Tidal Calculations (Secondary Ports)
In the days when Tide Tables were crunched by hand, it was only possible to produce a few tables for the major ports. Even as computing took over, it was still practice to produce tables for a small number of 'Standard Ports' and, for the smaller ports produce 'Secondary Port Corrections' based on those Standard ports Nowadays, with wonderful software like the Admiralty's 'Easytide' Tidal Heights are available for many small ports but, as conscientious mariners we should all be able to understand secondary port corrections and calculate tidal height predictions manually . Secondary Ports are much feared... and the bane of yachtmaster theory students everywhere, but actually, they are fairly straight forward... especially if you are organised in your approach. For this exercise, let us look at a Solent Port - Warsash on the River Hamble. First stop, our Reeds Almanac:
Now, this shows us that Warsash is a Secondary Port and the corrections (in the red box) are based on the Standard Port of Southampton. The even helpfully shows us that the Southampton pages are before this page in the book (arrow shows the way!). The next thing to do, then is to go to the Southampton pages:
The first thing to notice is that the Tidal Curve for Southampton, as with many Solent Ports, is 'upside down'... based on Low Water. (We'll discuss why in another top tip but it largely due to a distorted tidal curve that gives an indistinct HW point... largely due to the geography of the area and its position near the 'middle' (looking W-E) of the English Channel. Let us say that we are going to arrive at Warsash at 1630 on Thursday the 26th of November.... The tidal details for that day are shown in the excerpt (right). Let us extract the details and lay out our calculations:
We now need to correct these figures to find the data for Warsash. First we need to extract the relevant correction times... our LW time is between 1200 and 1800 so we only need those corrections:
There are many ways to interpolate between corrections (i.e. to find the correction at 1352 when we have 1200 and 1800 tabulated). In many cases you can easily approximate by inspection. However, I find the most straightforward way, especially when working on a small chart table pitching around on a darkened boat, is the 'crocodile' technique. Draw a line divided into any units of length (I have used cm) and label 1200 to 1800 (i.e 6 units long). In this case do the same for the corrections... again I used cm, 10 units long. Then join correction with time - i.e. 1200 goes with +10 mins and 1800 goes with 0 mins:
Then draw a parallel line through the LW today (1352):
We can no read off the correction for LW time on the lower scale - in this case plus 7 minutes. This gives us a Low Water time of 1359 (1400 for government work)!
we now need to work out the calculations for HW and LW heights... we could use a crocodile but, in this case, it is unnecessary as the actual LW and HW heights are close to heights we have corrections for and the difference are only small:
We can now enter LW Time (1400), LW Height (1.8m) and HW Height (3.9m) into the Southampton Tidal Curve. I always relabel it to remind me I am interested in Warsash:
All that remains is to enter the curve with the time in which we are interested - our arrival time of 1630, go up to teh curve (noting we are just off neap range) and then across to our height line:
This gives us a tidal height for our planned arrival time of 2.5m above chart datum. As you can see, it is not as complicated as most people will lead you to believe. All it needs is a bit of organisation, some neat layout... and for you to stay calm!