Secondary Ports Explained


Secondary Port information is provided in Nautical Almanacs such as Reeds’ and also in the Admiralty Tide Tables. These publications can’t possibly give complete Tide Tables for all of the hundreds of small ‘Secondary Ports’ around the coast. Instead they give complete Tide Tables for all of the larger ‘Standard Ports’ like Dover and Devonport (Plymouth). They then also provide ‘Secondary Port Differences’ so that times and heights for any Secondary Port can be easily worked out.


Secondary Port Time and Height Differences are always presented in a uniform and logical way. Have a look at the diagram below. The top part shows us the presentation of the information in the Almanac or the Admiralty Tide Tables. Underneath we take a look at what that information means.

Secondary Port information layout explained image
Secondary Port Information Explained

How are we going to use that information? Look at the example below. We want to find out the time of High Water at Starcross on the afternoon of the 13th. The Standard Port is Plymouth (Devonport).

This image explains you how to work out time differences for a secondary port.
Secondary Port Time Differences

This diagram will show us how we use the same process to find the Height of the evening Low Water at Starcross.

This image shows you how to work out tidal Height Differences for a Secondary Port.
Secondary Port Tidal Height Differences

It is also helpful to have our handy encapsulated Secondary Port Calculation Sheet reminder with the explanation and the angles ready drawn on.

Secondary Port Calculation Sheet Link to product
Secondary Port Calculation Sheet

Learn More

Tide Tables Click Here.

Rule of Twelfths Click Here.

Tidal Curves

Variation and Deviation

Tidal Streams and Tidal Diamonds


Cockpit Card Complete Collection product image
Cockpit Card Complete Collection