Tide curves for dummys

[Colin24]

Stupid question #1

OK here goes, I’ve a feeling I’m setting myself up for some abuse for being such a dummy, but if you don’t ask….

I’m currently ploughing thru an interactive training CD (“How to sail a boat”) or something like that. I’ve just finished the bit about; how to work out how deep the water will be in a particular spot at a particular time. One of the methods involved using a chart, a tide table and a tide curve. Anyway, I sort of got the gist of how this goes and think –I’ll have a few practice shots of this and see how I get on.

Here is my question: The tide curve used in the example on the CD was like a blank template which the user populated with variable info (times, high and low water heights from the tide table and what not) and from this one could work out the answer required and it all made sense, I’ve no problem with that. Where I’m confused is: does this same template work for all areas? I mean does the shape of the tide curve change from one place to another? If so, how do I know which one I need and where do I get them from? If they are all the same, where can I get a blank one so that I can practice? Is there a place on the net I can download one from?

I’ll get my coat…

Colin


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[Leisure27]

Yes, different areas have different tidal characteristics. You can find the tidal curves for different areas in an almanac. I'm afraid I don't know of anywhere on the web.


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[Solitaire]

Tidal curves do vary by port. So the quickest way to answer your question is no - one teidal curve "template" cannot be used for all calculations. To see how the tidal curves vary go a get yourself an Almanac and it will show all the tidal curves for the main what are termed "primary" ports. - Dover, Portsmouth, Poole Plymouth etc. What you will notice that each curve differs. For example, the tidal curve for Dover is near as makes no odds a perfect bell shaped curve. Compare this to say the Poole tidal curve and you will see a marked difference. Also in the case of Dover, you would base your calculatons on high water times whereas with Poole you would take low water as your base due to the high water blip which would effect and calculations.
The find out which port you should be using for you tidal calcualtions you should look on the chart. Now I am assuming here that you know about tidal diamonds? On the chart will be marked a series of letters A, B, C etc - from this you can asertain the speed and direction of tides at certain points. The chart will show you the speed and direction and will also tell you upon which port the times are based. For example, if you look on a Solent chart, the tidal diamonds are based on Portsmouth tide times.

So go and buy an alamanc - better still put one on your Christmas list - depending on where you based either the smaller Reeds Channel almanac - about £20 or the bigger back breaking version for abot £32! Buy a chart and then go and play with the different tidal curves.





<hr width=100% size=1>Boating is <A target="_blank" HREF=http://www.powerboat-training.co.uk>Serious</A> Fun

 

[wishbone]

You could go for the RYA Practice navigation tables and charts, doing things on paper as well as on screen will help you grasp things more easily, you could get for xmas the rya course notes etc for day skipper, and RYA Navigation by Tim Bartlet a very good book ( make sure it's the latest edition)
Good luck!

Wishbone
Rolling, rolling, rolling keep them doggies moving!
Where’s me chuck wagon gone?



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[VicS]

I won't repeat what others have said but have you looked at the "Rule of twelfths" which gives a pretty good approximation for places where the tidal curve is a nice symetrical sine wave kind of shape.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[mnm]

Had the same problems/questions myself. Found the RYA booklet "Practice navigation tables for yachtsmen" invaluable.Plenty of examples plus some tidal stream charts and weather information too.

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[StugeronSteve]

Not a stupid question at all, but if you were looking for a bit of quality abuse and p**s taking you have posted in the wrong place, try Scuttbut!

As has been pointed out you cannot use one curve for every port, often one curve will work for several harbours in a locality, but that can't be assumed to be the case, each curve will clearly state which harbours it can be used for.

Ask Santa for an 2004 Almanac for Christmas, there will be tons of other useful information in it, or just scrounge an old one as I'm pretty sure that the curves are not ephemeral.

If you have the time get yourself on an RYA Day Skipper theory course. It isn't just the syllabus stuff you learn, with a good lecturer you will get the benefit his/her wealth of experential knowledge, and the chance to think through the problems in company.



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[Guest]

Technically ...

No two places have same tide curve.

In the Admiralty Tide Tables all secondary ports are referenced to a Primary port and curve. That way you use the curve designated for the respective primary port to calculate your secondary.

If you don't have Adm TT ----- then its a bit harder to sort out !


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I came - cos they said was FREE Guinness !
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[AndrewB]

Sine curve and rule of twelths.

While others are right that the tide curve varies a little from place to place and between spring (large) and neap (small) tides, for all practical purposes you can use the same shaped curve everywhere around the British Isles and our near neighbours. There is one important exception, and that is the English coast between Poole and Chichester harbours, and round the Isle of Wight. There you need the actual curves, which aren't even the same from day to day in Poole and Christchurch.

This curve is a sine curve, which is printed in many text-books or can be generated quite easily from Excel. The time from the bottom to the top is 6½ hours.

