Slack water time - Corryvrecken example (pedantic)

Amulet said:
Lets try this question again.

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So, to "Squeezy Bit": The tide turns east at 1600. To me that implies that the stream becomes "organised" at 1600, and I'd therefore conclude that the writer was indicating a slack period from 1500 to 1600.
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At 1600 the direction of flow reverses, but until 1600 it's still heading west at a fair few knots.

Think digital, not analogue; it's like the overflow in a bath .. nothing happens until the water reaches that level, then it pours out at whatever rate you are putting it in.
 
agurney said:
consider this then ..
- you are heading up the Sound of Jura with a spring flood and want to head north via the Corryvreckan.
- with, say, HW Oban at 19:30, HW Easdale will be 19:00 and Crinan 18:30
- this means that Cuan, Corryvreckan and Luing will turn south/east at around 17:00 .. though the tide will still be flooding
- with a spring tide, the suggested slack period in the tidal gate is 15 minutes, so if you haven't made it through then you have lost a couple of hours of fair tide.
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Aren't you just saying that "slack water" is when there is negligible tidal stream, just like what I have been saying? OP seems to think that "slack water" and "tide running" can happen at the same time.

- note, even though the water flow has changed direction it will still be rising until HW.
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I'm well used to that - the effect of the river flow at Kirkcudbright is to cancel out the weaker parts of the flood, giving slack water for up to 2 hours before high water and after low water. The actual duration depends on tidal range and how hard Tongland power station is working and is more typically 1 hour.
 
I understand the question, and in going through the Corryvreckan (last time at springs in May) we have always assumed in our calculations that the slack water period has preceded the start of the flow. So in your example in #17, I would say that westward flow stops at 1500, slack water is 1500-1600, and the water starts to flow east at 1600.

In practice, it seems to have worked pretty well so far - we have always arrived a bit before the start of the flow and gone through fairly smoothly, but as you say, at springs slack is only for 15 min, so not much difference in the two interpretations anyway. Slack water is longer in neaps.

However, I agree that the wording is not clear, and I am happy to be proved wrong in our interpretation.
 
I've just bought a railway ticket to Shrewsbury for next weekend. According to Virgin trains, I have an hour's wait in Wolverhampton and my train leaves there at 13.00. Does this mean that I wait in Wolverhampton from 12.00 to 13.00 or from 12.30 to 13.30? It's all very confusing.
 
The consensus is that the slack is before the stated time of start of flow - which is what I'd assume. Different opinions have been expressed.

I do understand tides and how harmonics are used to model them. Mine was simply a question of the language used; it seemed ambiguous, and the lack of perfect agreement here supports the view that is ambiguous. My own guess was in line with the consensus that has emerged.

"Before the flow starts, for crying out loud." That is indeed the consensus.

Stupidly I changed the times in my second example, using a flow starting time of 1600
"....but until 1600 it's still heading west at a fair few knots." This comment on the second example seems to me to imply NO slack despite the statement that there is an hour.

"OP seems to think that "slack water" and "tide running" can happen at the same time."
No he doesn't, he was just trying to work out when the slack occurred.

Thanks everyone.
 
Amulet said:
"OP seems to think that "slack water" and "tide running" can happen at the same time."
No he doesn't, he was just trying to work out when the slack occurred.
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I think the problem is basically that the CCC books, having been put together by amateurs, are not terribly consistent. In general they seem to assume that there are four phases: ebb, slack water, flow, slack water. In the bits I know, south of Oban, that seems to work pretty well, because the effects of tides coming in and out on different side of, for example, Islay/Jura/Scarba/Lunga or Luing/Seil means that there often is, as agurney has nicely described, times when they catch up or even overtake, giving rise to prolonged period of slack water in the cross passages like Coirebhreacan (first example) or the Cuan Sound (second), during which they can still be going up and down. You'll have seen a few notes about these oddities in the CCC books: "the tide attains its full rate in the first half hour" or "most of the rise or fall occurs in the first half of the tide".

In other places, where the tide is more sinusoidal, "slack water" often just means "an hour (or whatever) each side of high or low water, when there isn't enough stream to worry about". Which, of course, assumes that the author knows what worries the reader and is therefore as potentially misleading as "Access is possible one hour either side of high water", the accuracy of which depends on whether the reader is in a racer with a 3m T-bulb keel, my boat with a 1.4m deep long keel or a Drascombe with its keel up.

So yes, having thought about it your question was a reasonable one, the answer isn't obvious and I apologise for the snarkiness of some of my contributions.

Finally... The CCC guide has been observed by amateurs over the years, and some of its statements are made with a certainty which is not justified by the evidence on which they are based. Some are very old indeed - the instructions for the Cuan Sound in my 1991 copy are more-or-less identical to those in my 1937 copy (useful - more anchorages!) and may well be based on what one contributor wrote after going through a couple of times in the late 20s. That, I imagine, is how they screwed up the Mull of Kintyre so comprehensively. I find it a useful guide, but when there is a conflict between the CCC books and NP218, my money is on UKHO every time.
 
