Reading Tide Tables for Safer Coastal Sailing

Why the tide governs everything afloat

On a calm day with a light breeze, the tide is still the biggest thing moving you around. It lifts you over the bar at the harbour mouth, it drains the approach channel four hours later, and it sets the direction of every tidal stream you will meet on passage. Skippers who read tide tables properly make quiet, unhurried decisions. Skippers who don't end up waiting for the flood with the kettle on while the mud dries around them.

The good news is that tide tables are not complicated. They are simply a forecast, written in numbers, of when water will be where. Once you can read them alongside a chart and your own depth sounder, planning a passage becomes a calm, confident exercise rather than a hopeful gamble.

Springs and neaps: the fortnightly rhythm

Tides are driven by the pull of the moon and the sun. When both are roughly in line — at new moon and full moon — their combined pull gives us spring tides: higher highs and lower lows, with the greatest range. Around the first and third quarters, the sun and moon pull at right angles and we get neap tides, with a much smaller range. In practice, springs arrive roughly two days after full or new moon, and neaps about two days after the quarters.

The difference is not trivial. A spring range in the Bristol Channel can top twelve metres, while the following neap may give you barely half of that. Somewhere gentler, like Portsmouth, you might see four metres on springs and two and a half on neaps. That extra 1.5 metres is often exactly the difference between crossing a shallow patch and touching bottom, so the first thing to note on any planning sheet is whether your day is spring or neap.

Chart datum and what soundings really mean

Depths printed on a UK chart are measured from chart datum — in British waters, roughly the level of lowest astronomical tide. This is deliberately pessimistic: it means that in normal conditions, the actual depth is always greater than the number on the chart.

The working sum is simple:

• Charted depth plus height of tide equals the depth of water you can expect.
• A drying height (shown underlined on charts) sits above chart datum and is covered as the tide rises. Two metres of tide over a drying height of one metre leaves one metre of water.
• Figures for bridge clearances work the same way in reverse: the clearance increases as the tide falls.

Always keep a margin. Half a metre under the keel in a flat calm is a comfortable passage; the same half metre with a swell rolling into the channel is a very different day.

Reading a tide table line by line

A tide table gives you high and low water times and heights for a standard port. Most of the coast is then covered by secondary port differences, which you apply to that standard port. The traps are worth knowing well.

• Time differences can be added or subtracted — check the sign, and check whether the difference is applied to high water only, low water only, or both.
• Height differences are not fixed offsets. They change with the size of the tide, so you usually interpolate between a springs figure and a neaps figure.
• Between high and low water, heights follow a curve, not a straight line. The rule of twelfths gives a decent approximation: after low water, 1/12 of the range enters in the first hour, 2/12 in the second, 3/12 in the third, then 3/12, 2/12 and 1/12 as you approach high water.
• Slack water is not high water. In many channels the stream keeps running hard for an hour or two after the height has turned.

Local anomalies the tables won't tell you

This is where local knowledge earns its keep. A few well-known quirks:

• Double high water around Southampton and Portsmouth, where the tide effectively stands for an hour or more — wonderful for a long lunch afloat, confusing if you assume it's already falling.
• Poole Harbour, where the entrance is shallow and the tide arrives later than the open coast.
• The Menai Strait, where the tide can differ by well over an hour across a couple of miles, and the Swellies demand respect.
• Morecambe Bay, where the flood sweeps across the sands faster than you can walk.
• Estuaries in spate, where river water holds levels up long after the tide has turned.

Weather matters too. A deep low pressure system can raise sea level by roughly a centimetre for every hectopascal it drops, and an onshore gale can push that water further up a funnel-shaped estuary. A strong anticyclone does the opposite, keeping the tide lower than predicted. On a big spring tide with a low and a south-westerly, expect more water than the table promises.

Planning a passage with confidence

Start with your constraints. Work out the minimum depth you need at the shallowest point of the route, add your safety margin, and then ask which hour of the tide satisfies that. Choose your departure so you arrive at a harbour entrance near high water, and let the tidal stream help rather than hinder you — a fair tide is worth a knot or two of free boat speed.

Write the plan down before you leave: standard port, secondary port differences, hourly heights, and any local notes. Keep a small notebook of what you actually observe — the time the mud dries at your mooring, the hour the ebb starts running past the headland. Within a season you will have your own tide table, and it will be better than any book.