An even cruder approximation, which also works well for most practical purposes, is the "rule of twelths". If you know how high the tide is going to rise, its range, then from low water imagine it will rise one-twelth of the way in the first hour, two-twelths during the second, three-twelths in the third, three-twelths again in the fourth, two-twelths in the fifth, and one-twelth in the sixth hour when high water is reached. With practice, approximate tide height is then a matter of mental arithmetic.

If you do work out tide heights to the second place of decimals, never forget that the tables and curves are FORECASTS and the actual tide is not uncommonly up to half a metre different, depending on the weather.

 

[tcm]

yeah, they do differ. You get the curves in an almnac. Each port for which you would need to a SPC directs you to the standard (best fit) port nearby in the almanac: the curve is similar, but HW lags or leads from time to time, and HW and low water is higher or lower (or the same) from time to time.

In real life (ie not in an exam nor in the solent) you can use the rule of twelfths from high water to low to rough the height. 1st hour after HW it drops a twelfth, 2nd hour anothe 2/12, 3rd hour 3/12, 4th hour 3/12, 5th hour 2/12, 6th hour 1/12.

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[samuel]

I have a little slide rule type thing called a " Tidler " Tide level calculator purchased from a firm called Seamark Navigation Equipment Ltd from Littlehampton
It cost 36 shillings & is marked in feet. I have stuck a paper strip on it marked in metres. I have used nearly every passage for years as it is so quick to use & is very " visual "
Does anyone know if they are still made I would like one with the metric graduations ?
It does not do the awkward plots for Isle of White etc but for the Thames it has helped me run aground loads of times.


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[Omatako]

You need help to run aground??!?

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[TonyD]

Re: Tide curves on the Web

Anyone wanting to see tidal curves on the web should try the UKHO's EasyTide <A target="_blank" HREF=http://www.ukho.gov.uk/easytide.html>http://www.ukho.gov.uk/easytide.html</A>

Hey, its free and from the Government! That has to be a first. Get there before they start charging

TonyD

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[philip_stevens]

Like the UKHO tides on-line, WXTide will show you all the curves for UK ports - that is, if you have the version before the UKHO made the compiler remove UK tides.

I have that version, but there is somewhere or someone that is on this site, that has it in his own website for download.

Do a search over the past 4 weeks, and you should find the post that gives the site.

If all else fails, I could email the program as an attachment to your home email address - but you would have to send me your email addy via a PM.

P.S. found it for you. Be aware that you are "not allowed by UKHO" to use it /forums/images/icons/wink.gif
<A target="_blank" HREF=http://www.bluemoment.com/downloads.html.>http://www.bluemoment.com/downloads.html.</A>

<hr width=100% size=1>regards,
Philip

<P ID="edit"><FONT SIZE=-1>Edited by philip_stevens on 07/12/2003 23:59 (server time).</FONT></P>

 

[Colin24]

Many thanks to all contributors, I now understand the way it works and where to get the info.
I do intend to do an RYA course when I return to the UK and also purchase an almanac.
I work on a US flagged Seismic survey vessel (that’s where I'm writing this from now.) Currently doing a survey offshore North West India. Present position -
LAT 21'37.9925 N
Lon 67'59.4785 E
Stuck on here until 29th Dec - Third Christmas in a row onboard.
My only source of info until then is the net, emails and you good people, so thanks again.

PS its a dry boat so have a drink for me at Christmas.

Colin


<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Colin24 on 08/12/2003 01:32 (server time).</FONT></P>

 

[charles_reed]

No, not all tidal curves are the same, a lot depends on the place and local geography (ie estuaries like Strangford and the Channel).
However the curve is nearly OK for about 80% of places, though I wouldn't commit to conning anything in grounding areas based on that.

If you want a good LEGAL, tide alogarithm for the computer I'd recommend downloading from pangolin.co.nz TideComp, which covers the whole world and gives all the tidal data for main ports until 2023.

It's only a 7-harmonic version but produces curves which are virtually indistinguishable from 13-harmonic versions which cost a lot more money for far shorter time. Round European shores I've found the times to be within 10 minutes of Admiralty or local hydrographic tables.
You can use it free for one month or pay $29.29 for it.

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[alant]

An alternative for thos who find it difficult to divide by 'twelfths', is the % rule - 10% Hr1, 15% Hr2, 25% Hr3, 25% Hr4, 15% Hr5, 10% Hr6.
Never checked which is the best approximation.

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[wishbone]

I'le have a glass of red for you as the 29th is my 52nd birthday !!Blimey where's the time gone!!!!!!!!!!

Take care and Merry Christmas It must be soon just been listening to the brass band whilst shopping at Asda Barnsley......

Wishbone
Rolling, rolling, rolling keep them doggies moving!
Where’s me chuck wagon gone?



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[TwoStroke]

Have also found most ports have their own web site - together with their tidal curve which you can download and obtain accurate results. /forums/images/icons/smile.gif

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