Tide tables?

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Quandary

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Just came down from Oban to Crinan this morning, a slow passage but not as slow as Leader who despite full sail of Hampden Park proportions was tacking back and forth in the Sound Of Luing all morning. We were motoring at 6.4 knots and at Pladda SoG was 3.5 until we took her close inshore in the Luing bays, despite searching for the eddies in the Dorus Mohr we were only making 2.5 there. We left about 1/2 hr later than planned but expected to at least have cleared Luing by slack water.
So I had another check on the tide tables, HW Oban today O4.38 and 17.14 Bst, but on the BBC weather website HW Oban 04.08 and 16.24. quite a difference! The tables I had been using were downloaded in the spring from the Anglers website which gives full lists for most harbours around the UK and there was no reason not to trust them. I had noticed daily intervals of around 1hr.40mins. recently which surprised me but I had always thought that tidal information was mathematical and consistent, almost an hour of difference surprised me. Our Seatrak nav. program gives live tidal streams but these info. points are too widely separated to be much use on short passages like this.
Anyone explain the difference?
 
I don't think the West Coast tides always read the almanac. They sometimes have a mind of their own.
And certainly enjoy punishing a late start!
PS And the U.K. Tides phone App I generally use gives 4:08 the same as the BBC site did, as does the U.K. Hydrographic Office EasyTide so presumably these all use the same UKHO model. I had previously noticed the Angling site was often slightly different
 
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Sandy said:
Tide tables are a mathematical model, there is more than one model and the real world is a lot more complicated.
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But a difference of 50 mins. in calculating a 12 hour cycle (around 7%) seems a lot in the world of mathematics and computers.
 
Quandary said:
Just came down from Oban to Crinan this morning, a slow passage but not as slow as Leader who despite full sail of Hampden Park proportions was tacking back and forth in the Sound Of Luing all morning. We were motoring at 6.4 knots and at Pladda SoG was 3.5 until we took her close inshore in the Luing bays, despite searching for the eddies in the Dorus Mohr we were only making 2.5 there. We left about 1/2 hr later than planned but expected to at least have cleared Luing by slack water.
So I had another check on the tide tables, HW Oban today O4.38 and 17.14 Bst, but on the BBC weather website HW Oban 04.08 and 16.24. quite a difference! The tables I had been using were downloaded in the spring from the Anglers website which gives full lists for most harbours around the UK and there was no reason not to trust them. I had noticed daily intervals of around 1hr.40mins. recently which surprised me but I had always thought that tidal information was mathematical and consistent, almost an hour of difference surprised me. Our Seatrak nav. program gives live tidal streams but these info. points are too widely separated to be much use on short passages like this.
Anyone explain the difference?
Click to expand...

These are also the times you get from the Admiralty on line "Easy Tide"
 
Quandary said:
But a difference of 50 mins. in calculating a 12 hour cycle (around 7%) seems a lot in the world of mathematics and computers.
Click to expand...
Given the amount of water and the topology of the West Coast of Scotland I think that is pretty accurate.
 
I too use the Planet Fishing tables along with the Imray tide app and Easy Tide. Apart from Corryvrekan which is a law unto its self, especially at the west end, the tides are there or there abouts. I think the PF tides are calculated on the Portland model or something like that as opposed to the UKHO. Why not ask them?
 
Porthandbuoy said:
And just to add to the confusion the tidal stream direction changes before actual low or high water, or so I've experienced.
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That's what happens when there is more than one source of water to an area. For example, think of Kirkcudbright: water coming down the River Dee and water coming in from the sea. Slack water happens when these two balance, at which point the harbour is filling from two direction: no current (bar the occasional swirl) but the tide is still rising. When the tide stops rising the influx from the river is balanced by the efflux to the sea, so there is a flow through the harbour and out to sea.

Much the same happens in the Sound of Jura (for example) which fills both from the south (between Jura and Gigha) and the north (through the Sound of Luing), not to mention subsidiary flows through Coirebhreacan, the Grey Dogs, Cuan Sound/Loch Melfort and the Sound of Islay.

It particularly shows with the cross-linking channels like all these minor ones, since flow through them isn't a major tidal movement but the result of two major movements (in the case of Cuan, for example, it's the Firth of Lorn and Loch Melfort) being out of sync. If they were in sync, the Cuan Sound would simply rise and fall with little flow, but as Loch Melfort fills up and empties first, Cuan runs west on the flood and east on the ebb. If they were exactly 180o out of sync the currents would be huge but the depth in the sound would barely change.
 
