Secondary Port Tides  - Times and Heights of High and Low Water                Home Page

Click here for pdf handout of this page  secport.pdf      Secondary Port Example:  worked example

If we look at the progress of High Water up the Channel from west to east,  in general the time of HW is later in the day, and the height of HW and the range varies with the local geography.  If we knew the detailed height and time differences, which are consistent and predictable, between Dover and the other ports, we could determine all the other port tide times and heights from Dover only.  It is obviously easier to publish in the almanac the tide tables for the various Standard Ports, which are usually major centres of commercial shipping, and look up the tides directly. 

There are many other ports used by leisure and small commercial boats, where we still need to know the tides. It would be prohibitive in cost and sheer volume to publish the tide tables for all these ports, which are known as Secondary Ports.  Secondary Ports are related to the most suitable Standard Port.  This not necessarily the nearest, but the one with the most similar shape of tidal curve.

So in the almanac we find the details of these ports which show the 'DIFFERENCES' between the Standard Port and the Secondary Port, and we can use these to find out the times and heights of the Secondary Port. 

We then apply these new times and heights to the Tide Curve for the Standard Port, and it  becomes a Tide Curve for the Secondary Port. The shape of the curve for the Standard Port is similar to the shape for the secondary port.

An almanac entry will look like this:

HW Braye is approximately 4 hours before HW Dover; Mean tide height is 3.5 metres (average of MHWS, MHWN, MLWS, MLWN); Duration of rising tide from LW to HW is 5 hrs 45 minutes; Time zone 0  (UT).

The arrow next to St Helier just means turn the pages to the right to find the Standard Port.

The 'Times' are the times of High and Low Water direct from the tide table for St Helier, do not add the hour for Summer Time. The time differences only apply  at local standard time (UT in this case).  The differences are in 24 hour notation, thus 0105 is 65 minutes.

So looking at this secondary port Braye:
when HW St Helier is at 0300, the HW at BRAYE is 50 minutes later.
when HW St Helier is at 0900, the HW at BRAYE is 40 minutes later.

Between 0300 and 0900, interpolate the difference at Braye, so at 0600 the difference is +0045 minutes, and HW Braye is at 0645.  If required you can now add the hour for Summer Time - 0745

Similarly, from 0900 to 1500, the difference varies from +40 to +50 minutes, from 1500 to 2100 it is +50 to +40, and from 2100 to 0300 it is +40 to + 50.

One way of applying the differences is as follows:

The difference at Braye varies by 10 minutes over 6 hours, so each hour applies 1.66 minutes of difference between 0040 and 0050 minutes:

• 0300        50.0 mins
• 0400        48.3
• 0500        46.6
• 0600        44.9
• 0700        43.2
• 0800        41.6
• 0900        40.0 mins

My students find this tabular method the most reliable and easy way, and you can readily work to the nearest half hour.  Use a similar method for the heights.

You can also work it out by arithmetical ratios, or graphs (see below) or the RYA method is to draw similar triangles as shown below - see worksheet here - Secondary_port_worksheet.pdf  

The Tide Height differences are associated with the Spring and Neap levels of High and Low Water:

At MHWS for St Helier, HW is 11.0m, the difference at Braye is -4.8m,  HW at Braye is  6.2m.
At MHWN for St Helier, HW is   8.1m, the difference at Braye is -3.4m,  HW at Braye is  4.7m.

Again we interpolate the difference when HW is between 11.0 and 8.1m. You don't have to extrapolate the differences if HW is outside these limits, just use the 11.0 and 8.1 values.

Quite often the differences are trivial and the effort of calculating differences is not worthwhile, or you can do it by inspection.  But in an emergency, say you have to enter a difficult secondary port on a falling tide with a leak in the hull, it pays to be familiar with the process.

updated 28th November 2009