Differing Tide Tables !

[jacksaid]

An Imray tide table gave HW Portsmouth for 4th May as 12.59 UTC (ie 13.59 BST)
My other tide table gave the time as 12.59 BST !! ( this was the only entry for a day or so either side that differed)

The man at Imray says he has checked his table with the Admiralty Tables and it is correct.

I would be interested to know what your tide table says for this date and time and the source of the information.
Thanks

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[NigeCh]

13:02 GMT

Source: WxTide32*

The harmonics were uploaded before Groggie took umbrage.

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[VicS]

12:59 BST according to the tide table sent out by the boatyard. BUT this is clearly wrong when compared with the times of 13:26 and 01:36 for the preceding tides and 02:07 and 14:32 for the tides following. I guess it should have been 13:59.

Information said to be reproduced by permission of UKHO and HMSO; printed and published by Crossprint Ltd. who "will not be responsible for any inaccuracies that may occur".

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[LadyInBed]

ETPS Ver 1.00 Crown Copyright 1991
4/5/2003 (GMT)
Age of moon: 3 days
Portsmouth
00:32 4.45
06:02 0.97
12:51 4.39
18:16 1.19

WXTide32 Ver 2.8 Oct 9,2002 but with older version UK harmonic data
04 May 2003 (GMT)
H01:15 4.5
L06:22 1.0
H13:36 4.4 Interesting, different to NigelCh!
L18:36 1.1

Belfield Tide Plotter Ver 4.1
04 May 2003 (GMT)
00:54 4.5
06:10 1.1
13:16 4.4
18:25 1.3

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[DeeGee]

Tide tables are only as good as the data they run on. If you have an old copy of WxTide for example, it will be using older harmonic constants. WxTide seems to be still available on www.wxtide32.com. Reading the notes suggests nothing to confirm where the harmonic constants came from or whether they can be expected to be better or worse than droggie. I have v2.6 and it gives different results to both those quoted above....

droggie's EasyTide or TotalTide uses the most up-to-date harmonic constants and gives:

00:36 4.5
06:07 1.0
12:58 4.4
18:20 1.1

(ps. TotalTide - which is the retail version of EasyTide - has just been reduced to £82 and covers one helluva lot of territory, including the whole of Europe, Scandinavia, Med, N.Africa, Canada, NRussia...)

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[DeeGee]

apropos of my slightly earlier posting, WxTide is still available but not covering UK
I think your harmonics are dated up to 8/2/98, Nige.

From John Hopper's site....

May 2001: I was notified by the United Kingdom Hydrographic Office (UKHO) that the Crown claimed copyright ownership of all tidal products for Great Britain, Ireland and Scotland. To avoid being sued, my options were to stop distributing tidal data for those areas or purchase a license (with a recurring fee) for use of the official UKHO data. WXTide32 has never been a commercial endeavor so I declined their offer of buying a license. It appears that when the International Hydrographic Office (IHO) disbanded in 1999, tidal data ownership reverted to individual countries and the blanket approval IHO gave for non-commercial use of world-wide tidal data was rescinded. Since Great Britain has now claimed copyright ownership, other countries may follow suit. This means that only tidal data for countries that have explicitly released their data to public domain may be used free of risk of copyright infringement. Data for the U.S. is not a problem. That data was gathered using public (taxpayer) funds so no copyright is or can be attached to NOS or NOA tidal data. That still leaves lots of world though, and the database now distributed with WXTide32 version 2.7 shows that new greatly reduced area of coverage.



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[Apetts]

Have a look at
www.hydro.nl
from where you can download their CNAV40 for free. If you have a copy of the tidal constants publication you can easily create new areas of prediction. I am told by the Dutch hydro office that the prog should work quite accurately for a number of years on the built in constants. It will also reduce sun and star sights if anyone knows what these are in this modern electronic era!

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[LadyInBed]

Yes, you just need to buy a copy of NP 160.
Note how CNAV40 opt out of predicting Poole!

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[BrendanS]

Nigel

What's the translation in English
NP 160 ?
CNAV40 ?

We're not all as experienced as you!

Brendan

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[LadyInBed]

NP160 - Tidal Harmonic Constants (European Waters)
See http://www.hydro.nl/pgs/en/software_en.htm
The beauty of this program is that the Harmonic Constants can be edited, so data from NP160 (UK Hydrographics data) can be substituted.
The Dutch opted out of predicting for places such as Poole where the 'IofW' effect give double Highs.


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[rhinorhino]

UKHO Totaltide 1359 BST

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[BrendanS]

Thanks!

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[qsiv]

OOOh - it's not so much the Isle of Wight (as evidenced by the fact that double lows / double highs exist well outdide the bounds of the IOW.

As I understood it the effect is more due to a standing wave resonance cused by the tidal flow (which is a wave travelling around the world) sees the English Channel somewhat akin to a pipe, and as such the 'reflected' wave augments the rising or falling tide at certain points. The myth of it being to do with the Solent having a Western and Eastern entrance is, I believe, a myth.

There are times I wish I had spent more time at the hydrology model of the Solent when I was at (Southampton) University - but then the regular lectures interfered enough with sailing as it was!

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[LadyInBed]

Re: \'IoW\' effect

I bow to any greater knowledge on the subject. It will be great if someone can shed more light on it.
It always seemed quite reasonable to me that on a Westerly flow, current through the Solent arrives at a different time to current that came South of the IoW, giving rise to two highs West of the IoW.


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[montezuma]

Does it really matter? I appreciate the science of getting it right a la YM, but for practical purposes surely at the top of the curve the height and flow differences are going to be slight?

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[peterb]

Double high waters (long and a bit technical)

It's covered in fair detail in NP120, the Admiralty Manual of Tides.

