Understanding Tides for UK Boating: A Beginner's Guide to HW, LW and Springs

Tides are the one part of boating you cannot talk your way out of. You can guess at the weather, coax a reluctant engine, tidy a fender — but if there is not enough water under the keel at the moment you need it, your day is over. The knowledge required is straightforward and free: a tide table, a chart, and a little arithmetic. What follows covers high water, low water, tidal curves, springs and neaps, and the local quirks that catch people out around the UK coast.

What a tide table tells you, and what it assumes

Every tide table gives two things for a given day: the time of high water (HW) and low water (LW), and the height of the water at those times. Heights are quoted above chart datum, the level below which the sea rarely falls in that area. Add the tide height to the depth printed on the chart and you have the actual depth of water — in principle, at least.

Check three details before you rely on any table. First, the time zone. Most UK tables are published in local time, switching to BST in summer, but some forecasting tools stay on UTC. An hour's error is a serious error. Second, the reference port. Tables are calculated for a small number of standard ports, with secondary ports given as time and height differences. A note reading "HW Dover +0040, height −0.3 m" means exactly that, but you must interpolate the height difference between springs and neaps. Third, the datum: chart datum is not identical everywhere, and a harbour's own datum may differ again.

The tidal curve is not a straight line

If the tide rose and fell evenly, you could halve the range and know the depth at any moment. It does not. The curve of height against time is roughly a sine wave — steep in the middle, flat at the top and bottom. Around HW and LW the level barely shifts for a while, which is why high water in places such as the Solent can appear to stand for an hour or more.

The practical consequence is that the first and last hours of a rising tide give you far less depth than the middle hours. Assuming a straight line will flatter you into shallow water near low water, exactly where the mistake costs most.

The rule of twelfths: quick, close enough, easy to remember

This rule approximates the curve with a simple sequence. Take the total range — the difference between the height of LW and the height of HW — and divide it into twelve parts, then distribute them across the rise.

1. Hour one after LW: 1/12 of the range
2. Hour two: another 2/12 (3/12 in total, a quarter)
3. Hour three: another 3/12 (6/12, half the range)
4. Hour four: another 3/12 (9/12, three quarters)
5. Hour five: another 2/12 (11/12)
6. Hour six: the last 1/12 — high water

Read it backwards for a falling tide. Now try it with real numbers. Suppose low water is 1.0 m and high water is 4.6 m, so the range is 3.6 m. Three hours after low water the rule gives 6/12 of 3.6 m — 1.8 m — so the height of tide is 2.8 m. Two hours after low water it is only 3/12 of 3.6 m, or 0.9 m, giving 1.9 m. Add your draught, add a margin, compare with the charted depth. That is the whole method.

Two cautions. The rule assumes a roughly six-hour rise, which is close enough in most UK ports (the real figure is nearer six hours and twelve minutes) but not everywhere. And it is an approximation, so treat the answer as a guide rather than a promise. Where the stakes are high, use the printed tidal curve for the nearest standard port or a tidal height calculator, which will give a better figure.

Springs, neaps and the fortnightly cycle

Ranges are not constant. Roughly twice a month, around the new and full moon, the sun and moon line up and their pull combines. UK ranges are then at their greatest — spring tides. Around the first and third quarters the pull is partly cancelled and ranges shrink: neap tides. Springs usually arrive a day or so after the moon phase, so check the calendar rather than the sky.

What matters afloat is the size of the range, not the label. If springs give a 4.0 m range and neaps 2.0 m, then at the same state of tide, neaps can leave a metre less water over a shallow patch. Write the range into your plan and the decisions become much clearer.

Local effects no table will warn you about

• Wind. A sustained onshore gale pushes water up the coast and can hold the tide back; a strong offshore wind does the reverse. Expect tens of centimetres in exposed estuaries, sometimes more.
• Air pressure. As a rough rule, one hectopascal of pressure change is worth about a centimetre of sea level. A deep winter low can add half a metre or more; a settled anticyclone can take it away.
• Rain and river flow. After heavy rain an estuary may run high and fast, with debris afloat and less clearance under bridges.
• Double high water and stands. Some stretches of coast hold high water for a long time, the Solent being the classic example. The stream turns before the level drops noticeably.
• Tidal streams. Height and flow are different things. Headlands, narrows and islands can accelerate the stream well beyond a general tidal diamond's figure, with overfalls in the worst spots.
• Silting and shifting channels. Charted depths near harbour entrances and river mouths can be out of date. Local notices to mariners, harbour websites and a word with the harbour office are worth ten minutes of your time.

A simple routine before you cast off

You do not need a qualification in oceanography, just a habit. Once the routine is established, tide planning takes a few minutes and removes a whole category of worry.

1. Write down HW and LW times and heights for your reference port for the day.
2. Note the range, and whether it is a spring or neap tide.
3. Apply secondary port corrections for your destination and confirm the time zone.
4. Work out the height of tide at the times you will actually arrive and leave, using the rule of twelfths or a curve.
5. Subtract your draught, add a safety margin, and compare with the charted depth.
6. Check the forecast wind and pressure for anything that may add or remove water.
7. Note the tidal stream direction for the hours you will be underway and adjust the plan.

Allow a margin you would be comfortable explaining later. A metre of water under the keel is worth more than the ten minutes it cost you to wait.

Write the plan down and leave it where the crew can see it. If the engine refuses to start or the wind dies at the wrong moment, knowing when the tide will float you off again is the difference between a short wait and a long, expensive afternoon.

Photo: 999kasya999 / Pixabay