Secondary Ports Explained


Secondary Port information is supplied in Almanacs like Reeds and the Admiralty Tide Tables. They can’t give full Tide Tables for all of the hundreds of small ‘Secondary’ ports around the coast. Instead they give full Tide Tables for a number of ‘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 worked out.


Secondary Port Differences are always presented in a logical way. See the diagram below. The top part shows how the information is shown in your nautical almanac or the Admiralty Tide Tables. Underneath we can see what that information means.

Secondary Port information layout explained image
Secondary Port information explained

How do we 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 how to work out time differences for a secondary port.
Secondary Port Time Differences

This diagram shows us how we can 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 always 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

To Learn about working with Tide Tables Click Here.

To check out The Rule of Twelths Click Here.