The tide in the English Channel can be thought of as a long slow wave progressing up the Channel. One feature of any surface wave is that its speed depends on the depth of the water; as the water gets shallower the wave slows down. So as the tidal wave comes from the Atlantic into the shallower water of the Channel, it will slow down.

However, the slowing will be greater at the shallow part of the tide (low water) than at the deep part of the tide (high water); the crest of the wave will start to catch up with the valley and the rise becomes faster than the fall. (Look at the tidal curve for Dover; at springs, the rise takes less than 5 hours while the fall takes more than 7.) The nice smooth curve of the oceanic tide becomes distorted as it passes up the Channel. Distorted curves of this sort produce higher harmonics; listen to an overloaded sound system. The fundamental tide curve has a frequency of about 2 cycles per day (cpd); the harmonics will be at 4 cpd, 6 cpd, 8 cpd and so on.

Not only do we get the tidal wave passing up the Channel; some of it gets reflected back to form a standing wave system. It's best seen on Chart 5058, which shows the tides all round the British Isles, but one of the effects is that the range of the tide varies as it goes up the Channel. Roughly speaking, a 5m tidal range at Lands End becomes 4m at Falmouth, 3m just west of Portland Bill, 2m at Portland, 1.5m at St Albans Head and not much more tha 1m at Poole. Then it starts to increase again: 1.5m at Hurst, 2m at the Needles, 3m at Shanklin, 4m at Portsmouth, 5m at Littlehampton, 6m at Brighton and nearly 7m at Dungeness. Not just the range changes, the timing does too; between Christchurch and Lulworth Cove the times of High Water change by about 3 hours over a distance of about 20 miles. It's rather like swinging a hand in a bath tub: the range at the ends is high, while the middle hardly changes at all, and when it's "High Water" at one end it's "Low Water" at the other.

But this standing wave effect with its reduction in range only applies to the fundamental components of the tidal curve. The harmonics which have been generated in passing up the Channel will have quite different standing wave effects (which may include amplification by resonance effects in the Solent). This means that near to Poole and the Solent the harmonics increase relative to the reduced fundamental, and may be sufficient to show up as a double hump in the tidal curve.

Incidentally, the shape of the curve will be different depending on whether the 4cpd or the 6cpd components are dominant. If it's the 4cpd, then if there are double high waters then the low water will be sharpened (as at Poole) while if there are double low waters (as at Portland) then the high water will be sharpened. If the 6cpd component is dominant, then double high waters will be accompanied by double low waters (see the tidal curve for Hoek van Holland).

Remember, the effect started because of the difference in depth between high and low water as the tidal wave came into shallower water in the Channel. Obviously this difference will be greater at spring tides than at neaps, so we would expect that the resultant phenomena will be more significant at springs. Have a look at the tidal curves; you'll find that the neaps curve is always smoother and more regular than the springs curve.

The story about the double high water being due to one tide entering at Hurst while the other comes round Bembridge is a myth. If the tides at Hurst and Bembridge were regular and undistorted but of different timings, then the effect in the Solent would be to give a similar undistorted curve everywhere, although its range and timing would vary.



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[qsiv]

Re: \'IoW\' effect

This wasnt the text I was after - but it's close and the analogy seems useful....

To try to understand the reasoning behind the description 'Double High Water', one has to look first at the tidal flow throughout the English Channel. When it is High Water at Dover it is Low Water at Land's End and vice versa. Imagine the English Channel as a rectangular tank 300 nautical miles in length and having a uniform depth of 36 fathoms pivoted at its mid-length. If inclined in either direction the water flows towards the lower end, thus giving the effect of High and Low Water at opposite ends. At the point of pivot, however, the level remains constant. Of course the English Channel does not tip, but external forces created by the position of the moon and sun relative to the earth create the same effect, originating from the Atlantic Pulse which keeps the English Channel alternating between High and Low Water with the time of High Water at one end coinciding approximately with the time of Low Water at the other. This effect is called an oscillation and occurs twice daily. If the actual physical features conformed to this ideal pattern there would be no tidal rise or fall at mid-length, but though the tides at each end of the Channel do conform approximately to this pattern the friction, irregular depths and restrictions in width of the Channel between the Isle of Wight and the Cherbourg Peninsula result in a further four oscillations daily within an area bounded by Portland, Cherbourg, Littlehampton and Le Havre. Combined with the natural twice daily oscillations, this produces the 'Double High Water' curve as experienced in the Port of Southampton. In the shallower waters within the Isle of Wight and in the Port of Southampton up to thirty further oscillations of varying magnitude again vary the 'Double High Water' curve to produce the ultimate Southampton tidal curve embodying the local tidal features, namely, the short duration of the ebb tide, the 'young flood stand' and the pronounced fall between first and second High Water stands.

Thanks to Captain Ridge for that ..

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[Twister_Ken]

Well said that man!

The last hour of last week's spring rise meant, at most. 40 cm more water.

As I like to have a spare metre under me at all times, the tables being out by an hour wouldn't really matter, especially as the tables are a mathmatical prediction of actual conditions, not the conditions themsellves (insert usual caveat about wind, weather, atmospheric pressure, etc)


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[DeeGee]

I may be wrong about this, as I cannot find the reference, but I think I saw some blurb from droggie, to the effect that TotalTide/EasyTide are a bit more sophisticated than the simplified harmonic method in NP160, ie adding some more harmonics, or summat?

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[bedouin]

Sorry - I have to disagree.

The tidal prediction curves are all referenced to HW - so if the prediction of HW is out, then so is the whole curve.

Whereas an hour of difference at HW does not make that much difference - an hour at HW+3 is significant (roughly a quarter of the total range). Certainly when navigating up to the boatyard at HYC you can't afford to allow an extra 1m clearance!

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