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) Look up the standard port HW and LW times and heights. (2) Apply the time differences to get secondary port HW/LW times. (3) Apply the height differences to get secondary port HW/LW heights. (4) Calculate the range and use the standard port's tidal curve as normal.
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.
Key points
- 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
- Then use the standard port’s tidal curve method as normal
Tip: 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.
Continue studying Tides & Tidal Streams
This topic is part of Module 11. Open the full module for lessons, quizzes, flashcards, and revision tools.