Avoid 1m Tide Errors: Secondary Port Worksheet for Skippers

Skipper calculating secondary port tide heights

Produce a secondary-port prediction by starting with a named standard-port HW/LW time and height, applying the published signed time corrections, then scaling the standard-port height deviations by a range ratio and adding the secondary port’s own mean sea level and chart-datum correction. Interpolate any time in between using the supplied tidal curve. Skip the range ratio and you get a time that’s close but a height that’s wrong, sometimes by more than a meter.

TL;DR:

Using the range ratio is essential because heights are scaled according to local tidal swing differences, which can be less than one, sometimes by more than half.

Confirm whether your location is semi-diurnal or diurnal to select the correct worksheet path; applying the wrong one can produce inaccurate turning points.

Digital predictions with proper metadata avoid double corrections and provide more reliable data, but manual checks for datum and time basis are still critical.

Interpolating heights between HW and LW relies on approximate tidal curves, which can be inaccurate in narrow or funnel-shaped waterways, so conservative margins are advisable.

Estimating secondary port tides involves starting with standard port predictions, applying time shifts, scale deviations with the range ratio, and then adjusting for local datums and corrections.

1. Step-by-step worksheet method
2. The height formula and why the range ratio matters
3. Interpolating heights between HW and LW
4. Worked example: standard port to secondary port
5. Checks, common mistakes, and verification before navigation
6. Why validated tide inputs and metadata matter
A trade-off worth keeping in mind
Getting validated tide inputs into your planning
FAQ
Sources

1. Step-by-step worksheet method

Every secondary-port calculation starts with the same four boxes filled in before you touch a pencil: the standard-port HW/LW times and heights for the day in question, the secondary port’s mean time difference, its mean sea level (MSL), its spring and neap range figures, and the chart-datum correction. The Australian Hydrographic Office worksheet lays this sequence out explicitly, and it’s worth printing rather than improvising from memory.

The sequence runs in this order:

Copy the standard-port predicted HW and LW times and heights straight from the almanac or digital source.
Add the published signed time difference to each HW and LW time to get the secondary-port times.
Subtract the standard port’s MSL from each standard-port height to get the deviation at HW and at LW.
Calculate the range ratio by dividing the secondary port’s spring (or neap) range by the standard port’s spring (or neap) range.
Multiply each deviation by the range ratio, then add the secondary port’s MSL and its chart-datum correction to get the final secondary-port heights.

One branch point matters before you start: check whether the location is semi-diurnal or diurnal. The MSQ guidance on secondary port tide times and the AHO worksheet both split into separate paths for each regime, and forcing a semi-diurnal worksheet onto a diurnal port invents turning points that never happen. If your secondary port’s pattern looks irregular across a lunar month rather than twice a day, follow the diurnal section instead, and treat the table’s MHWS, MLWS, MHWN and MLWN entries as the trusted anchors rather than the nearby semi-diurnal station’s numbers.

2. The height formula and why the range ratio matters

The practical formula for a secondary-port height is:

secondary height = secondary MSL + (standard height − standard MSL) × range ratio + chart-datum correction

Each term does a specific job:

Standard MSL is the average sea level at the named reference port, the baseline your deviation is measured from.
Secondary MSL replaces that baseline once you move to the secondary location, since mean sea level itself shifts between ports.
Range ratio scales the standard port’s swing up or down to match how big the tide actually gets at the secondary port.
Chart-datum correction reconciles the level you’ve calculated with the datum printed on the chart, which is what you need for clearance.

A range ratio below 1.0 means the secondary port’s tides compress relative to the standard port, so a 3-meter swing at the standard port might become a 2-meter swing a few miles away. The AHO worksheet derives this ratio from each port’s own spring range rather than a fixed offset, because the relationship changes shape across the tidal cycle, not just in timing.

A quick sense check: if the standard port’s deviation at HW is positive and the range ratio is 0.8, the scaled deviation should shrink, not grow. If your arithmetic produces a bigger number after scaling by a ratio under 1.0, you’ve made a sign or division error somewhere in the worksheet.

Range ratio scaling of tidal deviation

3. Interpolating heights between HW and LW

Once you have corrected secondary HW and LW times and heights, you still need the height at any point in between, say, the moment you plan to cross a shoal. Start by computing the local range (HW height minus LW height) and the elapsed time from the nearest turning point.

Find the tidal-curve fraction for that elapsed time, read from the curve or table supplied for the secondary port.
Multiply the local range by that fraction.
Add the result to the LW height, or subtract it from the HW height, depending on which turning point you’re working from.

Many almanacs use a straightedge method across height bands rather than a smooth curve, and for practical local-tide timing and tide-pool access advice, the Laguna Beach tide pools timing guide offers a relevant example of tidal-curve interpolation in action. It’s an approximation, built for a generic curve shape that doesn’t perfectly match every estuary or inlet.

