Tidal calculations — Rule of Twelfths and secondary ports

Modified on Wed, 20 May at 10:02 PM

KnotWise includes two tidal calculators: the Rule of Twelfths estimates tidal height at any time between low and high water, and the Secondary Port calculator derives tidal predictions for secondary harbours from standard port data. Both run fully offline and are available on every plan, including Free.

Important: Tidal results in KnotWise are harmonic approximations that depend on the tide table data you enter. Always verify against official tide tables — Reeds, Admiralty Tide Tables, BSH or the national hydrographic source — before making pilotage decisions, especially in tidal harbours.

How the tidal calculators work

KnotWise does not include a built-in tidal database. Every input — high water and low water times, heights, time and height differences for secondary ports — comes from your own tide tables. The calculator handles the interpolation, not the prediction.

This is deliberate. Tidal data is licensed, regional and updated annually. By keeping the inputs manual, the calculator works with whichever source you trust most — Reeds for European waters, BSH for the German Bight, NOAA for US coasts, the Admiralty for the rest.

Rule of Twelfths

The classic approximation for tidal height between low and high water. The tide is assumed to rise by 1/12, 2/12, 3/12, 3/12, 2/12 and 1/12 of the range in each successive sixth of the cycle.

Input	Source
Low water time and height	Tide table
High water time and height	Tide table
Target time	Time you want a height for

The calculator returns the tidal height at the target time, plus a graphical tidal curve showing the full cycle and your target moment.

The Rule of Twelfths assumes a sinusoidal tidal curve. It is reliable for typical semi-diurnal tides — most of the Atlantic and Mediterranean coasts. It is unreliable in waters with significant shallow-water distortion (Solent, Bristol Channel, Cook Inlet) or non-standard tidal patterns. For these, use the official tidal curve diagrams from your almanac.

Secondary Port Tides

Predicts high water and low water at a secondary port by applying standard time and height differences to the standard port for the same day.

Input	Source
Standard port HW time and height	Tide table for the day
Standard port LW time and height	Tide table for the day
HW and LW time differences	Secondary port table (Reeds, Admiralty)
Height differences (MHWS, MHWN, MLWN, MLWS)	Secondary port table

The calculator interpolates between mean spring and mean neap height differences based on the standard port range for the day, then applies the time differences to derive secondary port HW and LW. The output includes a graphical curve for the secondary port.

For most cruising purposes — choosing a departure window, planning a marina entry, anticipating bridge clearances — this is sufficient. For marginal pilotage in tidal harbours where a few decimetres matter, refer to the official curve diagrams.

Shared features across the tidal cluster

Both calculators offer a height unit toggle (metres or feet) and graphical tidal curves. The source note in each calculator names the recommended tide tables. Inputs do not persist between sessions — to preserve a calculation, use Export PDF or Save to Logbook.

Accuracy and limits

Rule of Twelfths: Typical error around ±10 percent of the range in mid-cycle. Larger errors near LW and HW slack, and in shallow-water-distorted tides.
Secondary Port: The interpolation between MHWS / MHWN / MLWN / MLWS is linear. Sufficient for planning, less accurate for marginal pilotage where official harmonic prediction methods are preferred.
Springs and neaps: The calculator interpolates automatically from the standard port range. No spring/neap factor needs to be entered separately.
Atmospheric effects: Tide table predictions assume average atmospheric pressure. A persistent high pressure depresses sea level by approximately 1 cm per mbar above 1013 mbar; a deep low does the reverse. Storm surge can override both predictions and approximations entirely.

Frequently asked questions

Why are tide tables not built in?

Tidal data is licensed and varies by region. Rather than locking the calculator to a single licensed source, KnotWise works with whichever tide table you already trust. This also keeps the app fully offline and free of regional restrictions.

What about non-standard tide patterns?

For harbours with significant tidal distortion — the Solent, the Bristol Channel, parts of the Dutch Wadden Sea — the Rule of Twelfths is not reliable. Use the official tidal curve diagrams from Reeds or the Admiralty for those locations.

Can I save a tidal calculation?

Yes. Export PDF generates a formatted document including the curve. Save to Logbook creates a log entry pre-filled with the calculation summary.

Which tide tables are recommended?

For European waters: Reeds Nautical Almanac or Admiralty Tide Tables. For the German Bight and Baltic: BSH Gezeitentafeln. For US coasts: NOAA. For other regions: the national hydrographic office or equivalent.

Does the calculator account for storm surge or atmospheric pressure?

No. Both calculators work from the predicted astronomical tide. Storm surge, persistent pressure anomalies and strong onshore or offshore winds are not modelled — you must allow for them manually when planning critical clearances.