Module 11 - Tides & Tidal Streams
Secondary-port tidal calculations: the correct sequence
What you will learn
Apply the publication's separate HW, LW and height correction method to the named standard port before calculating an intermediate secondary-port height.
A secondary-port correction is a source-controlled conversion, not one reusable offset. Time and height methods can differ between HW, LW, bands and publications.
ADMIRALTY Tide Tables provide time and height differences for secondary ports relative to a named standard port. Begin with the correct standard-port HW and LW for the date; the named standard port is not necessarily the nearest harbour.
Apply the published HW correction to HW and the LW correction to LW. Use the height band, columns, ratio or interpolation instructed by the publication. Keep every sign visible and do not assume one correction applies to both waters or every range.
Only after establishing corrected secondary HW and LW should you use the supplied local curve for an intermediate height. If an authorised digital service already gives a direct prediction for that secondary location, follow its displayed time and datum and do not correct it again.
Worked example
Standard-port HW is 0535 at 5.2 m. The applicable secondary HW corrections are +0015 and -0.4 m.
- 1Secondary HW time = 0535 + 15 minutes = 0550.
- 2Secondary HW height = 5.2 m - 0.4 m = 4.8 m.
- 3Use 0550 at 4.8 m as the HW input; calculate secondary LW from its own row.
Sense check: A plus-time correction must make the time later and a minus-height correction must make the height lower.
| Stage | Control question | Evidence recorded |
|---|---|---|
| Relationship | Which standard port is named? | Port and date |
| HW | Which time/height row applies? | Signed HW corrections |
| LW | Are its corrections different? | Signed LW corrections |
| Height | Band, ratio or interpolation? | Printed argument and working |
| Curve | Which local method applies? | Corrected HW/LW first |
Make the correction auditable
The table has different HW and LW time corrections and two standard-port height bands.
1. Copy the relationship
Record the named standard port, date and separate signed corrections.
2. Select the printed argument
Use the height band or interpolate exactly as the table instructs.
3. Write corrected waters
Establish secondary HW and LW before using the relevant curve.
Sense check: If another navigator cannot see which row, band and signs were used, the result is not ready for a passage plan.
Common mistake or limitation
- The nearest geographical port is not necessarily the standard port named by the publication.
- One correction does not automatically apply to both HW and LW or every predicted height.
Recap
- Use the named standard-port values.
- Apply each signed time and height method exactly once.
- Curve only after the secondary HW and LW are established.
Optional quick check
Section 6 of 11
Standard HW is 0535 at 5.2 m; corrections are +0015 and -0.4 m. What is secondary HW?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Practice the complete tidal-height method
The tides module links standard ports, secondary corrections, interpolation, curves, anomalies and safe-depth decisions.