Tides, currents and weather: build the passage clock correctly

Chapter 6 / 12 · 5 min reading + practice

Separate water level from horizontal flow, check datums and time zones, calculate arrival and clearance transparently, and recognise meteorological residuals.

Tidal planning is both a depth problem and a timing problem. Enough predicted water beneath the keel does not mean a manageable stream; a favourable stream does not prove enough depth. Weather adds another layer. Prepare water-level and current information separately, join them to the yacht’s route timeline, then assess wind, waves and local restrictions. This process supports the skipper’s judgement; it is not an automatic permission to enter a harbour or cross a bar.

Vertical height and horizontal flow are separate products

A tide is a changing water level. A current is horizontal movement of water and can be tidal, wind-driven or part of larger ocean circulation. High or low water does not generally identify slack current. NOAA explicitly publishes tide and tidal-current predictions separately because the relationship is location-specific. Use the relevant current station, tidal-stream atlas or chart diamond—not the nearest town’s tide widget—to obtain direction, speed and slack timing for a restricted passage.

Springs and neaps concern the astronomical cycle, not the seasons. Spring tides generally have greater range; local current predictions determine the associated stream, rather than a universal multiplier. Diurnal, semidiurnal and mixed regimes need different treatment. The rule of twelfths is only a rough approximation for a suitably regular, approximately six-hour rise or fall. It does not reproduce a local tide curve with double high water, strong inequality or meteorological disturbance.

First check the reference system

Depths, predicted heights and observed levels must share a compatible vertical datum before addition. Chart datum is not necessarily mean sea level. NOAA coastal predictions commonly use MLLW; other chart systems may use a different reference. Read the chart and station metadata, units and any documented conversion. Do not combine an airport elevation datum, a tide gauge’s arbitrary station datum and charted depths because each happens to be labelled metres.

Check the time basis just as carefully. NOAA distinguishes GMT, local standard time and local time adjusted for daylight saving. A table’s printed time may therefore differ from the phone’s clock. Keep the passage calculation in explicitly labelled UTC, retain the original product time, and record the local conversion for the date. A tidal atlas tied to high water at a named reference port uses that port’s specified event, not high water at whichever harbour seems geographically closest.

Predicted water is not necessarily observed water

Astronomical predictions do not incorporate every short-term weather effect. Wind setup or setdown, pressure, runoff and storm surge can alter actual levels. Compare a relevant gauge’s observed level with its prediction at the same time and datum, and consult official surge or coastal forecasts where available. This difference is a residual, not a correction guaranteed to remain constant at your destination. A pressure-only shortcut must not replace a local assessment.

Bathymetry constrains currents as well as waves. Constrictions, headlands, banks and outfalls can produce complex flow and rough water. Wind against a stream deserves a separate wave assessment. A northerly wind comes from north and blows generally south; a current setting north moves toward north, so they oppose. Verify conventions before comparing two arrows. A coarse offshore current field is not a substitute for an entrance atlas, a pilot book or harbour instructions.

Worked depth and timing exercise

The following port, times and numbers are invented. A shallow entrance has a charted depth of 1.8 m. A compatible prediction gives 1.2 m above chart datum at the planned arrival. Yacht draught is 2.1 m. The simple static calculation is 1.8 + 1.2 − 2.1 = 0.9 m. That is not an approved under-keel clearance: chart uncertainty, siltation, yacht motion, heel, squat where relevant, wave troughs and the operator’s required margin remain unresolved.

A nearby relevant gauge is currently 0.4 m below its astronomical prediction. If that deficit persisted locally, the same arithmetic would give 0.5 m before additional allowances. Record this as an adverse scenario, not an exact transferred correction. Obtain appropriate local evidence and restrictions. At a bridge, rising water reduces air clearance even as it improves nominal depth: one condition can help one constraint and worsen another.

