Module 11 - Tides & Tidal Streams
Secondary Ports
What you will learn
Convert a standard-port prediction into secondary-port high and low waters using the exact published time and height correction method.
A paper secondary-port calculation starts from a named standard port and applies published differences. High- and low-water corrections, and different height bands, may not be interchangeable.
Standard ports have full daily high- and low-water predictions. A paper tide publication can provide secondary-port differences from a named standard port: separate time corrections for HW and LW and height corrections or interpolation methods tied to the standard-port heights.
Begin with the correct standard-port values for the date. Apply the secondary port's HW correction to HW and its LW correction to LW. Apply height corrections using the band, columns or interpolation instructed by the publication; do not assume one constant correction fits every tide.
Once the secondary high and low waters are established, use the supplied local curve or method if an intermediate height is required. Some authorised digital products provide direct predictions for the selected secondary location; in that case follow the product's stated method and time basis instead of correcting the value a second time.
Worked example
Standard-port HW is 1140 at 4.8 m. The exercise gives secondary-port HW +0020 and height -0.3 m for this height band.
- 1Secondary HW time = 1140 + 20 minutes = 1200.
- 2Secondary HW height = 4.8 m - 0.3 m = 4.5 m.
- 3Use 1200 at 4.5 m as the corrected HW input; calculate LW separately from its own corrections.
Sense check: A +20-minute correction must make the time later, and a -0.3 m correction must make the height lower.
Keep the correction trail auditable
The harbour is secondary to a standard port, but the HW and LW rows show different time corrections and the height table has two standard-port bands.
1. Copy the source row
Record the named standard port and the separate HW and LW corrections with their signs.
2. Select the height method
Use the column or interpolate between bands exactly as the publication instructs.
3. Calculate before curving
Write the corrected secondary HW and LW values, then use the relevant curve for the time required.
Sense check: If the final sheet does not show which standard port, band and signs were used, another navigator cannot verify it.
Common mistake or limitation
- The nearest port is not necessarily the named standard port; use the relationship printed by the source.
- One time or height correction may not apply to both high and low water or to every predicted height.
Recap
- Start from the named standard-port prediction.
- Apply signed HW, LW and height corrections by the published method.
- Use the corrected secondary values before any intermediate-height calculation.
Optional quick check
Section 7 of 8
Standard-port HW is 1140 at 4.8 m; the applicable secondary correction is +0020 and -0.3 m. What is secondary HW?
Continue studying Tides & Tidal Streams
This public lesson is part of Revision Module 11. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.