Tidal Height & Under-Keel Clearance
Tide height at any hour by the rule of twelfths, with the water actually under your keel once draft and a safety margin come off.
Under-keel clearance
Hour by hour
| Hour | Rises by | Total risen | Height |
|---|
The twelfths are 1 · 2 · 3 · 3 · 2 · 1 — slow at both turns, fastest through the middle two hours.
Where the rule comes from and when it breaks
A clean tide approximates a sine curve, and the twelfths are that curve chopped into six hourly slices: one, two, three, three, two, one. It is slow at the turns and quickest in the middle, which is why the third and fourth hours between them move half the entire range.
It assumes a clean semi-diurnal tide on a six-hour cycle. Somewhere with a double high water, a strong river influence, a shallow estuary or a significant surge will not follow it. The tide tables are the authority; this is arithmetic for the cockpit when you want a number quickly.
The mistake that grounds boats is forgetting that charted depth is measured from chart datum, so the tide height is added to it. Read a chart at 1.8 m and assume that is what is under you at low water and you will be fine; assume it at any other state and you are guessing in the wrong direction.
How to use
- Enter the low and high water heights from the tide tables.
- Slide to the hour you care about for the height and the curve.
- Add charted depth, your draft and a safety margin for clearance under the keel.
Frequently asked questions
What is the rule of twelfths?
A tide roughly follows a sine curve, and the rule slices it into six hourly parts: 1, 2, 3, 3, 2, 1 twelfths of the range. Slow at both turns, fastest in the middle — the third and fourth hours between them move half the entire range.
When does the rule not work?
It assumes a clean semi-diurnal tide on a six-hour cycle. Double high waters, strong river influence, shallow estuaries and storm surge all break it. The tide tables are the authority; this is cockpit arithmetic when you want a number quickly.
Do I add the tide to the charted depth or subtract it?
Add it. Charted depth is measured from chart datum, which is close to the lowest astronomical tide, so the tide height sits on top of it. Getting this backwards is how boats end up aground, because the error always runs in the dangerous direction.
How much clearance should I leave?
Half a metre is a common minimum in calm water, and more where there is swell — a one-metre swell moves your keel half a metre down as well as up. Add margin again for an uncertain survey, soft mud you would rather not test, or a tide prediction taken from a distant port.
Can I use it for a falling tide?
Yes. Enter the high water as the starting height and the low as the finish, and the same twelfths run in reverse. The rate is symmetric, which is why the middle hours are the ones that catch people out in both directions.
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