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Robert Wilson said:
Forgive my potential ignorance, but isn't there an area of less affected/calm water to the south of the race? i.e. close in to Jura?
I understand from a few who have transited the area that there is indeed a safe (?) passage.
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Sort of. The southern half of the gulf is generally the preferred route; it can be flat calm but still moving at 6 knots, and if that's the direction you want to go then great. It's all a safe passage if you pick your weather and times.

The race is a very different beast .. on the flood it extends a fair few miles to the west and sets up a nasty steep sea, even in relatively benign conditions (as SWMBO's mother is wont to remind me .. ).
 
agurney said:
Sort of. The southern half of the gulf is generally the preferred route; it can be flat calm but still moving at 6 knots, and if that's the direction you want to go then great. It's all a safe passage if you pick your weather and times.

The race is a very different beast .. on the flood it extends a fair few miles to the west and sets up a nasty steep sea, even in relatively benign conditions (as SWMBO's mother is wont to remind me .. ).
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Thanks.
I take it that the 6kts flow is in one direction or another, depending of the timing of the tide?
At least SWMBO's mother has "done it". I have only sailed past, down from Shuna, and even then the noise was alarmingly apparent !
 
Robert Wilson said:
At least SWMBO's mother has "done it". I have only sailed past, down from Shuna, and even then the noise was alarmingly apparent !
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Years ago I spent the night in Bagh Gleann nam Muc, just off Coirebhreacan. It was very remote-feeling and very noisy, what with the tide doing its stuff across the entrance.
 
Robert Wilson said:
Thanks.
I take it that the 6kts flow is in one direction or another, depending of the timing of the tide?
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If it's an eddy, pick the appropriate side of the roundabout, but often it's an upwelling which is flat, fast, and moving out from the centre .. but don't expect your rudder to work as you expect.

If the weather's decent, just do it. Baigh nam Muc has a couple of excellent anchorages, and you can be fairly sure that that nobody else will join you once the tide turns.
 
agurney said:
If it's an eddy, pick the appropriate side of the roundabout, but often it's an upwelling which is flat, fast, and moving out from the centre .. but don't expect your rudder to work as you expect.

If the weather's decent, just do it. Baigh nam Muc has a couple of excellent anchorages, and you can be fairly sure that that nobody else will join you once the tide turns.
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That sounds logical :highly_amused:
 
Basically nothing is ever quite what it seems.
A few years ago I broke my golden rule where instead of planning a trip to fit in with the tides I tried to fit it to the Inverness Kyle train times.
As a result I was late on board my boat in Plockton to begin to move it down to Arisaig.This combined with a lot of growth on the bottom and a strong head wind meant that instead of reaching The Skye Bridge at low water I was about 30 minutes late and the north bound tide was already running under the Skye Bridge.
Going under the Skye Bridge at one knot on the GPS and getting ready to abandon my attempt I remembered the many holidays as a child waiting for the ferry which ran at all states of the tide and what it used to do.
I got in as close to the mainland side as possible which can literally be described as almost alongside as the rocks drop sheer and caught the south bound current which persists along the shore finally coming alongside the floating pontoon at Kyle.
Shortly thereafter I was speaking to the skipper of the Glass bottomed boat who was surprised to find that I wasn't a local.
 
Interesting question and some interesting answers too.
For my part, and maybe I'm over cautious, but I reckon winds, atmospheric pressure and perhaps other factors affect time and height of tides by enough to make any prediction just that, a prediction. I would treat slack water times with similar care.

Thread drift but almost as pedantic - I once asked the Met Office what the difference was between occasional showers and rain. Apparently down to the type of cloud it is coming from.
 
ffiill said:
As a result I was late on board my boat in Plockton to begin to move it down to Arisaig.This combined with a lot of growth on the bottom and a strong head wind meant that instead of reaching The Skye Bridge at low water I was about 30 minutes late and the north bound tide was already running under the Skye Bridge.
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Once, years ago, I was rather late for the Dorus Mor and had to head for my mooring at Crinan with the westward flow underway. I crawled through at perhaps 1/2 knot over the ground, passing as I did so a bigger motor-sailor on which I had perhaps 1/2 knot. Once out of the current things improved. I sailed across to Crinan, picked up my mooring, tidied the boat, rowed to the pontoon, transferred my stuff to my car, carried the dinghy up to the racks, got in my car and drove off. As I passed the Crinan Hotel I slowed down and looked out to see. The motor sailor was still in the Dorus Mor.

Humblebee said:
Thread drift but almost as pedantic - I once asked the Met Office what the difference was between occasional showers and rain. Apparently down to the type of cloud it is coming from.
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I think they user "showers" to mean wet stuff falling from fair-weather cumulus (the fluffy, sheep-shaped ones) and "rain" for wet stuff due to wider-area effects.
 
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