Secondary Ports - differences data?

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scotty123

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An example of the typical differences for High Water times between a Standard Port & a Secondary Port, could look like the following:-

0000 0600
1200 1800

Diff +0015 -0010

Why, if these times of 0000, 0600, 1200, 1800, represent HW times at the Standard port, doesn't the time difference for the secondary port, stay the same , instead of differing from +15 minutes to -10 minutes as in this example?
 
One column probably coincides with H Water times for spring tides and the other with HW times for neap tides. Or something close to that.
 
There is just a variation between the tide difference at springs and neaps, which varies from port to port. The 'bulge' of the tide will move at slightly different speeds around the coast depending upon its size. It is itself a waveform. All the almanac does is approximate this by giving the difference when HW (& LW) is at a certain time and assuming a linear relationship to the differences at other times.

Of course in reality the secondary port differences are not going to be triangular waveform (as per the alamanac calculations) but will be a sinusoidal waveform, probably with harmonics superimposed.

The thing is you don't need to understand the theory to use the technique to calculate the tidal height.

You should be aware that the prediction is itself an approximation, and anyway the tides don't always exactly conform to the prediction, but by allowing a margin under the keel you'll be ok.
 
scotty123 said:
Why, if these times of 0000, 0600, 1200, 1800, represent HW times at the Standard port, doesn't the time difference for the secondary port, stay the same , instead of differing from +15 minutes to -10 minutes as in this example?
Click to expand...

At the secondary port it's not only the height of tide but also the times of HW & LW that differ from those of the standard port. The differences vary between springs and neaps.

In some cases the time differences are insignificant. Five or ten minutes either way can usually be ignored, in my view. However, elsewhere the differences are much greater, for both heights and times. Take Cardigan as an example. The time difference from the standard port (Milford Haven), for LW, ranges from +1h30 to +2h20, i.e nearly an hour.

As Vic says, the round-number times quoted at the tops of the columns correspond approximately with the standard port springs and neaps. Presumably most people here know that springs in a particular location occur at about the same time of day every time, and neaps likewise.

The setting out of the differences (in Reeds at least, which is all I use) allows easy interpolation between the height and time differences. Often they're simple enough to do with mental arithmetic.

I hope this helps.
 
If it is a guess, it is a good guess!

The first column is HW Springs, 2nd column is HW Neaps, 3rd is LW Neaps and 4th is LW Springs.
You do differentiate between Springs and Neaps when calculating the HW or LW times. It is just that the difference between Springs and neaps happen automatically as the time changes from one day to the next.
For example:
South coast of England HW Springs is at or around 0000 & 1200 (give or take around 50 mins) HW Neaps is at or around 0600 & 1800.
It takes 7 days to go from Springs to Neaps and you have a time difference of 6 hours.
6 hours divided by 7 days is a shift of 50 mins per day.
So the time of HW does automatically allow for the change from Springs to Neaps.
For example, if HW at Portsmouth is at 15:00 then you must be 1/2 way from Springs to Neaps.

Hope that this helps,

Simes
 
Having picked up a mooring in Wootten Creek approx 1hr before HW once and feeling that the boat was aground looked at the tide and it was clearly flowing out. Quickly ploughed our way out!!

Checked my secondary port calcs and also use Raymarine Chartplotter and both similar result it was approx 1hr before HW.

Since then always look at these tide calcs as an approximation. Mk 1 eyeball as always is best!
 
Sailfree said:
Having picked up a mooring in Wootten Creek approx 1hr before HW once and feeling that the boat was aground looked at the tide and it was clearly flowing out.
Click to expand...

Is there any significant river flow there? Depending on how strongly the Dee is running - which in turn depends on how wide open the taps are at Tongland Power station - the outward flow in Kirkcudbright starts an hour or two before high water. Jolly confusing.
 
webcraft said:
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Hmmmm . . . that appears to be five people at least who don't understand how to calculate the time of high and low water at a secondary port.

- W
Click to expand...

You may be misunderstanding my reply.

I moored approx 1hr before HW but while being aware that tidal heights and times are approximate and depend on other factors I never expected then to be 1hr out.

I double checked my secondary port calcs and read off the raymarine tidal info from the charplotter and they approx agreed (to within a few minutes)

Tidal info is at best only a best estimate so be aware it can be wrong so also use common sense.
 
Explaination from Head of Tides!

scotty123 said:
An example of the typical differences for High Water times between a Standard Port & a Secondary Port, could look like the following:-

0000 0600
1200 1800

Diff +0015 -0010

Why, if these times of 0000, 0600, 1200, 1800, represent HW times at the Standard port, doesn't the time difference for the secondary port, stay the same , instead of differing from +15 minutes to -10 minutes as in this example?
Click to expand...


Following extract obtained from 'Head of Tides, UK Hydrographic Office -

"Explanation of the Time and Height Differences published in Part II of Admiralty Tide Tables


· High Water Times at the Standard Port vs. High Water Time Differences at the Secondary Port
· Low Water Times at the Standard Port vs. Low Water Time Differences at the Secondary Port


Time Differences
On looking at the Time Difference curve diagrams it can be seen that there is one ‘peak’ and one ‘trough’ along their length, which effectively refer to the maximum and minimum time differences tabulated for the Secondary Port in the Admiralty Tide Tables. The times at which these maxima and minima occur at the Standard Port being the headers used in bold in the Standard Port header information (e.g. 0000 & 1200, 0600 & 1800 etc. etc.)

The above explanations of how the time differences have been originally derived at the Secondary Ports also account for the variations seen in the Standard Port header information."

Unable copy & paste actual tables/graphs.
 
webcraft said:
.
Hmmmm . . . that appears to be five people at least who don't understand how to calculate the time of high and low water at a secondary port.

- W
Click to expand...

I'll take the bait.

I know how I'd do it. I just double checked Reeds introduction to remind myself how they suggest it's done. Most of the people above seem broadly in agreement with both that and what I'd do. It's pretty self explanatory.

So I'm not sure I can count five people who are materially wrong. Put me out of my misery.
 
webcraft said:
.
Hmmmm . . . that appears to be five people at least who don't understand how to calculate the time of high and low water at a secondary port.

- W
Click to expand...

And one person who doesn't understand that the thread isn't about how to calculate the tides at secondary ports but rather why there is a variation in the time differerence between HW/LW at a primary & a secondary port.

Just because you've posted thousands of times doesn't mean you can't be a troll.
 
Basically tidal flows in different areas do not always have a linear relationship to each other, thus you get these strange variations. I was once taught that the relationship could be described with complex mathematical formulae but for us normal mortals droggy's table are easier to live with.
 
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OK, I apologise - having read through some of the posts again. To an extent the two columns can be thought of as representing Springs and Neaps, but the correspondence isn't important for practical purposes and of course you don't need to know or worry about which is which to perform a secondary port calculation.

I had done a detailed illustration of how to do a simple secondary port calculation but you do all probably know and I didn't want to patronise anyone so I have removed it.


- W
 
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maxi77 said:
I was once taught that the relationship could be described with complex mathematical formulae but for us normal mortals droggy's table are easier to live with.
Click to expand...

It's called Fourier Analysis (named after a French mathematician). You can break any waveform down into sine waves made up of a fundamental and harmonics of that.

As you say, easier to stick with the simplistic system and accept it is an approximation.
 
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