This is bugging me (tide tables)

[boat37]

For today (30th April) the BBC Tide Tables for Lymington show High Tide occurring at 01:33 and the following Low Tide 05:08.

The same web site shows Tides for Portsmouth as High Tide at 00:00 and next low Tide as 05:36.

However my Reeds Almanac says that when High Tides are at 00:00 (and 12:00) then the difference at Lymington compared to Portsmouth is -01:10 i.e. the High Tide should be 1 hour 10 mins before Portsmouth not 1 hour 33 mins afterwards. I looked at another locally supplied Tide Table (also based on UKHO supplied data) in Lymington today and found similar discrepancies at several randomly chosen Spring Tide dates throughout the year. There is a double high tide at Lymington on Springs and all Tables and Reeds say they publish the time of the first High Tide.

The High Tide times in Reeds for Portsmouth match those on the BBC site and the locally published Tide Table.

Does anybody know what is going on or am I completely misunderstanding my Tide Tables. There is a potential error here of more than 2 hours in the time of High Tide.

 

[timbartlett]

There is a double high tide at Lymington on Springs and all Tables and Reeds say they publish the time of the first High Tide....There is a potential error here of more than 2 hours in the time of High Tide.

I think you may have hit the nail on the head.
If you look at a graph such as http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/ShowPrediction.aspx?PortID=0040&PredictionLength=1 you'll see that today, the higher of the "double high waters" is the second one.
The first "high water" is little more than a reduction in the rate of rise, and it happens about two hours before the true high water.

 

[boat37]

Thanks Tim

The Admiralty 'Easy Tide Prediction' site shows High Tide today (2nd May) for Portsmouth being approx 14:00 (UT) which matches my Reeds Almanac.

The link you posted for the same web site but for Lymington shows the top of High Tide as being 15:05 (UT) which agrees with my locally issued Tide Table.

The bit I find misleading is that the Reeds Almanac says the time difference on Portsmouth is -1:10 when High Water occurs around these times (HW at 00:00 or 12:00). That must be referring to the first rise of tide at Lymington not the top of High Tide which according to these graphs arrives almost 3 hours later and which is what is published locally.

The reason this came about is that I was planning to go to Bembridge on Friday and almost assumed that High Water in that area would be about 1:10 later than that published for Lymington. That would have been a mistake and a good reminder not to 'assume' anything. In the end we didn't go cos of the weather.

Anyway thanks again.

 

[oceandrive]

This is imposible to answer easily.

But consider the following which may help. Lymington is a secondary port usualy using Portsmouth as its standard port. you will have to carry out a secondary port calculation to reach a satisfactory answer. Don't just presume that becouse the almanac says that a HW is so many mins befor Portsmouth ( Standard Port) that this is the same every day you have to calculate further (Secondary Port Calculation) This is not easy to explain and if I tried this reply would be pages long however if you look up secondary port calculations in the Reeds almanac you will find a couple of pages on the subject which I think may answer you question.

Hope this helps

 

[l'escargot]

...The reason this came about is that I was planning to go to Bembridge on Friday and almost assumed that High Water in that area would be about 1:10 later than that published for Lymington...

Bembridge is a secondary port just like Lymington. You should always do calulations for a secondary port using a standard port, not another secondary port.

 

[Solitaire]

Bembridge is a secondary port just like Lymington. You should always do calulations for a secondary port using a standard port, not another secondary port.

Correct, and as an addition secondary port calculations for the various ports in the Solent should be done using the low water curves as shown in the almanac.

 

[boat37]

Thanks everyone for your replies. Fair point about using Bembridge differences rather than assuming Portsmouth will do. I will read up about secondary port calculations again. Just to be clear - I do not assume that Lymington is always 1:10 behind Portsmouth it just so happens that on the date in question with High Water around midnight (UT) then the stated difference is -1:10. Obviously an accurate calculation would be more complex but i am not sure it explains the discrepancy.

Thanks again.

 

[l'escargot]

Thanks everyone for your replies. Fair point about using Bembridge differences rather than assuming Portsmouth will do. I will read up about secondary port calculations again. Just to be clear - I do not assume that Lymington is always 1:10 behind Portsmouth it just so happens that on the date in question with High Water around midnight (UT) then the stated difference is -1:10. Obviously an accurate calculation would be more complex but i am not sure it explains the discrepancy.

Thanks again.

It is as Tim says (that's why he's a good teacher) to do with the double tide in the Solent. Easytide only recognises one highwater and defines it as when the tide reaches it's highest point - this Monday it gives Portsmouth as 0252 and Lymington 0355, over one hours difference (Bembridge 0243, 9 mins different from Portsmouth). Next Sunday it gives Portsmouth as 0838 and Lymington 0835, just 3 minutes difference (Bembridge 0831, 7 mins different from Portsmouth). That is why as Solitaire says, do your calcs on low water - there is no "double low". Great fun Solent tides, people under-rate them.

It is all pretty academic though if you are looking at tidal flows which tend to be based on +/- Portsmouth anyway, not secondary ports. If on the other hand you are looking at depth for entering somewhere, cheat and use the tidal curves from Easytides - not the purist approach but nice and easy!

 

[beached]

Yes the Solent area is "interesting" with tides; and listed tide times have to be taken like an impressionists painting stroke. Different tables do list completely different times. Read on.

I sold my sailing boat and equipment last year; but a lot of my tide "education" has actually come from swimming around the marshes at Lymington. I do triathlon training and sea swim all year around in just swimming shorts (no wet suit). I know that I need to swim when the tide is above 2.6m chart datum; otherwise I have to walk out a long way. Fine in the summer; but not so good in the winter.

Today (March 5th 2015) just happens to be what I nickname a "5 tide day" referring to the number of high and low tides.
information supplied by the UK Hydrographic Office, for Lymington on the BBC website shows these as ;-
high 0044, low 0409, high 1030, low 1626, high 2259.

However Lymington figures from the National Oceanography Centre show that first Lymington high tides as being 2238 on March 4th 2015.

So a whole 2 hours 6 minutes difference between the timetables for the same tide. (Yes both are on UTC, and both timetables adjust with the clockchanges between UTC and British Summer Time besides).

As you are all sailors you are all aware of the impact the weather has on altering and modifying the tides.

What (surprisingly) appears to be less well applied is the of atmospheric pressure upon tide height.

Today the high pressure was projected to be 1045 hpa. So that is 32 hpa above atmospheric standard.

This means that sea level and tides were therefore 32cm lower than they were projected to be.
Handy the metric system isn't it. No hard maths in this example. The number stays the same.

Today I missed my swim because of this. After my beach run in the sun; I considered it too low to comfortably go.

For my sailing environments around the Solent, and swimming locations; I have found that the weather and atmospheric pressure are more important (to me in my situations) than published tide times. Besides the tide tables are often more than 2 hours adrift from each other; depending on which you read.

Hence I treat them as an impressionists paint brush stroke...only for rough guidance.