Tidal predictions?

Ian_Edwards

Ian_Edwards

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I'm doing a some background planning for a trip south from the Oban area in September.
I'd like to arrive at the tidal gate of the Sound of Luing at high water slack, so I have the ebb tide going south for a full tide.
It'll take me about 3 hours from my mooring to the Sound of Luing, so I'm looking for slack water around 09:00, that'll be an early start, but not too early, and the possibility of getting to Gigha in day.
Slack water at the sound of Luing is HW Oban -1hr 55min.
All straight forward so far.
I check with two tide tables, the Absolute tides App on my phone and MetOceans website.
For the 17th, 18th and 22nd the predictions are similar, to close to worry about, but for the 19th, 20th and 21st, they are significantly different.
HW Oban
Date Time (absolute tides app) Time Meto Consultants
17/9.......10:17..............................................10:18
18/9.......11:03..............................................11:05
19/9......13:08...............................................12:20 ..... difference 48min
20/9.....15:51................................................14:36 ..... difference 1hr 15min
21/9.....16:35................................................16:18 ......difference 17min
22/9......17:05 ..............................................17:04
The 19th is neaps
Why is there such a big difference in predictions around the 19th and 20th?
The difference in the predicted time of slack water on the 20th is 1hr 15min! And there is still a strong tide in the Sound even at neaps.
It looks like the 17th or 18th are optimum dates for sailing south,
All this is subject to a good weather forecast.
I'm interested to know why the Oban HW are so different around neaps, and which one is likely to be right.
 
You are over thinking it, IMHO. Absolute Tides does use UKHO data I believe, so good enough for me.

But the tides don’t tend to read the tide tables, and can vary by much more than this. The variability within 1 mile of Dorus Mor yesterday was substantial - 1 knot adverse instead of 2 knots positive. But that’s Scottish tides, sail with what you have got.

The weather is massively more important to me for choosing a date, and a start time - particularly by September. Often sailed through Sound of Luing against the tide with a good reaching wind. But can be a major bashing getting South to Fladda from Oban if it is a brisk SW and a big sea state from previous gales.
 
I did say subject to a good weather forecast.
And
For me planning to get a fair tide is just common sense and good seamanship.
Sailing south on the 20th July to Craighouse, we had consistent SOG of over 8knots with peaks of 10knots. If we'd had a flood tide that SOG would would have been more like 5 or 6 knots 7knots at best. That's a significant difference.
I've also struggled when I've arrived late going south to get through the Sound of Luing against a flood tide, at times going north.
The post was more out of curiosity, as to why the difference in prediction are so large.
 
There was quite an interesting article in last year's CCC Bulletin about the tides in the Dorus Mor area which will have some relevance on the Sound of Luing. One of the conclusions was that using Oban HW times (relative to Dover) can introduce errors, especially at neaps - the ebb can start later than predicted (by 25 mins?)

Being technical, there is certainly an anomaly in the calculation of the harmonic S2 which is particularly significant at neaps.

One suggestion was, to reduce errors, to use the time of the NEXT HW at Dover.

On the 19th July i headed south through the Sound (HW Oban10.35, HW Dover 03.57, near springs). I entered the Sound before local HW, and the ebb was already in good flow - I measured it at something like 4-5 kns.
 
What is the MetOceans website? My google found a lot of references to Metocean but not tide tables.
Absolute Tides is exactly UKHO data so that would be my benchmark.
 
The different tide times has been subject for quite a few years ... as UKHO and another source ... which I am trying to remember the source ... they produced a Computer app ... ? WXTide ??

The times were different by a few minutes in one location .. could be an hour in another. I think the tables were based on Edinburgh or Aberdeen based group ??
 
It has been a standing joke in Aberaeron for many years that the tide tables issued by Ceredigion Council often have no relationship to reality, no one has ever identified what they base them upon.
 
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