Tidal Coefficients
TIDAL COEFFICIENTS
We Brits (well some of us) use Range, the difference in metres between the heights of successive HWs and LWs, to measure the size of a tide. This in turn allows us to call a tide a big one or a small one, or to say that it is a Spring or Neap tide respectively – important information.
In order to arrive at this simple situation you have to look up the heights of HW and LW at the Port and on the day in question and this takes a little time. Such figures will be different whether you are in Oban or off Shoreham, so do not readily stick in the brain cell. There is a better way, much better, for which we have to thank the French (along with good wine, food and cheese). That is Coefficients ...
A Coefficient, whatever the application, is simply a non-dimensional way of expressing size. Non-dimensional means that it doesn’t matter whether we are measuring in feet, metres, lbs avoirdupois or in peanuts or bananas. So the people at SHOM, the French Hydrographic office in Brest, produce a table of tidal coefficients for the year, every day, morning and afternoon.
You, the hard-pressed Vasco, have to remember only 3 figures: 95, 70 and 45. Tack on 120 and 20 to cover the extremes and we can produce a small but meaningful table, comme ça:
120 = a very big Spring tide
95 = a Spring tide (vive eau)
70 = an average tide
45 = a Neap tide (morte eau)
20 = a very small Neap tide
These coefficients are valid whether you are in Leith, Dunkerque or Tilbury. If you doubt that, then think BIG: think Earth, Moon and Sun and little old Europe. Astronomical distances are so great that it is easy to accept that the whole of Europe is subject to the same lunar and solar influences at any one time.
In practical terms, just look up and record in your logbook the day’s coefficient, eg 43/42, just below Neaps – and plan accordingly. In France the times of access to a tidally limited port are often expressed in Coefficients, eg at Binic:
Coefficient 40-50, Access HW –1 to HW; 50-60, HW –1½ to +1¼ ... and so on.
In the Wiley Almanac we colour Spring dates in red, conventionally big and positive; and Neaps in blue, small and negative. Once you’ve got used to Coefficients (allow 5 minutes), you’ll never look back.
Neville Featherstone is the Editor of the Wiley Nautical Almanac.
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