Entry & progression
Exam module: Q163 — Chart Plot. 10 questions, 90% pass mark. You may miss one. Chart and instruments permitted.
The problems you will be given
- Plot a fix from two or three bearings, and label it.
- Find set and drift from a DR and a fix.
- Find the course to steer to make good a track against a current.
- Find speed made good and ETA.
- Convert between true, magnetic and compass, in both directions.
- Measure course and distance between charted points.
- Running fix — advancing a single line of position to cross with a later one.
The running fix
You have one object and one bearing. Take a bearing, run on, take a second bearing on the same object, and advance the first line of position by the course and distance run in between. Where it cuts the second bearing is your running fix.
Working a running fix
- Take the first bearing and plot the line of position. Label it with the time.
- Run on. Note the course steered and the distance run.
- From any convenient point on the first LOP, lay off the course and distance made good during the run.
- Draw a new line parallel to the first LOP through the end of that vector. This is the advanced LOP.
- Take the second bearing and plot it. Where it crosses the advanced LOP is the running fix.
The running fix is only as good as the course and distance you used to advance the first line — and if there is an unaccounted current, the fix inherits that error. It is a real position, not a guess, but it is a weaker position than a simultaneous two-bearing fix. Label it as a running fix, with the time, and treat it accordingly.
Special-angle bearings
| Method | How it works |
|---|---|
| Doubling the angle on the bow | When the relative bearing of an object doubles, the distance run between the two bearings equals your distance from the object at the second bearing |
| Bow and beam bearings | The special case: take a bearing at 45° relative, run on until the object is abeam (90°). The distance run equals the distance off at the beam |
These are the elegant part of the module and they turn one object and a stopwatch into a distance off. They also fall out of exactly the same isosceles-triangle geometry, which is worth understanding once rather than memorising twice.
TIP — The marks are lost in the arithmetic, not the navigation 60D = ST. Time in minutes. Distance measured on the latitude scale, at your working latitude. Variation from the nearest compass rose. Every one of those is a place candidates throw away a question they actually knew how to answer — and at 90 percent, one thrown-away question is the whole margin.