Day Skipper theory guide · Tides
Day Skipper tides: chart datum and depth calculations
Tidal height becomes useful when you connect it to a charted depth or drying height. The first step is to check that the figures share the same reference level: chart datum.
Practise this topicWhat to look for
Chart datum is the reference
Charted depths and predicted heights of tide are normally referred to chart datum. For a charted sounding, add the height of tide to estimate the water depth at that time. Check the chart's notes and the tide source before combining figures.
A drying height goes the other way
A drying height describes a feature above chart datum. Subtract the drying height from the height of tide to estimate water over it. If the tide is lower than the drying height, the feature remains exposed in this simple calculation.
Keep depth and clearance separate
Water depth is not under-keel clearance. Subtract your vessel's draught from the estimated depth, then consider the extra margin needed for uncertainty and movement.
Worked example: water under the keel
The chart shows 1.8 m of depth. The predicted height of tide is 2.4 m above chart datum, so the estimated water depth is 4.2 m. If the boat draws 1.5 m, the simple under-keel clearance is 2.7 m. That figure is before any allowance for squat, waves, forecast error or chart uncertainty.
Charted depth + height of tide − draught = simple under-keel clearance.
A common mistake
Adding a drying height to the height of tide. A drying height is above chart datum, so subtract it when calculating water over the feature.
Try three questions
These are independent practice questions. Read the answer and explanation after making your choice.
01Charted depths are referenced to…
Answer: Chart Datum (approximately the lowest astronomical tide)
Charted depths are referenced to Chart Datum, which on UK Admiralty charts is normally approximately Lowest Astronomical Tide. Adding the height of tide gives an estimated water depth before local and weather effects.
02If charted depth is 1.8 m, height of tide is 2.4 m and your boat draws 1.5 m, the clearance under keel is…
Answer: 2.7 m
Actual depth is 1.8 + 2.4 = 4.2 m. Subtract draught 1.5 m to give 2.7 m clearance.
03A rock has a drying height of 1.2 m and the height of tide is 3.4 m. How much water is over the rock?
Answer: 2.2 m
A drying height is above chart datum, so depth over it is height of tide minus drying height: 3.4 - 1.2 = 2.2 m.
Sources and further reading
Examples checked against these primary sources on 27 September 2026. Check the latest editions before using any rule or calculation afloat.
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