Worked tide example

Drying Heights: How to Work Out When You Can Cross a Drying Bank

A drying height (underlined on the chart) is how far the seabed sits above chart datum, so you add it to your draught instead of subtracting it: height of tide needed = drying height + draught + margin. To cross a 1.4 m drying bank in a 1.2 m boat with 0.4 m to spare, you need 3.0 m of tide, here from 1628 until 2152.

The question

A channel dries 1.4 m (charted as 14 underlined). Your draught is 1.2 m and you want 0.4 m clearance. LW 1310 (0.7 m), HW 1910 (4.7 m), next LW 0110 (0.7 m). Between what times can you cross?

What you are given
ItemValue
Drying height1.4 m
Draught1.2 m
Clearance wanted0.4 m
Low water1310, 0.7 m
High water1910, 4.7 m
Next low water0110, 0.7 m
0 m1 m2 m3 m4 m5 m6 m3.0 m needed1310 LW1910 HW0110 LW16282152
Tide curve drawn with the Rule of Twelfths. Real curves vary from port to port; use the almanac curve for real passages.

Working, step by step

  1. Height of tide needed = drying height + draught + clearance = 1.4 m + 1.2 m + 0.4 m = 3.0 m.
  2. Range = 4.0 m; one twelfth = 0.33 m.
  3. Rising: the tide must rise 2.3 m from LW.
  4. In the first 3 hours the tide changes 1 + 2 + 3 = 6 twelfths, which is 2.00 m.
  5. That leaves 0.30 m to go. In hour 4 the tide changes 3 twelfths (1.00 m), so it takes 0.30 of that hour, about 18 minutes.
  6. Earliest: 1310 + 3 h 18 min = 1628.
  7. Falling: the tide can fall 1.7 m from HW before it is too shallow.
  8. In the first 2 hours the tide changes 1 + 2 = 3 twelfths, which is 1.00 m.
  9. That leaves 0.70 m to go. In hour 3 the tide changes 3 twelfths (1.00 m), so it takes 0.70 of that hour, about 42 minutes.
  10. Latest: 1910 + 2 h 42 min = 2152.

Answer

You can cross between about 1628 and 2152. Aim for the rising tide: if you touch, the tide will soon lift you off.

The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.

Try it yourself

Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.

Learn the method

More worked tide examples

Frequently asked questions

How is a drying height shown on a chart?

As an underlined figure in the green (drying) area, such as 14 underlined for 1.4 m. It means the seabed there is that far above chart datum, so at a height of tide of 1.4 m the water is just reaching it.

Why add the drying height rather than subtract it?

Because the seabed is above chart datum. The tide must first cover the bank (the drying height) and then provide your draught and clearance on top.