Amiralty Easytide and other incorrect tidal predictions?

[James Marinero]

I had a quick search and couldn't find a thread on this, so here goes:

We were checking tide times in Lymington for a haulout, and I noticed that the times looked strange - eg pm HW on Tue 21 June at 16:05 UTC and the next HW at 02:55 UTC on the morning of Wed 22 June (i.e. regresssing). Seemed odd, so we nipped along to the chandlers and they told us that the tide tables they were selling were incorrect too. Thanks. what's the point in sellng those then? So, back home goes I, and checks the Admiralty Easytide site and this is what I see (confirming earlier doubts):

Tue 21 Jun
HW LW HW LW
02:05 07:48 16:05 20:16
2.8 m 1.0 m 3.0 m 1.3 m

Wed 22 Jun
HW LW HW LW
02:55 08:31 16:29 21:03
2.8 m 1.2 m 3.0 m 1.4 m

Life's too short so I haven't checked outside this week, but it does seem strange and, dare I say, incorrect. LW times seem to be OK though. Intervals between successive HWs vary from 10.3hrs to 14 hrs. The tide tables posted outside the Lymington Harbour Office seem to make sense.

AFAIR (and all my sailing has been based on this premise), successive high waters are usually later, and not earlier then later then earlier. I always thought that HW time was tightly coupled to the time of the moon's meridian passage. Is this problem peculiar to Lymington this week, or is it a systemic fault in Easytide. Alternatively, am I missing something? Is the moon behaving? I wait to be enlightened.

If the HW times were more civilised then I could carry out direct observation from the pub.

FWIW I realise that the table of digits printed above, though incorrect, is Crown Copyright, but trust that I will not be sent to the Tower...

 

[lustyd]

I think they are all worked out from low water so that's the one to take note of. The times you posted seem reasonably consistent within the limited set you provided although the afternoon times are like you say odd.

EDIT: just looked at BBC and the difference seems to be there all week, could it just be the way the tides work there?

 

[Plevier]

The tide calculator in SOB Digiboat gives 16:01/2.9m, 20:12/1.2, 03:50/2.75 and 08:41/1.06 (UTC not BST)

 

[Ubergeekian]

AFAIR (and all my sailing has been based on this premise), successive high waters are usually later, and not earlier then later then earlier. I always thought that HW time was tightly coupled to the time of the moon's meridian passage. Is this problem peculiar to Lymington this week, or is it a systemic fault in Easytide.

Even I, a confirmed West Coast of Scotland sailor, know about the weird double tides in the Solent! Here are the Lymington tides for this week (see you in the Tower )

At springs in particular you'll see that there is an almost-hold at the top of the tide, and that the highest point alternates between the beginning and end of the almost-hold. Hence big gap, little gap, big gap, little gap - and hence the use of low tides to do calculations in the Solent.

You'll see, by the way, that the effect is even more pronounced at Southampton, where at springs there are two high tides with a very small dip between them. The higher of the two high tides alternates between the first and second.

 

[James Marinero]

Yes but...

I'm OK with Solent double tides and stands. The 'authorities' usually quote the first HW in the tables, and that's fine. The problem is that Easytide (and some of the published tables) is/are being inconsistent. Hence the recognition by locals that the times are wrong from some sources (BUT correct in the harbourmaster's office).

 

[Ubergeekian]

I'm OK with Solent double tides and stands. The 'authorities' usually quote the first HW in the tables, and that's fine. The problem is that Easytide (and some of the published tables) is/are being inconsistent. Hence the recognition by locals that the times are wrong from some sources (BUT correct in the harbourmaster's office).

I suspect that the EasyTide algorithm will simply look for the highest predicted sea level and use that as "high tide". Which is quite reasonable, as is using the first high. You just have to know which it is. Still, looks like 15 minutes and 10cm most of the time, so perhaps not a biggy.

 

[Gelfie]

If you are using Easytide set the graph for 1 day and that should give you a more acurate graph of the tides for each day. It's pretty good for Wareham which is a bitch to predict.

 

[Bosun Higgs]

I'm sure I remember being told in my YM that high water for the solent area is unreliable because it holds at times and you should always work from the low water times if, for example, calculating tidal heights for harbour entry.

 

[Angele]

I'm sure I remember being told in my YM that high water for the solent area is unreliable because it holds at times and you should always work from the low water times if, for example, calculating tidal heights for harbour entry.

Quite so.

All the tidal curves from Poole to Selsey Bill use low water as the mid-point. Not only does the stand make high water indistinct, making it hard to do calcs around HW accurately, but the shape of the curves vary significantly between neaps and springs - in some places and on some occasions with a long stand and in others with two high waters of different heights.

The single point of low water is not so affected and so provides the only reliable starting point for tidal height calcs from these curves.

 

[Kawasaki]

Peeps, tides do strange things
Look at the thread about 'Spring Bulges'!!
Do yer flippen head in!
Oop North in 'Swellie Land' on the Menai Strait tides are 'different' like the Solent etc
Today for instance
Whilst out training peeps.
Noticed the predicted HW (Easytide and Lavers Liverpool Tide book,had different predictions
Both were wrong by 30 mins ish either way in the real world
Wind direction and atmospheric pressure as we all know play a large part in the actual times of HW and LW
Plus the real Range.
I think the word 'Predictions' should be adhered to plus local knowledge.
By local knowledge I mean the knowledge you have built up yourself in your region over the years
Tomorrow the wind is forecast to veer locally
Lower pressure is forecast too
Based on that info I expect the ebb locally to be 'lower' ie the tide will go 'further out
The 'height' of HW won't be affected much I reckon
This is no Scientific study by the way Chaps
Just as I see it in me local patch
What I am trying to say in a rather long winded way is.
Use all the info available plus yer noggin and experience etc!