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Coastal Navigation  Secondary Port Tides  Calculations of Times and Height Differences of High and Low Water
Click for pdf handout of this page Secondary_port_tides.pdf Secondary Port Exercise Example: worked example . Time zones and secondary ports : timezones.pdf
CAUTION
Tide tables assume a standard atmospheric pressure of 1013 millibars.
Atmospheric pressure creates
a down force on the sea. A cubic metre of air at sea level weighs about one
kilogram. A change in pressure of one millibar will change sea level by roughly
one centimetre. This means that a pressure of 1040 mb, not unusually high,
could give a sea level lower by nearly 0.3 metres than at standard pressure.
990mb would raise tide heights by 0.2m. Tide heights are also affected by
prolonged strong winds and storm surges. So do not trust tide height
calculations to less than 0.5 metres, maybe more! Tidal streams are
similarly not as precise as the tables indicate.
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.
Secondary Port Calculations
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:
My navigation 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.
HW 

HW Ht (m) 
LW 

LW Ht (m) 

Standard Port 
St Helier 
1131 
9.4 

Differences: 
+44 
4.0 

Secondary Port 
Braye 
1215 
UT 

1315 
BST  5.4m  
updated 1st January 2012