Worked example
A vessel at 34°00.0′N runs due east at 12 kn for 1 h 30 min along the parallel. Distance = 12 × 1.5 = 18 NM. Departure is 18 NM. Change of longitude = 18 / cos 34° = 18 / 0.829 = 21.7′. New longitude = 129°00.0′E + 21.7′ = 129°21.7′E. Latitude is unchanged because the track is east-west.
Traps
Do not treat one minute of longitude as one nautical mile; it shrinks with latitude. Check whether variation is east or west before converting compass to true. Do not add current to speed through the water without vector addition. In ECDIS, check datum and safety depth, not just the display.