The approach leg is 18 nautical miles. With a simplified steady 5-knot speed through water and a directly opposing 2-knot stream, ground speed would be 3 knots and the leg would take 6 hours, not the 3.6 hours computed without current. Actual streams change through time, so calculate short legs using local predictions rather than applying this example to an entire tidal cycle. Recompute arrival, depth, daylight, weather validity and remaining options together.

A vector exercise, not a ready-to-steer instruction

For a separate simplified case, the desired ground track is 090° true, speed through water is 5 knots and a steady cross-current sets 000° at 2 knots. Ignoring leeway, the boat must provide a 2-knot southward component. Elementary vector geometry gives a through-water direction about 113.6° true and eastward ground speed about 4.58 knots. This calculation illustrates why ground track, heading and course through water differ. Real steering must also consider leeway, changing flow, instruments, traffic and charted dangers; do not copy the answer as a navigational instruction.

Departure and arrival checklist

Select the correct water-level and current products and geographical coverage.
Check station, reference port, year, datum, units, time basis and daylight-saving treatment.
Calculate each leg’s arrival with plausible speed loss and changing stream.
Assess depth and air clearance independently with vessel-specific margins and chart uncertainty.
Check weather residuals, wind-against-stream exposure, sea state and entrance rules.
Identify the last practical decision point and a waiting or alternative destination option.
Recheck close to arrival; log the information actually used.

What good post-passage evidence looks like

Keep predicted and observed values in separate columns, including time, location, datum and sensor quality. Record why arrival changed and whether the current prediction covered the actual channel. An unexplained GPS-speed difference is not automatically a faulty current model: steering, leeway, engine output and sea state also matter. Preserve these caveats when giving feedback or reviewing an automated briefing.

Quick chapter check

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1. A tide table gives high water at 12:00 for a nearby port. What does that establish about slack current in your narrow channel?

Nothing definite about channel slack; obtain the applicable local current prediction or tidal-stream reference. Slack current must occur at exactly 12:00 in the channel. Current is weakest exactly three hours before high water at every location.

2. An exercise gives charted depth 1.8 m, predicted height 1.2 m in the same datum and draught 2.1 m. What does the result 0.9 m represent?

An approved operational under-keel clearance with all motion and chart uncertainty already included. A simple static arithmetic margin before allowances, actual-level differences and local restrictions are assessed. The maximum depth through the whole tidal cycle.

3. For a deliberately simplified 18-nautical-mile leg, speed through water is 5 knots and a steady current directly opposes it at 2 knots throughout. What is the moving time?

About 2.6 hours, using 7 knots over ground. 3.6 hours, because current does not change arrival time. 6 hours, using 3 knots over ground; real changing currents require a more detailed leg calculation.

Check your understanding

First explain your answer in your own words, then open the explanation. These are original Sailing Weather exercises, not official exam questions.

1. A tide table shows high water at noon. Is the channel current necessarily slack then?

No. Obtain the local current prediction or atlas. Water-level timing cannot generally substitute for stream timing.

2. Why is the exercise's 0.9 m arithmetic result not an approved clearance?

Motion, chart uncertainty, weather residuals and the yacht's required margin remain. The numbers are hypothetical, not local advice.

3. Do a wind FROM 000° and a current TO 000° move in the same direction?

No: that wind blows south while that current sets north. Compare movement only after decoding conventions.

4. What must be rechecked when a current adds two hours to arrival?

The local tide/current phase, depth and air clearance, daylight, weather validity and available alternatives—not just ETA.

Sources and verification scope

NOAA Ocean Service: tides versus currentsVertical tide versus horizontal current; tidal, wind and density-driven contributors.
NOAA CO-OPS: tide/current FAQSlack and high/low water are location-specific; tide predictions do not predict all weather residuals.
UKHO ADMIRALTY EasyTide: explanatory FAQTidal regimes, reference-port stream information and weather effects on observed levels.
NOAA tide-prediction user guideStation selection, datum, units and distinctions among GMT, standard and daylight-adjusted local time.
NOAA: navigating tidal datumsMLLW and other vertical datums are not interchangeable without an established relationship.