tidal calculations - sanity check please

Ian_Edwards

Ian_Edwards

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Hi All,
I'm nearing the end of an annual delivery trip from Inverness to Loch Linnhe via the Pentland Firth and Cape Wrath, currently waiting in Loch Aline, sound of Mull, waiting for the wind to quieten down a bit, before I do the last few miles to Linnhe.
Yesterday, I came through Kyle Rhea and needed HW slack to give me the ebb tide down the Sound of Sleat.
Both Reeds and the current Clyde Cruising Club/Martin Lawrence, Ardnamurchan to Cape Wrath Pilot give HW slacks as Dover -4:20 or HW Ullapool.

So from Reeds for the 14th April 2016:
HW Dover 16:27 UTC, 17:27 BST
HW Ullapool 13:09 UTC, 14:07 BST

When I subtract 4:20 from Dover HW I get 13:07 BST ...... a One Hour Difference.

In the event I was early at the entrance to Kyle Rae, so I took it very slowly, looking at the difference between SOG and water speed. When the difference was small I motored through at about 6 knots on the log, monitoring the difference in speeds.

I entered the narrows at 13:18 and exited at 13:41.

My estimate is that it was more or less slack water, just a hint of the end of the flood, but no more than 0.5knots of adverse tide, generally 0.2 'ish knots, and that could easily be the error in the calibration of the log.
So it looks like the tide reference to Ullapool is a better estimate that the tide reference to Dover.
AND
Both Reeds and CCC pilot have an error. Both Reeds and the CCC pilot are long established publication, and I don't expect to find errors in tidal data, hence the request for a sanity check.

Have I made a school boy mistake?
 
My 2016 Almanac confirms you picked out the HW times correctly.

I have no local knowledge, but is it possible that the difference with Dover actually varies a bit between neaps, midtides and springs?

May be rubbish but good for a bump until someone knowledgeable comes along.
 
Yes, it occurred to me that it could the difference between springs and neaps, but an hour is a big difference when going through a tidal gate like Kyle Rhea, where the tidal stream will easily reach 8 knots.
On the day my experience suggest it turned somewhere between the 2 times suggested by Reeds and the CCC pilot.
In addition we've just had a week of strong E and NE winds, so there could be some form of storm/wind driven affecting the actual time of shack water, but that doesn't explain the discrepancy in the predictions.
When I get chance I'll look at other dates and see if the difference in predictions is consistent.
 
Thanks for this question; I have always assumed that the tide time offset from Dover is approximate - given the complicated and often asymmetric tidal flows around the UK it is hard to see how it could be a constant. Reeds (4.3.2) says that the Dover offset is 'on average' the difference.

It is interesting to compare the Ullapool table with Dover. On the 9th (Springs) applying the -0415 given in Reeds to Dover gives nearly the correct high water at Ullapool but over an hour in error at low water; on 16th (Neap) the reverse is true - the low water prediction is close but the high water is over half an hour in error.
 
Yes, that makes sense, the tidal flow around the UK is very complex, so to think that a prediction based on an offset and a tidal curve at the other end of the country is going to be accurate, is being a little optimistic and perhaps more that a little naive.
I guess the lesson learnt, this time around is use the closest geographic offset available.
As they say, you live and learn.
 
Ian_Edwards said:
So it looks like the tide reference to Ullapool is a better estimate that the tide reference to Dover.
AND
Both Reeds and CCC pilot have an error. Both Reeds and the CCC pilot are long established publication, and I don't expect to find errors in tidal data, hence the request for a sanity check.
Click to expand...

My experience in the Irish Sea is very similar. I now treat the tidal chartlets or tidal references in the almanacs that are referenced to Dover as approximate and if I need figures I have to rely on I use tidal diamonds referenced to a nearby primary port.

I've been in a position where I've obtained times using the two alternatives that were 60 mins apart - went with the tidal diamonds for a race and timed out almost all of the rest of the fleet. :)

I also read a few years ago that Reeve-Foulkes chose not to use Dover as the basis of his tidal atlases because the tides there were so irregular. Not that the predictions for Dover were any less accurate, but that the were less suitable to use as a basis over a wider area than the tides for Cherbourg that he eventually picked.
 
All my notes, charts etc. for that area are packed away. However, I seem to remember that the times were wrong last time I went through, probably 2011. I'd have checked Reeds and CCC so sounds as if I picked up the same error.

I've just looked at the Admiralty TotalTides app. and get similar figures for Ullapool and Dover.

HW Ullapool 13:08 UTC
HW Dover 16:27 UTC
Subtract -4:20 gives slack at 12:07 UTC and not 13:08 UTC

Totaltide gives HW at Glenelg bay (just past the narrows) as 12:08 UTC and that ties up with Dover -4:20

It does look as if arriving at HW Ullapool wouldn't be slack water.

Only of academic interest as I'll be back on my boat in a few days. I think there's a big tide just after I get back. The range might be as much as 0.3m :D :D.
 
