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West coast Scotland & Dover based tides anomaly

CFarr

New member
Joined
19 Aug 2010
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404
Location
Me: Well inland. Boat: Bellanoch, Crinan Canal
Might be a bit long winded this so apologies in advance but, I have an issue with correlating times for the Corryvreckan with the Admiralty tidal stream atlas.

I got the information from various sources stating that the West going stream commences +0410 HW Oban.
HW Oban 28th March was 1502 (easytide) so therefore West stream starts 1912

Then I look in the tidal stream atlas with tides based on HW Dover (and Ullapool)
At 2 hrs before HW Dover the stream is shown running East.
At 1 hr before HW Dover the stream is shown running West.
Therefore I assume the change of direction happens somewhere on the cusp of the two.

HW Dover on the same date was 1828 so the HW tidal hour was 1800 to 1900 (ignoring the 2 mins).
Therefore 1 hour before is 1700 to 1800 and 2 hours before is 1600 to 1700.
The cusp of the two you would assume to be 1700 then (or somewhere 15 mins either side).

But that is over 2 hours different to the time based on HW Oban!
Assuming the Oban based times are correct due to it being the local port does that then mean that the entire Tidal Atlas based on HW Dover is then 2 hours out for that day?
I’m sure HW Oban and HW Dover are not a constant time apart and that this error will change but if you’re sailing in the area it does call the Atlas into doubt doesn’t it?
Or am I missing something here?

P.S. If you base it on the Ullapool times you get to 1800, so that's 1700, 1800 and 1912! Take your pick.
 

Mark-1

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22 Sep 2008
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2,703
Might be a bit long winded this so apologies in advance but, I have an issue with correlating times for the Corryvreckan with the Admiralty tidal stream atlas.
From memory of a trip this time last year the Pilot Book (Lawrence or CCC don't recall which) details differing offsets between Springs and Neaps (up to several hours) relative to Oban, both at the Corryvreckan, and that whole area. So if my memory is correct you can't accurately pin in to HW Oban without taking those large offsets into account.

Having said that, we used "Claymore's Definitive Tidal Gate" chart which does pin it to Oban and we went through the Corryvreckan without an drama fighting a weakish tide. (We were expecting the tide with us but as long as we got through we were happy).
 
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CFarr

New member
Joined
19 Aug 2010
Messages
404
Location
Me: Well inland. Boat: Bellanoch, Crinan Canal
From memory of a trip this time last year the Pilot Book (Lawrence or CCC don't recall which) details several hour offsets in the time of the change of tide between Springs and Neaps at the Corryvreckan and that whole area. So if my memory is correct you can't accurately pin in to HW Oban, or anywhere else, alone.QUOTE]

So it's just the times of the streams in the Corryvreckan that may not be accurate in the atlas and not all the other areas on the same page?

What I'm trying to get at is not a time to go through the Corry but how accurate the atlas is based on Dover tide times if the streams shown for my data were over 2 hours out?
 

Ubergeekian

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Joined
23 Jun 2004
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9,905
Location
Me: Castle Douglas, SW Scotland. Boats: Kirkcudbri
From memory of a trip this time last year the Pilot Book (Lawrence or CCC don't recall which) details differing offsets between Springs and Neaps (up to several hours) relative to Oban, both at the Corryvreckan, and that whole area. So if my memory is correct you can't accurately pin in to HW Oban without taking those large offsets into account.
As I recall the CCC book and the Admiralty publications have - or had - wildly different ideas about tide times round the Mull of Kintyre as well.
 

Mark-1

Well-known member
Joined
22 Sep 2008
Messages
2,703
From memory of a trip this time last year the Pilot Book (Lawrence or CCC don't recall which) details several hour offsets in the time of the change of tide between Springs and Neaps at the Corryvreckan and that whole area. So if my memory is correct you can't accurately pin in to HW Oban, or anywhere else, alone.QUOTE]

So it's just the times of the streams in the Corryvreckan that may not be accurate in the atlas and not all the other areas on the same page?

What I'm trying to get at is not a time to go through the Corry but how accurate the atlas is based on Dover tide times if the streams shown for my data were over 2 hours out?
The atlas will be fine, and it's perfectly acceptable to base it on HW Dover. You can totally trust it. (Well, as far as you can trust any tide information!)

However, if my memory is correct, there are some specific areas where the time of the change of tides varies a fair bit and these are mentioned in the Pilot Book with appropriate offsets.

Claymore's Definitive Tidal Gate Table:
http://fattie.freehostia.com/Scotland_April_2010/target59.html
http://www.yotblog.co.uk/Claymore/
 
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Sgeir

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Joined
22 Nov 2004
Messages
14,711
Location
Stirling
It's all a bit approximate, but would agree that the UKHO tidal streams chart more or less captures it; however the surface can vary with wind strength and direction, and, possibly, air pressure.

The easiest and safest passage (both ways iMHO) is in deep water close to the Jura side. Best to get there at about the start of slack water - if you're too early you'll soon find out; you'll either be able to make no headway, or you will have an exciting but fairly safe passage at speed (and you may need the engine running to steer through the eddies). Haven't checked the chart, but from memory there's a rock to watch out for on the west side, somewhere near the entrance to Bagh Gleann nam Muic (spelling?), the anchorage atn the north side of Jura.

I'm told that the Corryvreckan is a pussy cat compared to the Gray Dogs; Webcraft and some other brave souls on Scuttlebutt have gone through the Dogs, but I haven't.
 

bikedaft

Active member
Joined
16 Dec 2008
Messages
3,105
Location
tayvallich
have almost always found HW dover and admiralty tide charts are accurate enough eg to half an hour. always remembering BST! have never seen much of a reason to go by HW oban, when i use admiralty tide chart. tho obviously CCC tends to use this
 

agurney

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Joined
10 Jun 2009
Messages
1,496
I keep a set of pre-calculated corrected tides for the area on the internet so that I can access them using laptop, kindle, or whatever.

I haven't done the calculations for this year yet, but last year's are here: http://agurney.com/tides.pdf .. they'll give you some idea of the corrections I use (based on Oban), and I've found them to be pretty accurate over the years (making allowances for effects of pressure and wind direction).

Bear in mind that the tidal gates in this area are due to the differences in the height of the water either side of Jura, rather than the tide turning at HW/LW, and that the rule of twelfths doesn't apply.
 

agurney

Active member
Joined
10 Jun 2009
Messages
1,496
I keep a set of pre-calculated corrected tides for the area on the internet so that I can access them using laptop, kindle, or whatever.

I haven't done the calculations for this year yet, but last year's are here: http://agurney.com/tides.pdf .. they'll give you some idea of the corrections I use (based on Oban), and I've found them to be pretty accurate over the years (making allowances for effects of pressure and wind direction).

Bear in mind that the tidal gates in this area are due to the differences in the height of the water either side of Jura, rather than the tide turning at HW/LW, and that the rule of twelfths doesn't apply.
The arrival of the rain gave me the impetus to do the calculations .. help yourself if you are interested: http://www.agurney.com/oban-tides

They are in a format that suits me, but maybe not everyone's cup of tea.

Caveat emptor.
 
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