jamesdestin
Well-Known Member
Morning all, does the rule of twelfths work in the BC?
Rule of twelths
- Thread starter jamesdestin
- Start date
graham
Well-Known Member
Yes,it works very well.Sometimes it will be a bit out within an hour of low tide but the rest of the time its usefull.
wotayottie
Well-Known Member
the rule of 12ths works reasonably anywhere where the tidal curve approximates to a sine wave. doubt that it would be worth much by the time you got to Lydney / Sharpness
PCUK
Well-Known Member
A new European directive is now making it illegal as it is Imperial. A new rule of tenths is now mandatory for all EU countries. Gordon Brown is particularly keen on it, being the leader of Euro-Labour!
tonybarebones
Well-Known Member
This for those who may be unfamilier with the Rule of Twelths... /forums/images/graemlins/confused.gif
TIDES: Estimating tide rise and fall. The simplest rule of thumb for the guestimation of the height and speed of flow of the tide is the twelfths rule:
1 Hr. after low/hi water 1/12 of the tide height will rise/fall
2 Hr. after low/hi water 2/12 of the tide height will rise/fall
3 Hr. after low/hi water 3/12 of the tide height will rise/fall
4 Hr. after low/hi water 3/12 of the tide height will rise/fall
5 Hr. after low/hi water 2/12 of the tide height will rise/fall
6 Hr. after low/hi water 1/12 of the tide height will rise/fall
The speed of flow of the tide is approximately proportional to the amount of rise per hour, thus the fastest tide flows will occur at mid tide.
For greater detail consult a local chart and tidal atlas.
TME DIFFERENCES ON CARDIFF
Avonmouth +15 min
Barry - 15 min
Ilfracombe - 60 min
Milford Haven - 60 min
Minehead - 20 min
Newport +10 min
Swansea +45 min
INTERESTING FACTS: When during a year can we expect to find the largest tides? A day or two after the full or new moon nearest to the equinoxes. The spring equinox is usually the 21st March, and the autumn equinox, the 23rd September.
Some years have tides that are notably higher than other years. 1997 was a significant year, as will be the year 2015. For really favourable conditions - you will have to wait around until the year 3182. Even then, the tides may only be 1 or 2 cm higher than in 1997.
TIDES: Estimating tide rise and fall. The simplest rule of thumb for the guestimation of the height and speed of flow of the tide is the twelfths rule:
1 Hr. after low/hi water 1/12 of the tide height will rise/fall
2 Hr. after low/hi water 2/12 of the tide height will rise/fall
3 Hr. after low/hi water 3/12 of the tide height will rise/fall
4 Hr. after low/hi water 3/12 of the tide height will rise/fall
5 Hr. after low/hi water 2/12 of the tide height will rise/fall
6 Hr. after low/hi water 1/12 of the tide height will rise/fall
The speed of flow of the tide is approximately proportional to the amount of rise per hour, thus the fastest tide flows will occur at mid tide.
For greater detail consult a local chart and tidal atlas.
TME DIFFERENCES ON CARDIFF
Avonmouth +15 min
Barry - 15 min
Ilfracombe - 60 min
Milford Haven - 60 min
Minehead - 20 min
Newport +10 min
Swansea +45 min
INTERESTING FACTS: When during a year can we expect to find the largest tides? A day or two after the full or new moon nearest to the equinoxes. The spring equinox is usually the 21st March, and the autumn equinox, the 23rd September.
Some years have tides that are notably higher than other years. 1997 was a significant year, as will be the year 2015. For really favourable conditions - you will have to wait around until the year 3182. Even then, the tides may only be 1 or 2 cm higher than in 1997.
graham
Well-Known Member
[ QUOTE ]
you will have to wait around until the year 3182. Even then, the tides may only be 1 or 2 cm higher than in 1997
[/ QUOTE ]
Could you remind us a bit closer to the time.I'll take some pictures and post them on here. /forums/images/graemlins/grin.gif
you will have to wait around until the year 3182. Even then, the tides may only be 1 or 2 cm higher than in 1997
[/ QUOTE ]
Could you remind us a bit closer to the time.I'll take some pictures and post them on here. /forums/images/graemlins/grin.gif
damo
Well-Known Member
A YM instructor once told me that it wouldn't be adviseable to use the twelfths rule in the BC, because of the big ranges. So being a contrary bu**er I took about 20 tides from 2007, with a range of springs and neaps, and found some averages.