Pro Tip: When the port sits in a river mouth or a sharply funneled bay, treat curve interpolation as a rough guide and lean on a digital harmonic prediction with stated metadata instead, since the generic curve shape can miss local distortion.

4. Worked example: standard port to secondary port

Say a standard port predicts HW at 0600 with a height of 4.2 meters, and LW at 1230 at 0.8 meters, with standard MSL at 2.3 meters.

The secondary port’s published time difference is minus 35 minutes at HW and minus 20 minutes at LW, giving secondary HW at 0525 and secondary LW at 1210.
The standard HW deviation is 4.2 minus 2.3, or 1.9 meters. The standard LW deviation is 0.8 minus 2.3, or negative 1.5 meters.
The secondary port’s spring range is 3.6 meters against a standard-port spring range of 4.5 meters, giving a range ratio of 0.8.
Scaled HW deviation: 1.9 times 0.8 equals 1.52 meters. Scaled LW deviation: negative 1.5 times 0.8 equals negative 1.2 meters.
With secondary MSL at 2.0 meters and a chart-datum correction of negative 0.3 meters, secondary HW height is 2.0 plus 1.52 minus 0.3, or 3.22 meters. Secondary LW height is 2.0 minus 1.2 minus 0.3, or 0.5 meters.

If you needed the height at 0830, roughly three hours after secondary HW, you’d read the curve fraction for that elapsed time against the local range (3.22 minus 0.5, or 2.72 meters) and apply it from the HW side. The compressed range (2.72 meters against the standard port’s 3.4-meter swing) is exactly what a 0.8 ratio predicts, which is your sense check that the arithmetic held together.

Before you commit a clearance decision to this number, confirm the vertical datum and time basis printed in your source, since mixing a chart-datum height with a different publication’s MSL reference quietly breaks the formula.

Confirm whether your digital source already predicts for the secondary location by name, in which case applying separate corrections on top double-counts them.
Check that signed time differences moved the secondary time in the direction the table states, earlier or later, not the reverse.
Ask whether the resulting range magnitude is plausible for that stretch of coast, since a ratio error often produces a swing that’s obviously too large or too small.

Pro Tip: Keep chart datum, not MSL, as your reference for any underkeel clearance calculation. They’re rarely the same number.

6. Why validated tide inputs and metadata matter

Paper corrections assume you know exactly which product you’re reading. The IHO guidance on digital tide and tidal-current predictions recommends digital predictions carry metadata stating the depth of prediction, time basis, and datum, precisely because a prediction without that label invites double correction.

Our tide feature carries metadata through to the planning step, so a secondary-port prediction arrives already flagged with its source and datum rather than as a bare number.
That doesn’t replace a local sense check: treat any clearance figure with a conservative margin, on paper or on screen.

A trade-off worth keeping in mind

Worksheets earn their place because they teach you what’s actually happening under the numbers, and that understanding doesn’t go away when the chartplotter does. We’d trust a validated digital prediction over a hand worksheet only when its provenance and datum are stated plainly, never when a number just appears with no label attached. Either way, leave yourself a conservative margin on any depth that matters.

— Andrea

Getting validated tide inputs into your planning

Our AI connector delivers secondary-port tide figures with source and datum already attached, helping prevent the double-correction mistake described above. It integrates with AI assistants you are already using, requires no new app, and draws from verified marine datasets rather than generated guesses.

Nausika

That means less time reconciling which worksheet path applies and more time on the actual passage plan. For sailors who want that validated tide and harbor data sitting inside their existing assistant, the Nausika landing page has the details on how the connector works.

FAQ

What is the difference between a standard port and a secondary port?

A standard port has full daily tide predictions published directly, while a secondary port is estimated by applying time and height corrections to a nearby standard port’s prediction. The method exists because publishing full daily predictions for every harbor isn’t practical.

Can I use a fixed time offset instead of the range ratio?

No. A fixed offset might approximate the HW and LW times reasonably well, but heights need the range ratio scaling because the relationship between the two ports changes shape across the tidal range, not just in timing.

Why does my secondary port use a different worksheet section?

Secondary ports are classified as semi-diurnal or diurnal, and the AHO worksheet provides separate procedures for each. Using the semi-diurnal path at a diurnal port can invent turning points that don’t actually occur there.

How do I avoid double-correcting a digital tide prediction?

Check the prediction’s metadata first: if it already targets your secondary location by name, the IHO guidance on digital predictions notes that applying separate secondary-port corrections on top produces an incorrect result.

Is curve interpolation accurate enough for underkeel clearance?

Curve and band interpolation give a reasonable approximation between corrected HW and LW, but the generic curve shape can miss local distortion in rivers or funneled bays, as sailing training guides note. Keep a conservative margin for any clearance that matters.

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