How to Work Out the Depth of Water from a Charted Depth — Worked Example

Depth of water = charted depth + height of tide. A spot charted at 2.4 m, with 3.1 m of tide, has 5.5 m of water. If your echo sounder reads depth below the keel, subtract your draught to know what it should show.

The question

You are over a charted depth of 2.4 m at 1230. HW was 0930 (5.3 m), LW is 1530 (0.9 m). How deep is the water, and what should your echo sounder, set to read below the keel, show? Draught 1.6 m.

What you are given
Item	Value
Charted depth	2.4 m
High water	0930, 5.3 m
Low water	1530, 0.9 m
Draught	1.6 m

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

Height of tide at 1230, 3 hours after HW: range 4.4 m, fallen 6 twelfths, so 5.3 m − 2.20 m = 3.1 m.
Depth of water = charted depth + height of tide = 2.4 m + 3.1 m = 5.5 m.
Echo sounder below the keel = 5.5 m − 1.6 m = 3.9 m.

Answer

There is about 5.5 m of water, and the echo sounder should read about 3.9 m under the keel.

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

What if the echo sounder reads differently from the calculation?

Check its offset first: it may read below the transducer, below the keel or below the waterline. A big difference can also mean you are not where you think, which is a useful check on your position.