Secondary port calculations question....

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dukes4monny

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Hi all, having recently completed my RYA YM theory course, I decided to purchase a REEDS Channel almanac and have a go at using some 'real' calculations.
On my first calculation i used my home port of Teignmouth on a secondary port calculation for the HW time.
The Standard port is Plymouth, HW time 0901 (yesterday 29th March), did the calcs and got a HW time Teignmouth of 0940..........
To double check this I went online and checked the actual HW times for Plymouth and Teignmouth........Plymouth 0902 Teignmouth 0920.........
I'm fairly sure I have done my calculations correctly, my question is, would you expect there to be this much difference between the calculated times and the 'actual' times?
I'm just wondering if I need a lot more practice /forums/images/graemlins/shocked.gif
 
There is no such thing as actual (except on the day obviously). Times are predicted and different sources get different answers.

It might be a bit out but although they teach you to be within a few minutes and 0.1 metres it would be foolhardy to do anything based on those margins given the effect of wind and pressure. As you said - it is THEORY!
 
There are two tide times for Teignmouth - Teignmouth approach & Tegnmouth Shaldon Bridge. It's not a precise science as predicted tide hights and times are estimates. I use the rule of 12 which I find accurate enough even for Teignmouth. Failing that get a free teignmouth tide table from the local chandlers.
 
Yes!

One of the things that the YM theory doesn't teach is that the reality is often quite different from the theory... not only in timing, but also in height.....

20 mins is nothing.... and only in the last month we had tidal heights out by over 1m in Harwich..... albeit an exceptional event....

Watch your sounder closely... /forums/images/graemlins/grin.gif
 
Thanks all for your replies. It is now clear to me that the calculated figures are only 'ball park'.........but being the first 'real' calculation that I had done I was hoping for the proverbial 'bullseye' /forums/images/graemlins/grin.gif
I was using the 'approaches' tide time for Teignmouth.......I'll give the 12ths rule a try too.

This was only an exercise to prove the theory.......I now know that the goalposts are a little further apart than I was expecting.
 
I don't mean to be rude and I hope you will take this in good spirit, but I'm a bit surprised by this question as you say you've done the YM Theory. The questions you ask should be well covered in the classroom but you show a naievety about tides - <<To double check this I went online and checked the actual HW times for Plymouth and Teignmouth>>. The actual time is when the water arrives. The predicted times are what the almanac will give you and as several people have pointed out, there are various ways the prediction could be out, sometimes by quite a long way. I'd go and have a look in one of those Tom Cunliffe books or similar if I were you - certainly before relying on a calculation.
 
If you have done the recent RYA courses you will know that they use theoretical data: charts and alamanac which do not bear any relationship to real places.
As I said, I was putting some of the 'theory' into practice for the first time, and to double check my calculations I can for the first time use 'real' data. Did you never question the accuracy when you first did it?
I know that air pressure wind direction etc can all affect the 'actual' height and time of the tide, I was simply trying to ascertain the accuracy of the supplied data.
I now know that the predictions used in the almanacs use a different method to the predictions in the tide tables.
I'll have to remember that one for when I'm asked what time high water is "when the water arrives......" brilliant ;o)
 
Not only do I look for several sources of information (including my eyes) but I was taught to, was my point, whereas you seem to have been left to discover this for yourself. Actually the bit you've discovered for yourself is the important bit, whereas the ability to deliver a prediction within 10 cm (RYA style) is of only passing interest.

You're dead right, the goalposts are further apart than they appear. Also they mingle with the crowd sometimes. Did the ball cross the line? (enough extended metaphors). I find sitting on my own boat wondering whether it will go aground makes me a ferociously accurate calculator of tides, no matter how knackered I am. Sitting at a desk somewhere doesn't have quite the buzz.
 
That's a tempting thought but, like other areas of navigation, errors tend to pile up and it's probably good practice to aim for the best possible accuracy.
 
The RYA reckon that you should be within 10 minutes and 0.1 metre in your calculations. That's not suggesting that the predictions are that accurate; it's a help to the instructor who then knows that you have done the calculation properly. If you're out by more than that then the instructor should investigate to find out where the slip-up has occurred.

