Question 1

During your oral exam you are asked: what is the difference between a great circle track and a rhumb line track, and which is typically used for short coastal passages on a yacht?

Accepted Answer

A rhumb line is a track of constant compass bearing, appearing as a straight line on a Mercator chart, and is normally used for shorter coastal passages where the difference from a great circle is negligible A rhumb line maintains a constant true course and plots as a straight line on a Mercator chart, making it simple to steer and plot, while a great circle represents the shortest distance between two points on the Earth's surface but appears curved on a Mercator projection and requires course changes en route. For coastal yacht passages, the distances involved are short enough that the difference between the two is negligible, so the simplicity of the rhumb line makes it the practical choice; great circle routes become significant on longer ocean crossings.

Question 2

When taking a visual position fix using two compass bearings of charted objects, why is a third bearing often taken if a suitable object is available?

Accepted Answer

Two bearings give a position at their intersection, but a third bearing forms a small triangle ("cocked hat") where the three position lines meet — the size of this triangle gives a visual indication of the likely accuracy of the fix and any compass error, helping the navigator judge how much to trust the result A two-bearing fix gives a single intersection point with no indication of error; a third bearing reveals a "cocked hat" whose size is a practical, visual guide to the fix's reliability.

Question 3

Using the "rule of twelfths" to estimate the height of tide between low water and high water, what fraction of the total tidal range is gained during the first hour after low water?

Accepted Answer

1/12 of the range The rule of twelfths approximates the rise/fall over each hour of a (roughly) 6-hour tide as 1, 2, 3, 3, 2, 1 twelfths — so the first hour accounts for 1/12 of the total range, the slowest rate of change near the turn of the tide.