Tides Explained for Boaters

Tides are the slow, rhythmic rise and fall of the sea, and for anyone navigating in coastal waters they are impossible to ignore. Tides decide whether there is enough water to float your boat, whether a harbour entrance is passable, and how strongly the sea pushes you along or holds you back. A skipper who understands tides can turn them into an ally; one who does not can run aground or fight a current all day. This makes tides a favourite topic in boating exams.

The subject can seem daunting because it involves both the height of the water and the horizontal flow of the tidal stream. This guide separates those two ideas and explains the practical points you actually need on the water.

What Causes Tides

Tides are driven mainly by the gravitational pull of the Moon, with a smaller contribution from the Sun. As the Earth rotates, the pull of the Moon raises the sea into a bulge, and most European coasts experience two high tides and two low tides in a little over twenty-four hours. This is known as a semi-diurnal pattern.

Because the Moon orbits the Earth, the timing of the tides shifts slightly later each day, typically by around fifty minutes. This is why yesterday’s tide times cannot simply be reused today. Accurate tidal predictions are published in tide tables and almanacs for specific ports, and these are the trusted source every navigator relies on.

Springs and Neaps

Twice a month the Sun and Moon line up, at new moon and full moon, and their combined pull produces the largest tidal range of the cycle. These are spring tides: the high water is higher and the low water is lower than average, and the tidal streams run faster. Despite the name, spring tides have nothing to do with the season.

About a week later, when the Sun and Moon pull at right angles, their effects partly cancel and the tidal range is at its smallest. These are neap tides, with a gentler rise and fall and weaker streams. Knowing whether you are near springs or neaps tells you how much water to expect and how strong the currents will be, which affects everything from anchoring to timing a harbour entrance.

Tidal Heights and Chart Datum

The height of tide is how far the water has risen above chart datum, the low reference level from which charted depths are measured. To find the actual depth of water at any moment, you add the height of tide to the charted sounding. This calculation, sometimes shown as a rule of twelfths for estimating the rise between low and high water, keeps you off the bottom.

The rule of twelfths is a handy approximation. It assumes that between low and high water the tide rises by one twelfth of its range in the first hour, two twelfths in the second, three in the third, three in the fourth, two in the fifth and one in the last. This lets you estimate the height at any point in the cycle without a detailed graph, though official curves are more precise. You can practise these height calculations in the boatingexamine course.

Tidal Streams and Their Direction

While the height of tide rises and falls, the water also flows horizontally, and this flow is the tidal stream. The stream floods as the tide comes in and ebbs as it goes out, but the timing of the stream does not always match the timing of high and low water; in many places the stream keeps running for a while after the water level has turned.

Tidal stream information is published as charts or tables giving the direction and rate of flow for each hour relative to high water at a reference port. A stream of two or three knots is significant for a small boat and must be built into your course calculations. Used cleverly, a fair tide can add speed for free, while a foul tide fought needlessly wastes time and fuel.

Planning Around the Tide

Good passage planning starts with the tide. Look up the times and heights of high and low water for your area, check whether you are at springs or neaps, and consult the tidal stream information for your route. Then plan your departure so that shallow sections are crossed with enough water and, where possible, so that the stream flows with you.

Timing a harbour entrance for around high water gives the greatest depth and the smallest error if your calculations are slightly off. Where a strong stream runs through a narrow channel, aiming to pass near slack water, when the flow is weakest, makes for a calmer and safer transit. Tides reward those who plan ahead.

FAQ

How do I find the actual depth of water at a given time? Add the height of tide at that time to the charted depth, or sounding, shown on the chart. The charted depth is measured from chart datum, a low reference level, so the tide almost always gives you extra water above it.

What is the difference between spring and neap tides? Spring tides occur around the new and full moon and give the largest range, with higher highs, lower lows and stronger streams. Neap tides occur around the half moon and give the smallest range with weaker streams. The cycle repeats roughly every two weeks.

Does high water mean the tidal stream has stopped? Not necessarily. The horizontal flow of the tidal stream often continues running for some time after high or low water, so slack water, when the stream is weakest, may not coincide exactly with high or low tide. Always check the tidal stream data, not just the height times.