When I taught on shore-based courses I always told students about the Annual Notices to Mariners. One of them (see here) deals with the under-keel allowances in the Dover Strait, and with the warning service for those occasions when the tidal height is less than predicted (ie when big ships have less water than they might expect). Notice that they don't even start giving warnings until the level is a metre below prediction. I'm told that the warning is given about half-a-dozen times a year.
 
The secondary port tidal height differences were actually spot on, it was only the times that differed.

Some of us on this RYA YM course did get the feeling that we were being taught to pass the exams rather than actually being taught the subject......I still learned a lot so a minor point really.

There have been a variety of interesting opinions raised in this thread, which just goes to show it's far from a precise science.
I think that I will now be allowing for a larger safety margin....
Oh well, just got to put it into practice this year.......so much to learn.........can't wait to get out there!
 
Apologies if I have missed it but no one seems to have tried to repeat your calculations.

According to my tables the differences for Teignmouth approaches are

HW Devenport 0100 and 1300 ............ + 0025
<span style="color:white">HW Devonport </span>0600 and 1800 ............ + 0040

Interpolating between the differences for 0600 and 1300 I get a difference of +0034 (to the nearest minute) for a HW at 0900 at Devonport

Hence if HW Devonport is 0901, HW at Teignmouth (approaches) will be 0935. That's 5 minutes different to the time you calculated.

Perhaps you can see why our results are different . I can't. Did you use the same difference figures or have you not got the same difference for 0900 by interpolation. You seem to have applied a "difference" of +0039 not 0034.

I agree it is all academic, especially as different sources for tidal data will give quite different predictions at times. But IMHO it is important to use the data you have with the best possible precision to avoid introducing errors due to poor calculations although a 5 minute error is really not important.

The calculations that you really need to practise are those for tidal heights at secondary ports at times between HW and LW. I always have to put my thinking cap on for a while to deal with those.

In practice the solution is to use Michael Reeve-Fowkes' Tidal Atlases and put all this arithmetic behind you!
 
Ok that's interesting, thanks for doing that.
Based on the figures that you are using, I get the same result, 34 minutes.
However, in my Reeds Channel Almanac they give the time differences Teignmouth approaches:

High Water
0100 0600
1300 1800
+0020 +0050

I am surprised at the difference between the 'differences'. So it would seem that different almanacs also use different data? The plot thickens.....
Is this a common occurrence?
 
I was using figures from an old PBO almanac that's lying around . It looks as though the diffrences may have been revised.

Interpolating between your figures of +0050 for a HW at 0600 and +0020 for HW at 1300 I make the diffrence to be applied +0037 giving a final answer of 0938hrs.
 
That clears that one up, thanks. I agree with your calculations, I had just 'eyeballed' it first time around, but using a calculator I get +0037.........
At least I know my calculations were correct.........just got to keep practicing now.......as they say "use it or lose it".

I will have to get myself one of your recommended tidal atlases too.
 
Can't comment on your example but I assumed all predictions would be reasonably accurate until I once picked up a moorinmg at Wooten Creek ,I of W and realised we were ploughing throug mud - no problem as there was another 1hr to go to HW - then noticed that tide was definately going out.

Abandoned mooring for lunch and quickly ploughed our way out through the silt. Sat down to recalculate HW and got same answer, checked with tidal prediction on chartplotter and it was close to my almanac based prediction. After that experience I assume that they are all just that - best predictions.
 
[ QUOTE ]
The Standard port is Plymouth, HW time 0901 (yesterday 29th March), did the calcs and got a HW time Teignmouth of 0940..........
To double check this I went online and checked the actual HW times for Plymouth and Teignmouth........Plymouth 0902 Teignmouth 0920.........

[/ QUOTE ]

Did you check the "actual" times, or the "actual predicted" times. If the latter, it looks like you have made a mistake - which is really easy to do, adding rather than subtracting or similar.

How do you do it? With a crocodile, or mathematically?
 
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