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Tidal range is the vertical difference between high and low tide. Spring tides (full/new moon) produce maximum range; neap tides (quarter moon) produce minimum.
Range = MHW − MLW (mean high − mean low water)Tidal height follows sinusoidal harmonic components (M2, S2, K1, O1). Simplified: interpolate between known high and low times using rule of twelfths for planning.
Rule of twelfths: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 per hour of 6-hour tide phaseMaximum current occurs mid-tide. Peak speed correlates with range and channel geometry. Spring tides produce roughly 1.5× neap current in many locations.
Current max ∝ tidal range × channel constriction factorUpdated: July 2026
Low tide 2 m depth at datum; high tide adds 3 m = 5 m depth. At 7:1 scope, rode need varies 21 m (low) vs 35 m (high) — set anchor for low tide depth.
Bridge 12 m clearance at MHW. Vessel air draft 10 m with 1.5 m spring tide above MHW: 12 − 1.5 = 10.5 m — passes with 0.5 m margin. Check tide table for exact hour.
Channel max flood 2 knots at 2 hours after low tide. Depart 1 hour before for slack-to-flood transition, riding favorable current through narrow pass.
At low tide, insufficient depth grounds the boat or steepens rode angle until anchor breaks free. Always plan scope and depth at lowest astronomical tide.
Secondary ports may differ 30+ minutes and different range from reference station. Apply local corrections from tidal almanac or chart.
Tides affect anchoring depth, bridge clearance, and current strength. This calculator estimates tidal range from harmonic constituents or reference station data, and predicts high/low times for voyage planning.