Module 11 - Tides & Tidal Streams
Secondary Port Calculations
Most ports are listed as secondary ports, with differences applied to a nearby standard port. The almanac provides time differences (e.g., HW is 20 minutes later) and height differences or ratios to convert standard port predictions to the secondary port.
To find tides at a secondary port: (1) identify the named standard port, date and time zone; (2) obtain its HW and LW times and heights; (3) apply the separate secondary-port HW and LW time corrections; (4) apply the stated height corrections, ratios or interpolation; and (5) use only the curve or interpolation method identified by the publication for that secondary port.
When differences vary with the height or time of tide, you must interpolate. For example, if the height difference at MHWS is −0.5 m and at MHWN is −0.2 m, and your HW height is between those, interpolate to find the correct difference. The same applies to time differences that change between HW and LW.
- Secondary ports use differences from a standard port
- Time differences: applied to standard port HW/LW times
- Height differences: applied to standard port heights
- Interpolate when differences vary with height or time
- Use only the tidal curve or interpolation method identified by the publication
Watch for this: Always check which standard port is referenced for your secondary port — it may not be the nearest geographical port. The almanac secondary port tables specify which standard port to use.
Sources and factual review
Reviewed 12 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.