Under-Keel Clearance: When Is There Enough Water to Get In?

Height of tide needed = draught + safety margin − charted depth. For a 1.8 m draught, a 0.5 m margin and a 0.6 m bar, you need 1.7 m of tide. Then use the Rule of Twelfths to find when the tide reaches it: here about 0720.

The question

A harbour entrance has a charted depth of 0.6 m over the bar. Your draught is 1.8 m and you want 0.5 m under the keel. LW is 0600 (1.0 m), HW 1200 (6.0 m). When is the earliest you can cross?

Working, step by step

1. Depth needed = draught + clearance = 1.8 m + 0.5 m = 2.3 m.
2. Height of tide needed = 2.3 m − charted depth 0.6 m = 1.7 m.
3. Range = 5.0 m; one twelfth = 0.42 m. Rise needed from LW = 0.7 m = 1.68 twelfths.
4. In the first hour the tide changes 1 twelfth, which is 0.42 m.
5. That leaves 0.28 m to go. In hour 2 the tide changes 2 twelfths (0.83 m), so it takes 0.34 of that hour, about 20 minutes.
6. Earliest time = 0600 + 1 h 20 min = 0720.

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

• Rule of twelfthsGuide
• How to read a nautical chartGuide

More worked tide examples

• Height of tide at a given time
• Time the tide reaches a height
• Depth from charted depth
• Crossing a drying height
• Bridge clearance
• Anchoring depth and scope
• Springs or neaps from the range
• Tidal stream rate from a diamond
• Secondary port: time of HW
• Secondary port: height of HW

Frequently asked questions