Can't really see what HW Oban has got to do with it. Granted the tidal diamond in the sound of Luing is not at Fladda(?) but it's as good as you'll get. It has an adverse stream from about 09:00 to 16:00 BST if my reading is correct. It looks to me as if your experience was fairly consistent with UKHO predictions.
 
Porthandbuoy said:
And just to add to the confusion the tidal stream direction changes before actual low or high water, or so I've experienced.
Click to expand...

Nothing surprising about that - heights and streams are different things you need to look up separately.

Around the Channel Islands, the change of direction is often halfway up or down the tide, nowhere near high or low water.

Pete
 
Quandary said:
Just came down from Oban to Crinan this morning, a slow passage but not as slow as Leader who despite full sail of Hampden Park proportions was tacking back and forth in the Sound Of Luing all morning. We were motoring at 6.4 knots and at Pladda SoG was 3.5 until we took her close inshore in the Luing bays, despite searching for the eddies in the Dorus Mohr we were only making 2.5 there. We left about 1/2 hr later than planned but expected to at least have cleared Luing by slack water.
So I had another check on the tide tables, HW Oban today O4.38 and 17.14 Bst, but on the BBC weather website HW Oban 04.08 and 16.24. quite a difference! The tables I had been using were downloaded in the spring from the Anglers website which gives full lists for most harbours around the UK and there was no reason not to trust them. I had noticed daily intervals of around 1hr.40mins. recently which surprised me but I had always thought that tidal information was mathematical and consistent, almost an hour of difference surprised me. Our Seatrak nav. program gives live tidal streams but these info. points are too widely separated to be much use on short passages like this.
Anyone explain the difference?
Click to expand...

I know where you went wrong - finding yourself off the bottom corner of Arran after starting from Oban that morning must have required bending the space time continuum.
Tide starts going south at Fladda @ about hw Oban -1:40 (-1:55 according to the rotund follicly challenged yin) so by any of your times the tide would have turned agin you by 09:00ish. Get up earlier!
 
JumbleDuck said:
Much the same happens in the Sound of Jura (for example) which fills both from the south (between Jura and Gigha) and the north (through the Sound of Luing), not to mention subsidiary flows through Coirebhreacan, the Grey Dogs, Cuan Sound/Loch Melfort and the Sound of Islay.

It particularly shows with the cross-linking channels like all these minor ones, since flow through them isn't a major tidal movement but the result of two major movements (in the case of Cuan, for example, it's the Firth of Lorn and Loch Melfort) being out of sync. If they were in sync, the Cuan Sound would simply rise and fall with little flow, but as Loch Melfort fills up and empties first, Cuan runs west on the flood and east on the ebb. If they were exactly 180o out of sync the currents would be huge but the depth in the sound would barely change.
Click to expand...
I'm not sure I follow your bit about the Sound of Jura & Sound of Luing.
My memory (and my Admiralty Tidal Stream Atlas NP218) tells me that the flood tide goes north up both the Sound of Jura and the Sound of Luing, north past Shuna to Loch Melfort and west through Coire Bhreacain, Cuan Sound and all the "gaps" north of Scarba. Sound of Jura does then "fill" from the north, but at the same time it is emptying to the south.:confused:

I "impressed" my crew one time by forgetting to open the engine seacock at Craighouse, so just sailed with the flood tide all the way to off the west end of Cuan Sound. They bimbled around for half-an-hour while I changed the impellor before catching the first of the ebb through Cuan to Ardinamir. They thought I knew what I was doing!;)
 
I have been calling it Pladda for about forty years ever since the first time we anchored in Ardnamir under the watchful eye of Ms. MacLachlan and walked our kids over to the 'buttery' at Cullipool for real lemonade.
Fladda, Pladda, pladdies in Strangford Lough, I always understood they were just different anglicised descriptions for a flat topped rock. However I can see how sailors from the Clyde might be exercised about it if they came west and found that their waypoints were not where expected.
Btw, regarding an earlier start, the dog was nibbling my ear to go ashore at 04-00 but once he had peed on Oban's new pontoons I threw him back aboard and got back in to my scratcher. One of the advantages of that cellular decking is that he does not regard it as wood so does not need to go out the gate.
 
Fladda:
4586708620_930x410.jpg
 
In all my time sailing the West Coast and FOC, I don't think I have ever seen so many queries on times of tide being wrong. Too many sources I say; damn the internet.
 
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