How to read tide tables before a harbour swim

Why the tide table comes before your swim kit

A harbour that looks like a millpond at nine in the morning can be a churning, fast-moving channel by noon. The single most useful thing you can do before a dip in a harbour, estuary or bay is spend five minutes with the tide table for that exact spot. Those tables are published free, usually a year ahead, and they tell you when the water arrives, when it leaves, and how much of it there will be. Read alongside a weather forecast and a chart, they turn a hopeful paddle into a planned swim.

Reading the columns without a glossary

Most UK tide tables list four things for each day: the time and height of high water, and the time and height of low water. There may be one or two high waters if the area has a double tide, as parts of the Solent and the Bristol Channel do. Heights are given in metres above chart datum, which is roughly the lowest tide you will ever see, so a low water of 0.4 metres means very little water indeed over the mud.

Three details catch people out:

• The time zone. Many tables are printed in GMT. Between late March and late October you need to add an hour for British Summer Time. Turning up an hour late for slack water is a common and occasionally serious error.
• The port. Tables often give a standard port, with corrections in minutes and metres for nearby harbours. Use the corrections, or find a table for your harbour itself.
• Drying heights. A chart will show how much a sandbank, slipway or rock dries at low water. A beach that is knee-deep at high water can be a kilometre of mud at low.

Spring and neap ranges: what the numbers really mean

Subtract low water height from high water height and you have the range. Springs, one to two days after a new or full moon, give the biggest range; neaps, around the quarter moons, give the smallest. A harbour might see six metres on a spring tide and two and a half on a neap. That difference matters more than the times do, because a bigger range means far more water shifting through the same six hours, and therefore stronger currents.

If you want to know how fast the depth is changing, use the rule of twelfths: in the first hour after low water the depth rises by one twelfth of the range, then two, then three, then three, two and one in the final hour. The middle two hours of a spring tide are when the water moves fastest.

Local currents, harbour mouths and what tables don't say

A tide table gives height, not speed or direction. In a narrow harbour entrance, the whole ebb can funnel through a gap only a few dozen metres wide, and that is where swimmers get pushed off course or pinned against a wall. Slack water, which usually falls close to high and low water, is your friend. Aim to swim in the hour either side of it.

Watch for these local quirks:

• Wind against tide. An onshore breeze running against the ebb builds short, steep, confused chop that is far more tiring than the forecast suggests.
• River outflow. After heavy rain, estuaries run fresh and fast on the ebb, carrying debris and cold surface water.
• Headlands and harbour bars. Water accelerating round a point can create eddies and, in places, rip currents. Bars shift after winter storms, so ask locally.
• Shelter. A wall that blocks the wind at high water may leave you exposed once the tide drops and you are standing on open sand.

Turning the numbers into a swim plan

Once you have your times, build the swim around them.

• Pick a window around slack water, ideally on a neap tide for a first visit to a new spot.
• Know which way the tide runs and where it will take you if you stop swimming for two minutes.
• Check that your entry and exit points are usable at the state of tide you will finish in, not just the one you start in.
• Allow a buffer of at least half an hour, because the tide rarely turns exactly when predicted.
• Tell someone ashore where you are going and when you expect to be back, and carry a tow float and a phone in a waterproof pouch.
• Check the harbour office noticeboard or local group for closures, works or events.

Weather can overrule the arithmetic

Tide tables are predictions, not promises. Sustained high pressure can knock twenty or thirty centimetres off a predicted high water; low pressure and a strong onshore gale can add considerably more. Water temperature also lags the season, so a bright June day may still mean twelve degrees. Read the table, check the forecast, then stand at the water's edge and look. If the current is running harder than you expected or the chop is building, save the swim for another tide. There is always another one along in about six hours.