How to check tides, and how to read them correctly

Tides decide more of a boating day than most people realise: whether there is water under the keel, which way the current runs, when the fish feed, and whether the boat you left on a beach is still floating. Checking them takes thirty seconds. Understanding what you are looking at takes slightly longer, and is the part that matters.

Where the data comes from

Tide predictions are computed from harmonic constants measured at a specific tide station. Everything you see — in an app, on a website, in a printed table — traces back to a national hydrographic authority: NOAA in the United States, the UKHO in Britain, and equivalent bodies elsewhere.

This matters for one practical reason: a prediction is only valid for its station. If the nearest station is twenty miles away in a different bay, the times at your location can be off by a substantial margin. Good sources tell you which station they are using; poor ones just show a graph.

How to check, in practice

• On an iPhone or Android, any decent tide or marine app will give times and heights for a chosen station, offline once downloaded. Offline matters: tides are what you check when there is no signal.
• On the web, go to the national authority directly — it is the source everything else copies.
• On a chartplotter, tide stations are usually built into the chart data and can be tapped for the day's curve.
• On paper, a local tide table costs almost nothing, works with a flat battery, and lives in the chart table.

Reading the numbers correctly

Heights are given above chart datum, the same reference used for soundings on your chart. So real depth equals charted depth plus the tide height at that moment. A chart showing 1.2 metres with a tide of 2.0 metres means 3.2 metres of water.

Negative tides exist. On a strong spring low, the tide height can be below datum, and then there is less water than the chart shows. This is when boats find bottoms that were never a problem before.

Spring and neap tides come from the moon: springs around full and new moon, with the largest range and the strongest currents; neaps around the quarters, with the smallest. Spring tides mean more water at high tide, less at low, and faster streams in between.

The rule of twelfths

Between low and high water, the tide does not rise evenly. Divide the range into twelfths and the rise is roughly 1, 2, 3, 3, 2, 1 twelfths per hour of the six-hour cycle. Half the total rise happens in the middle two hours.

The practical use: if you need another metre of water to cross a bar, this tells you how long to wait — and it tells you that the water goes away just as quickly on the ebb.

Tidal height is not tidal current

They are related but not the same, and the difference catches people out. Slack water — when the current stops — does not necessarily coincide with high or low tide, and in some channels the offset is more than an hour. If your plan depends on the current rather than the depth, you need a tidal stream source, not a tide table.

Finally, remember that predictions are exactly that. Strong wind and unusual atmospheric pressure can raise or lower actual water levels well away from the prediction: a sustained onshore gale can add a surprising amount of water, and a hard offshore wind can take it away.

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