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D135 — Tidal Currents — Flood, Ebb, Set, Drift, Rotary Currents, Slack Water, Navigation Effects & NOAA Current Predictions
Frozen parent: NAV.TIDES_CURRENTS · 11 selected source families · 21 source occurrences · 11 verified teaching facts
Separate vertical tide from horizontal current and treat prediction, real-time observation and vessel vector effects as different parts of the navigation problem.
✓ C1 Tide is the vertical rise and fall of water level; tidal current is the horizontal movement associated with tidal forcing. The two are related but are not the same phenomenon.
Flood and ebb current
✓ C2 A flood current generally flows landward or upstream; an ebb current generally flows seaward or downstream. Local geometry can make the actual direction more complex.
Current set and drift
✓ C3 Current set is the direction toward which the current flows. Current drift/speed is the rate of that flow, commonly stated in knots for navigation.
Cross-current changes course made good
✓ C4 A current nearly at right angles to the vessel’s heading has a strong cross-track effect on course made good; a current parallel to the heading mainly changes speed made good.
Stand of tide is not the same as slack water
✓ C5 A reversing current flows mainly back and forth along one axis. A rotary current changes direction continuously through the compass during its cycle.
Modern NOAA current-information workflow
✓ C6 In a semidiurnal East Coast setting, a rotary tidal-current cycle is roughly one semidiurnal tidal period, about 12.4 hours; exact station timing must come from predictions/observations.
✓ C7 Spring/perigean astronomical forcing tends to increase tidal-current speed; apogean/neap forcing tends to reduce it. Local hydrodynamics still control the actual current.
✓ C8 Stand of tide means the vertical water level is temporarily changing very little near high or low water. Slack water means tidal-current speed is near zero. They do not necessarily occur at the same time.
✓ C9 NOAA Current Predictions provides predicted maximum flood/ebb and slack times for official stations and can generate annual prediction tables digitally. Real-time current data are also available at equipped stations/PORTS locations.
✓ C10 The printed NOAA Tidal Current Tables books ended with the 2020 edition. For current operations use NOAA Current Predictions plus real-time/local pilot information where available.
✓ C11 Do not apply a fixed “best time” for a bar, inlet or strait without checking the actual predicted/observed current, sea state, wind, swell, local restrictions and pilot guidance.
Currentness gate: NOAA ceased annual printed Tide Tables and Tidal Current Tables after the 2020 edition. Current predictions are generated through NOAA Tide Predictions and NOAA Current Predictions; dated 2023 source-figure problems remain held unless their exact source illustration is verified.
MEMORY: DATUM FIRST → WATER LEVEL SECOND → CURRENT VECTOR THIRD → LOCAL PREDICTION / OBSERVATION CONTROLS THE OPERATION.
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