Alcyone
regular
Reged: 09/04/2008
Posts: 320
Loc: Bridgend, Wales, UK
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I can't explain Ramsey, but it does seem that the further offshore you go, the nearer 'the norm' the tide seems to be. Basically slack is 2.5 hrs after low 3 after high. It runs North either side of 'high' and south either side of low. Another strange fact, and a bit hairy if you are diving it, is the speed of tidal pickup as the North going run begins. I've seen the tide take a 2ft diameter buoy under water 7 minutes after the end of slack on a big spring. Scared the **** out of me, as I'd been in 10 minutes before that.
Golden rule - assume nothing, don't ever believe you know, and speak to everyone.
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Damo
regular
Reged: 22/02/2005
Posts: 2614
Loc: k keeper,Portishead
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Well, Tony_M got me thinking, so I have been having a play with some numbers tonight 
I used Belfield software to get those heights, Reeds almanac and the Avonmouth curve for those numbers, and Damo's Patent Twelfths Chart for comparison (My chart goes in 0.5m steps for ranges, but I didn't bother interpolating)
I couldn't work out how to post a text table, so I cheated and used a screen-grab Make your own mind up, but I think it reinforces my point about leaving plenty of margin for error. (Pressure was 1022mB BTW). I find it disconcerting that Reeds and Belfield disagree by a metre or so at some points, and it would make an interesting exercise (IMO ) to get comparisons with Arrowsmiths and other tidal software predictions.
Obviously Avonmouth is at the more extreme end of the scale - the big ranges and skewed curve will exaggerate differences using Twelfths. I would predict it would be a closer match further down the Channel (the curve for Milford Haven is a lot more amenable)
(Ooops - I've just noticed that the Reeds times aren't BST. I'm sure you can add the hour yourselves  )
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Never be at a loose end with the Yosemite bowline 
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