The twelfths agreed with Belfield, Reeds and WTides to within a few percent on time and height, so I now have a little excel table with ranges from 1m to 16m and the corresponding tide height for each hour.
I used to do all the calcs very carefully when anchoring to find minimum depths etc, but for the last couple of years, having seen what happens on a daily basis compared to prediction I think twelfths are as good as anything. ALL the different tables and programs are inadequate at some point, either the algorithms differ slightly so times/heights don't agree (eg Belfield is very accurate on Springs, and can be way out for LW Neaps), or the conditions on the day may surprise you.
For example, on the last neaps the tide turned an hour ahead of LW prediction, and the tidal heights on the ebb were about 1m below at predicted time. High pressure is said to depress the tide, but that's not always the case.
So if you want to calculate to the minute and centimetre in the BC, I would say don't bother. Using the Rule of Twelfths is sufficient, because IMHO if you plan to leave a few cms under your keel then you are asking for trouble. And if the tide goes against you when trying to get past that tidal gate (ie between Hartland and Sharpness /forums/images/graemlins/smile.gif ) it may be that you haven't miscalculated - it's what it has decided to do on the day!
The twelfths agreed with Belfield, Reeds and WTides to within a few percent on time and height, so I now have a little excel table with ranges from 1m to 16m and the corresponding tide height for each hour.
I used to do all the calcs very carefully when anchoring to find minimum depths etc, but for the last couple of years, having seen what happens on a daily basis compared to prediction I think twelfths are as good as anything. ALL the different tables and programs are inadequate at some point, either the algorithms differ slightly so times/heights don't agree (eg Belfield is very accurate on Springs, and can be way out for LW Neaps), or the conditions on the day may surprise you.
For example, on the last neaps the tide turned an hour ahead of LW prediction, and the tidal heights on the ebb were about 1m below at predicted time. High pressure is said to depress the tide, but that's not always the case.
So if you want to calculate to the minute and centimetre in the BC, I would say don't bother. Using the Rule of Twelfths is sufficient, because IMHO if you plan to leave a few cms under your keel then you are asking for trouble. And if the tide goes against you when trying to get past that tidal gate (ie between Hartland and Sharpness /forums/images/graemlins/smile.gif ) it may be that you haven't miscalculated - it's what it has decided to do on the day!
A
Alcyone
Guest
Ramsey Sound - the fastest rate of tide is at high/low water, and the slacks are around mid tide. As you move out, towards Grassholm and the Smalls, the slacks occur at 4 and then 5 hours after HW. Took me years to believe it, but it's true.
seedog
Well-Known Member
As a newbie to BC I have spent some time looking at what I need to know and 'Wotayatties' comments about the curve are definitely worth considering. In the upper reaches of the Severn and the Avon the curve is distinctly bigger on the right than the left meaning the ebb is 'larger' than the flood which means the rule of 12ths is not accurate in those situations.
Intrigued to understand what is happening in Ramsay sound. Can someone explain?
Intrigued to understand what is happening in Ramsay sound. Can someone explain?
damo
Well-Known Member
My main point earlier was that often the predicted times/heights from the curves don't agree with the reality on the day, so then the Rule of Twelfths is OK for practical purposes. You are right though that some areas are weird/extreme enough that it may be inadviseable. And the upper Severn Estuary is world-renowned for its wacky tidal behaviour!
And oddities such as Ramsay, the Solent, parts of the west coast of Scotland etc require a bit more local knowledge I understand /forums/images/graemlins/smile.gif
And oddities such as Ramsay, the Solent, parts of the west coast of Scotland etc require a bit more local knowledge I understand /forums/images/graemlins/smile.gif
A
Alcyone
Guest
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.
Golden rule - assume nothing, don't ever believe you know, and speak to everyone.
damo
Well-Known Member
Re: Rule of twelths (...numerophobes look away now!)
Well, Tony_M got me thinking, so I have been having a play with some numbers tonight /forums/images/graemlins/smile.gif
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 /forums/images/graemlins/smile.gif 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 /forums/images/graemlins/smile.gif ) 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 /forums/images/graemlins/grin.gif )
Well, Tony_M got me thinking, so I have been having a play with some numbers tonight /forums/images/graemlins/smile.gif
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 /forums/images/graemlins/smile.gif 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 /forums/images/graemlins/smile.gif ) 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 /forums/images/graemlins/grin.gif )