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The tidal curve

~12 minTidal heights — standard port

The tide tables give you high and low water — times and heights — and nothing in between. The curve is how you fill the gap. Most UK tides follow a familiar S-shaped rise and fall over roughly six hours each way, and almanacs print the measured curve for each standard port.

A rough rule before any drawing: the rule of twelfths. In the six hours from low to high water the tide moves 1, 2, 3, 3, 2, 1 twelfths of its range, hour by hour. Half the water arrives in the middle two hours — which is why a bar that is impassable at half tide can be comfortably open forty minutes later.

▸ Watch the twelfths happen — play the tide and see the bank cover and the dried-out boat float off.

The method

The almanac method runs the same way every time. First, pull the bracketing high and low water for your port from the tables. Second, work out the day’s range (HW minus LW) and compare it with the mean spring and neap ranges to see where today sits between the two curves. Third, find how many hours your time of interest is before or after high water. Fourth, read the curve at that offset: it gives a factor — the fraction of the range that has come in. Height of tide = LW height + factor × range.

That is the entire calculation. The worked example below runs it with real numbers — and they are computed by the same engine this app uses for passage planning, not copied from a book. Follow the link under it for a fresh set whenever you want another rep.

Live worked example

Worked example — height of tide at a time

Port Meridian (a standard port): HW 1135 UT 4.8 m, next LW 1820 UT 1.8 m. What is the height of tide at 1625 UT?

  1. 1Range. HW − LW = 4.8 − 1.8 = 3.0 m
  2. 2Springs or neaps?. Mean ranges here: springs 4.6 m, neaps 2.2 m. Today’s 3.0 m sits 33% of the way from neaps to springs — mid-range.
  3. 3Time from HW. 1625 is HW +4.8 h.
  4. 4Read the curve. At that offset the curve gives a factor of 0.09 — that fraction of the range has come in above LW.
  5. 5Height. LW + factor × range = 1.8 + 0.09 × 3.0 = 2.1 m above chart datum.
    6. HW−6 h+6 h1.00factor 0.09

Computed by the planner’s tide engine — these are not book numbers. New numbers ↻

▸ Now drive it yourself — drag the time cursor across the curve and watch the factor and height respond.

Check yourself

Roughly what fraction of the range moves in the middle two hours of a six-hour tide?

Tide tables for a standard port give times in…

Answers count towards your topic mastery on the exercises page.

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