How should I account for changing tides and tidal currents when sailing?

Plan for tidal height to keep enough water beneath the keel, and for tidal streams to calculate your departure time, course to steer and arrival time. Check predictions against conditions underway, because wind, atmospheric pressure and local geography can alter water levels and currents.

Check heights and streams separately

Start with the shallowest entrance or strongest tidal gate on your route. Either can dictate the whole passage. Tide is the vertical rise and fall of water; tidal stream is its horizontal movement, described by direction, or set, and speed, or drift.

• Use tide tables for the correct year, date and port, applying any secondary-port corrections.
• Use a tidal-stream atlas, chart tidal diamonds or local current predictions for stream direction, strength and slack water.
• Check whether times are UTC, local standard time or daylight-saving time. Add an hour only where the source requires it.
• Check which reference port controls the stream atlas. An arrow marked HW +3 means three hours after high water at that reference port.

Do not assume slack water coincides with local high or low water. In estuaries, narrows and around headlands, the difference can be substantial. Electronic tide gauges and current arrows may use separate datasets. Confirm that both are available and that the display shows your intended passage time.

Calculate clearance for arrival

For a charted depth below chart datum, predicted water depth is charted depth plus tidal height above the same datum. Subtract your draught to find under-keel clearance. A drying height must instead be subtracted from tidal height.

A channel charted at 1.2 metres with a predicted tide of 1.8 metres gives 3.0 metres of water. A yacht drawing 1.6 metres has 1.4 metres of predicted clearance. That clearance must also allow for waves, silting, prediction error and squat, the increase in draught that can occur when moving through shallow water.

Use the harbour's published minimum clearance where provided. Otherwise, choose a margin suited to the bottom, exposure and survey quality. A margin of 0.5 metres may suit sheltered, accurately surveyed water, but is inadequate in some exposed entrances. At an unfamiliar shallow harbour, arriving on a rising tide generally gives more recovery time if delayed.

Check whether your sounder reads depth below the transducer, keel or waterline before comparing it with your calculation. For bridges, use the chart's stated vertical-clearance datum rather than assuming it matches depth datum.