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OOW-1.1.15

Tidal heights for standard and secondary ports

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Standard Ports vs Secondary Ports

A standard port is one for which full tidal predictions — times and heights of every high and low water — are published directly in the Admiralty Tide Tables (ATT). A secondary port has no independent predictions; instead, its tides are derived by applying time differences and height differences (or height ratios) to the predictions of an assigned standard port.

The examiner will probe whether you understand why this distinction matters: at a secondary port you are always working from someone else's data, so errors compound. Recognise which port type you are dealing with before you begin any calculation.

Admiralty Tide Tables — The Three Volumes

ATT is published in three volumes covering different ocean regions. Volume 1 covers UK and Irish waters and is the volume most relevant to this certificate. Each volume contains the standard port predictions and the secondary port data tables.

Calculating Height at a Standard Port (Non-HW/LW Times)

When you need the height at a time between high and low water, use the tidal curve (or co-tidal curve) for that standard port, found in the ATT. The method:

  1. Extract the predicted HW and LW times and heights for the relevant day.
  2. Calculate the range (HW height minus LW height) and the mean level (HW + LW ÷ 2).
  3. Enter the curve with the time before or after HW.
  4. Read off the factor and apply: Height = Mean Level ± (Factor × Range / 2).

Alternatively, use the Rule of Twelfths as a rapid approximation when a curve is unavailable: the tide rises/falls 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of its range in successive hours. Know that this assumes a sinusoidal curve and a six-hour tidal cycle — it is an approximation, not a substitute for the curve where accuracy matters.

Applying Secondary Port Corrections

From the ATT secondary port tables, extract:

  • Time differences for HW and LW (applied to the standard port times).
  • Height differences for MHWS, MHWN, MLWN, MLWS.

Because height differences vary between springs and neaps, you must interpolate according to the actual range of the standard port on that day. Do not simply apply the springs difference to a neap tide — this is a common exam trap.

Once corrected times and heights are obtained, use the standard port's tidal curve to find intermediate heights — the secondary port does not have its own curve.

Critical Distinctions for the Oral

| | Standard Port | Secondary Port | |---|---|---|| | Predictions published? | Yes | No — derived | | Own tidal curve? | Yes | No — use standard port's curve | | Correction method | None needed | Time + height differences, interpolated |

Practice questions

recallcore

What is the difference between a standard port and a secondary port in the context of tidal predictions?

recallcore

When finding an intermediate tidal height at a secondary port, whose tidal curve do you use?

scenariocore

You need the depth of water at a secondary port at 1430 local time. The ATT gives you time differences and height differences for that port against its standard port. Walk me through the sequence of steps you would follow.

oralcore

The secondary port table shows different height differences for springs and neaps. The candidate has simply applied the springs height difference. What's wrong with that, and what should they have done?

scenariostretch

You have no tidal curves available and need a quick estimate of the height of tide two hours after high water at a standard port with a range of 4.2 metres and a chart datum LW height of 0.6 m. Using the Rule of Twelfths, what height would you calculate, and what caveat would you give?

Independent preparatory study aligned to the MCA OOW (Yachts <3000 GT) oral examination syllabus. Not an MCA-approved course and confers no credit toward a Certificate of Competency.