All you need to know about tides

If you live in the UK, then wherever you are based there are two tides a day. The timing of tides shifts forward by approx 53 minutes each day. Twice each month there will be an exceptionally big range between high and low tide called a spring tide (that´s spring as in boing or bounce, not the season) and there will also be an exceptionally small range between high and low tide called a neap tide.

The tide heights tend to follow a fairly smooth curve between springs and neaps, meaning that halfway through the cycle the tidal range will be at, or close to, its midpoint.

There, that was simple enough. I could have explained how the relative motion of the Moon, Sun and Earth causes the tides to be what they are hereabouts, but it would add nothing to your ability to work with tidal information. For many of us, the shape of the tidal curve is a simple bell shape:

The typical bell shape of the tidal curve

Unfortunately, there are things you need to understand that complicate this simple picture but they are all local phenomena. Some ports, for example, get a complicated tidal flow with double highs and even double lows. In our locality, Poole is an example of a complex curve shape:

The more complex tidal curve for Poole

There is both a myth and an explanation for complex curves. The myth is that they are caused by the proximity of islands such as the Isle of Wight, and that the tide comes in from two different directions as it flows around the island.

If such flows were to happen, they might happen somewhere in the solent, but not in Poole which is well to the west of the island. The real explanation is much more complex and has to do with standing waves in the channel and shallow water near the port.

The reason why you may see this variation may be interesting to some but does not help us to practically plan our passages. All we need is access to reliable information about future tidal movements in terms of heights and flows. We need to know the highs and lows will still leave us with enough water below our keel to allow us safe passage over shallow waters. Some knowledge about tidal flows can make the journey more comfortable by avoiding tidal races at their worst, and help predict fuel consumption.

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