If you cruise in France, you’re bound to have noticed that pilot books, harbour notices, etc, talk about ‘tidal coefficients’. This is a simple system whereby an operator can tell at a glance what size of tide to expect and, hence, what sort of time a lock can be accessed. For example, the sill at Douarnenez opens at high water plus or minus 2 hours when the coefficient is greater than 70. This means that with an average sort of tide, that’s when you’ll get in. It follows that if the tide is ‘neapier’ than 70, the flap will open later, and so on. The method is also used for interpolating stream rates, but for those operating under different arrangements, such as Brits, it’s mainly about heights. The crucial coefficients to remember are these:
20 – ‘mini-neap’
45 – mean neap
70 – average tide
95 – mean spring
120 – monster spring