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Looking at it in a slightly different way, the Admiralty Tidal Stream Atlas gives the main stream through Kyle Rhea at "5 hrs before HW Dover (0h 50m before HW Ullapool)" as northish 19,34 while at "4 hrs before Dover (0h 10m after HW Ullapool)" it is 02,04 southish. This therefore suggests that slack water is around HW Ullapool which is 4h 10m before HW Dover. Using the HW times from Reeds for 14th April this would give you slack at 1409 BST using Ullapool or 1317BST using Dover. This suggests that using Dover (which is the primary reference) would be better if using the Atlas.

The Admiralty leisure chart folio SC5616 has a tidal stream diagram for Kyle Rhea printed on the reverse of chart 5616.1 which shows the stream running north at 3 to 4 knots at 35 minutes before HW Ullapool and running south at 3 to 4 knots at 25 minutes after HW Ullapool, suggesting slack about HW Ullapool. For some reason, the chartlets are timed at 6,5,4,3,2 hours before HW, then 35 minutes before, 25 minutes after, and then 1h 25m, 2h 25m and so on until 5h 25m after HW Ullapool.
 
Far better to use the nearest Standard Port referred to. Dover must be ~ 1,000 sea miles away, with as mentioned complex variations due to Spring < > Neap and inter range variations. Another significant factor on any tidal prediction anywhere including at a standard port is that of weather, because both wind and barometric pressure can have a significant affect on both tidal height and local current flows associated with the height.
 
I've been sailing out of Arisaig since 1992, and whilst it's not much help to your calculations I've never referred to HW Dover. Always used either Oban or Ullapool.
For going through the narrows:-
North going stream starts at LW Ullapool
South going stream starts at HW Ullapool,
and if going from Kyle of Loch Alsh (or Totaig) leave at HW plus an hour and you have the tide running with you all the way down the Sound of Sleat (and, into Arisaig).
Mike

Editted bit, I suppose I should add that wind has a big effect on the times of slack water, and creates dangerous overfalls at either end if strong wind against tide.
It's safe to go through at any time with the tide with you subject to wind.
 
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I used to have a mooring actually in Kyle Rhea just by the ferry slip. Having been swept through like a cork on one occasion, I used to tide plan with great care after that. One thing I noticed, no matter what tide table I used or which pilot book direction, when I arrived to pass through I could actually have managed it earlier.
 
After bemoaning to my wife by phone yesterday that I had met a foul tide on entering Kyle Akin from under the Skye bridge she went and found this post. Which somewhat exonerated my tidal calculation not before she had lambasted me for getting it wrong. By my calculation I should have been two hours into the East Going which would give me a fast passage through all the narrows popping me out of Kyle Rhea like a cork out of a bottle. However I picked up a visitor mooring at the Kyle of Lochalsh to ponder my calculations even calling the inshore lifeboat I'd spotted out in the stream to ask which way the tide was running and to be advised it was against me if I was heading.south. So 5 1/2 hours after when I'd calculated the east going stream should start I decided to poke my nose out to see which way it was running . Yaberdaberdoo it was going my way which gave me a reasonably fast passage motoring through Kyle Rhea with about 3/4 knots of tide with me.
To quote Martin Lawrence - 'Tidal streams in Kyle Akin vary widely between springs and neaps, and are affected by barometric pressure, wind direction, rain and melting snow.
At neaps , the wind in particular may reverse the tide direction of flow, a southerly or southwest wind tending towards a west-going stream and a northerly wind tending towards an east-going stream'.
So the conditions yesterday for me were neaps, strong south westerly and buckets of rain for days if not weeks.
 
grandpaboat said:
After bemoaning to my wife by phone yesterday that I had met a foul tide on entering Kyle Akin from under the Skye bridge she went and found this post. Which somewhat exonerated my tidal calculation not before she had lambasted me for getting it wrong. By my calculation I should have been two hours into the East Going which would give me a fast passage through all the narrows popping me out of Kyle Rhea like a cork out of a bottle. However I picked up a visitor mooring at the Kyle of Lochalsh to ponder my calculations even calling the inshore lifeboat I'd spotted out in the stream to ask which way the tide was running and to be advised it was against me if I was heading.south. So 5 1/2 hours after when I'd calculated the east going stream should start I decided to poke my nose out to see which way it was running . Yaberdaberdoo it was going my way which gave me a reasonably fast passage motoring through Kyle Rhea with about 3/4 knots of tide with me.
To quote Martin Lawrence - 'Tidal streams in Kyle Akin vary widely between springs and neaps, and are affected by barometric pressure, wind direction, rain and melting snow.
At neaps , the wind in particular may reverse the tide direction of flow, a southerly or southwest wind tending towards a west-going stream and a northerly wind tending towards an east-going stream'.
So the conditions yesterday for me were neaps, strong south westerly and buckets of rain for days if not weeks.
Click to expand...
I understand those moorings have nor been serviced.
 
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