Seven Common Mistakes When Reading Tide Times (and How to Fix Them)

Tide tables look like the most straightforward document in the world: a harbour name, a column of times, a column of heights. Then you stand at a slipway at 06:00 with a boat on a trailer and discover the water is a metre lower than you allowed for.

The predictions are usually right. The misreading almost always comes from the assumptions we bring to them — which clock the times are on, what the heights measure, and whether the numbers actually describe the water in front of us. These are the seven mistakes that come up again and again, with the practical fix for each.

1. Reading the times on your own clock

Most UK tide tables are printed in local time, which means GMT in winter and BST in summer. Others stay on UTC or GMT all year and say so in small print at the top of the page. Both are perfectly normal, and mixing them up shifts every time by an hour — often the difference between crossing a bar with water beneath you and arriving there at a state of tide you didn’t plan for.

If you are comparing a printed table with a chartplotter, a phone app or a foreign source, the clocks may not match at all. Pick one standard — UTC is the safest — and convert everything into it before you compare. Then write your own working list in 24-hour time. A good share of misreadings are really 12-hour clock problems.

2. Thinking the printed height is a depth

Those heights are measured above chart datum: a low reference level set so the sea rarely drops below it. In UK waters it sits close to lowest astronomical tide. On its own it tells you nothing about the depth of water, because the soundings printed on the chart are measured down from the same datum. The shallow patch marked 1.2 m, with a tide height of 3.4 m, has about 4.6 m of water over it.

Different datums cause a second, quieter mistake. Ordnance Datum (Newlyn), used for land levels and flood maps, sits around mean sea level and is a different reference entirely — in many UK ports the two differ by a metre or two. Heights quoted against one are not tide heights quoted against the other, and swapping them without noticing is a common error when checking a causeway or a harbour wall.

3. Assuming one table covers the whole coast

Standard port tables apply to one gauge in one harbour. Secondary port corrections give a time difference and a height difference for other places, and they are approximations rather than measurements. Up an estuary, around a headland or across a shallow bay the tide can be an hour or more out of step with the standard port, and higher or lower with it. Some places behave oddly: Southampton Water and Poole Harbour both have double high waters, where the tide barely falls between two peaks.

The fix is unglamorous. Use the nearest standard port, then find out how the water in front of you differs. Ask at the harbour office, a sailing club or a boatyard. People who work the same stretch every day know more about it than any table.

4. Ignoring springs and neaps

Range changes through the fortnight. Spring tides arrive a day or two after a new or full moon, when high water can stand a metre or more above a neap high water; neaps fall near the quarter moons. The word has nothing to do with the season.

The consequence is that “high water” is not one fixed height. A passage that clears a sill comfortably on springs may not clear it at the same clock time a week later on neaps, in identical weather. If your table has a springs-and-neaps column, use it. If not, note the moon phase and expect a range somewhere between the two extremes.

5. Misreading the format of the table

Tables vary in ways that catch people out, particularly around midnight and in how times are rounded:

• Is the clock 12-hour or 24-hour? 12:20 am is 00:20, not lunchtime.
• Are times rounded to the nearest minute, five minutes or ten?
• Where does the day begin? A tide at 00:15 belongs to the new date, but it is often printed at the foot of the previous day’s column.

Read the table as a sequence rather than a grid. Work out what happens across the 24 hours you actually care about, then count backwards from the moment that matters.

6. Treating a prediction as a promise

Predictions come from the astronomical tide and are usually good to a few centimetres in calm weather. They contain no weather. Sustained low pressure and onshore wind push water up; high pressure and offshore wind hold it back. A rough working rule is about 1 cm of sea level for each millibar away from roughly 1013 mb, so a deep winter low can add half a metre or more, and storm surges add more still.

Timing moves too, especially in shallow water and estuaries. Allow 10–20 cm either side as normal, more in unsettled conditions, and never plan a marginal passage on the assumption that the table is exact.

7. Forgetting the rate of change, and what your boat needs

High and low water are instants, not states. The water moves fastest around mid-tide and nearly stops at the turns. The rule of twelfths is a useful approximation: in the first hour after low water about a twelfth of the range has come in, then two twelfths, then three, three, two and one.

That has a practical bite. If you need half a metre over a bar on a 4 m range, and there is 0.4 m at low water, waiting an hour gains you roughly 0.3 m — not enough. Wait longer, or go elsewhere. Remember too that the height is measured at the gauge, not over your obstacle, so add your draft, squat and under-keel clearance before deciding.

A routine worth keeping

Before you rely on any table, run through six checks:

1. Which clock are the times on?
2. Which datum are the heights on?
3. Does the table cover this harbour, or is it a secondary port correction?
4. Springs or neaps?
5. What is the tide doing at the time you need, rather than at high water?
6. What margin do the weather and your own draft demand?

Answer those and the table stops being a trap. For anywhere unfamiliar — a bar, a lock, a drying mooring — check with the harbour authority and take local advice before you set out.

Photo: Martijn Stoof / Pexels