Heights, streams, chart datum and the calculations that decide whether there is enough water.
Tidal work answers two questions: how much water is under the boat, and which way the water is moving. Heights and streams are different calculations from different tables, and mixing them up is the classic exam mistake — so these guides keep them firmly apart.
Worked examples use metres and the standard springs/neaps pattern. For real passages, always use the current year's tide tables or almanac for your area.
Ready to test yourself? The Day Skipper Theory, Actually Explained course turns these topics into 1,000+ practice questions with explanations.
Tides
What chart datum is, how charted depths, drying heights, heights of tide and clearance heights are measured from it, the datums used on UK and US charts, and how to combine them without mixing quantities.
8 min read
Tides
How the rule of twelfths approximates the tidal curve, a worked example checked step by step, what it assumes, why it says nothing about tidal streams, and when you must use the tidal curve instead.
8 min read
Tides
Why height of tide and tidal stream are separate quantities, where the timing of slack water and maximum stream comes from, how to read tidal diamonds and a tidal stream atlas, and how springs and neaps change the answer.
8 min read
