Question 1

A chart shows a feature with an underlined depth figure (a "drying height"). What does this indicate, and how would you determine whether the feature is covered by water at a given time?

Accepted Answer

An underlined figure indicates a drying height — the height of the feature above chart datum (typically Lowest Astronomical Tide) when it is exposed at low water; to determine whether it is covered at a given time, compare the predicted height of tide above chart datum at that time to the drying height — if the height of tide exceeds the drying height, the feature is covered Drying heights (conventionally underlined on charts) are referenced to chart datum and represent features that alternately dry out and are covered with the tide — comparing the predicted height of tide to the drying height tells you whether the feature is currently a hazard at or below the surface.

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.