Start with the right tide table and the right time zone

Every harbour entry starts with a tide table you trust and a time zone you have checked twice. UK tables are usually printed in local time — GMT in winter, BST in summer. A one-hour error can put you at the bar at the wrong state of tide. Read the date, the port name and any correction notes before you look at a height.

Standard ports have daily predictions. Secondary ports use time and height differences from a standard port, and those differences are rarely uniform. Write the corrected times and heights into your plan, and keep a paper copy aboard. Batteries fail; paper does not.

Chart datum: the zero your depths are measured from

Charted depths are measured from chart datum, roughly the lowest astronomical tide. Actual depth = charted depth + predicted height of tide. If the chart shows 1.8 metres and the tide table predicts 2.4 metres, you have 4.2 metres over the seabed. Subtract your draught — a boat drawing 1.5 metres leaves 2.7 metres under the keel in smooth water.

Drying heights are also measured from chart datum. A bank with 0.8 metres drying is above water when the tide is below 0.8 metres. Negative tidal heights occur in some places, so zero is not always the floor.

High and low water are not the whole story

High water time is useful, but the shape of the tide between high and low keeps you safe. Use the tidal curve or the rule of twelfths. For a 4.8-metre range, the first hour after low brings about 0.4 metres, then 0.8, 1.2, 1.2, 0.8 and 0.4 across the six hours.

Springs can have nearly twice the range of neaps. Wind and pressure move water too: a deep low can raise the tide, while a persistent onshore wind can pile it up or strip it away. Treat the table as a prediction, not a promise.

The harbour bar: where tide, swell and seabed meet

Harbour bars combine shallow water, breaking swell and a narrow safe window. Even when the table says you have enough water, onshore swell reduces effective depth and can turn chop into breakers. The bar may also have shifted after winter storms.

• Check the predicted height at the bar, not just inside the harbour.
• Keep at least 0.5 metres under the keel, more in swell.
• Prefer mid-tide to high water, and cross near slack if the bar is exposed.
• Ask the harbourmaster or read local sailing directions for the latest report.
• If in doubt, stand off and wait. A few hours is cheaper than a broken rudder.

Squat is easy to forget. Reduce speed before the bar and have an escape route planned if the swell builds.

Plan the entry backwards from the bar

Start with the minimum height you need at the shallowest point. If the bar has 1.2 metres charted and you draw 1.6 metres, you want 0.6 metres under the keel, so you need at least 1.0 metre of predicted tide. Find when the curve reaches and leaves that height — those are your gateway times. Add half an hour either side for a slow passage or foul tide.

Work backwards to set departure. Allow for tidal stream, boat speed and any lock or bridge. Never let the destination dictate the timing of a bar crossing.

Keep the boat and instruments honest

Tidal planning only works if your depth information is reliable. Check the echo sounder offset at the start of the season with a lead line or known depth. A fouled transducer can read shallow or drop out. Keep paper charts and a printed tide table aboard, update electronic corrections, and inspect keel, rudder and anodes before a coastal passage.

Keep a simple log of actual bar conditions, times and depths. Over a season you build your own local tide table. Read the numbers, watch the water, and give yourself room to be wrong.