Rule of Twelfths Spreadsheet

TIDAL HEIGHT CORRECTIONS RELATED TO PRESSURE

Sorry Sticky, you can print it twice but those classroom figures still don't convince!
Anyone berthed in a harbour behind a cill, or on a drying berth, soon learns to take them with a pinch of salt.
IMHO the differences are consistently much greater: it's not unusual to have a discrepancy of 0.3m when pressure is around 15mb from the norm.
At Chimet today the tidal height of 4.6m at morning HW was 0.4m above the Admiralty prediction, yet pressure was 1012mb, with no prolonged onshore winds.

Anyone care to comment? (apart from criticising the piece of kit on West Pole beacon!)
 
Like all the classroom simplifications, it does not impress in the English Channel.
Compare these two:
http://www.pol.ac.uk/ntslf/sadata_tgi_ntslf_v2.php?code=Weymouth&span=1

http://www.pol.ac.uk/ntslf/sadata_tgi_ntslf_v2.php?code=Portsmouth&span=1

I doubt the pressure has been ever so different between Portland and Solent today. I think this illustrates that the tide may be more complex than the YM syllabus would have you believe. Trying to predict height to better than a foot or so comes with a health warning.

I believe if you register with pol.ac, you can access some of their surge models and get real daily predictions, taking into account atmospheric pressure distribution.
 
Tidal heights and barometric pressure

Sorry squire, you've lost me!

FWIW, Chichester HW this morning was about 4.8m against Easytide's prediction of 4.4m, while pressure is currently 1021mb. And the wind has been consistently offshore.

But I wasn't going to take my Yachtmaster again anyway . . . .
 
It's hard to comment with any clarity on the tidal height this week.
The wind has been mostly in the north and west.
Portsmouth has been very close to predicted height apart from higher high tides, by about a foot on two mornings.
Mostly this tells me that the predictions are always to be treated with caution.
Maybe a north wind backs up the tide at Dover, and this influences the whole channel?
I'm inclined to avoid simplistic cm per millibar corrections and just trust the predictions a little less whenever we are not having 'average' weather. These corrections may be valid terms in a more complex model, but I feel they are out of context in the general scenario of the central English Channel.
If anyone has gathered lots of data, I'm interested, but practically I prefer a couple of metres of water below the keel whenever possible.

A semi-relevent factoid: There is a point in the North Sea where the tidal range is normally less than 200mm!

Bear this in mind when extrapolating tide at a port to some point offshore.
 
Just jumping in here: in this far-flung part of the dark continent, the examinations that were thought out years ago when we were the pariahs of the Known World were loosely based around the RYA's old curriculum- probably dating from when Nelson was studying for his tickets. The exams accumulated a whole lot of navel-fluff and assorted grot over the last 35-odd years, and so they really DO ask silly stuff like 1cm-accuracy when calculating tidal range. In reality, we have a very small tidal range out here, and the time-differences between ports are something like a minute or two. Whoever the candidate is who is getting befuddled by the exams would be better served by showing mastery of the concept, and loudly denouncing it as a time-waster. Tell him or her that the prudent sailor would KNOW whether there is enough water to leave the dock or not, and if not, would know whether to wait an hour or so, or to give the whole thing up for another day.
I used to get quite irritated when i was an instructor, because the exams were focussing on assinine junk like this, and the infamous Question 27, whilst they should be concentrating on teaching people things how to sail our coast safely...
 
Adamastor said:
Just jumping in here: in this far-flung part of the dark continent, the examinations that were thought out years ago when we were the pariahs of the Known World were loosely based around the RYA's old curriculum- probably dating from when Nelson was studying for his tickets. The exams accumulated a whole lot of navel-fluff and assorted grot over the last 35-odd years, and so they really DO ask silly stuff like 1cm-accuracy when calculating tidal range.
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I used to get very fed up with RYA exams which, for example, asked you to work out departure times - based on clearance over a destination sill thirty miles away - to the minute/

On reflect it's not quite as daft as it sounds. Doing the calculation to the same precision as the supplied data is an easy way to check that the right calculation has in fact been done. Anyone might guess "6am" but it would be much less likely to get "6.07am" by accident.

Of course you also have to stress that in Real Life tides aren't accurate to a centimetre, leeway isn't always exactly ten degrees (or was it four?) and boats don't stick to whole numbers of knots!
 
Ubergeekian said:
...Of course you also have to stress that in Real Life tides aren't accurate to a centimetre, leeway isn't always exactly ten degrees (or was it four?) and boats don't stick to whole numbers of knots!
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I think this could be stressed a whole lot more in the shorebased stuff.

I know HOT questions usually include some kind of 'clearance under the keel' but I don't remember any discussion of what safe clearance should be.

The same with predictions - accuracy is often stressed but the practicality of it and interpretaion is often skimmed over a little.

Precision isn't accuracy - you can be precisely wrong!

The biggest factor in whether a tide curve is sinusoidal is local geography, for some areas R of 12ths is pretty accurate. But the tricky bit is knowing where the R of 12ths is not accurate. Porthmadog for instance, easytide have given up on that one!

I think awareness of the factors involved is more important than knowing how to calculate everything to the nth degree. For example Re: lw395's factoid - I doubt I will ever use a Co-tide/co-range chart to calculate an offshore tidal height - but I'm glad I know what theyre for and how the HOT offshore is affected...
 
alant said:
There's a rule of tenths, which should be easier.


Rule of tenths.
The rule of twelfths works well enough for anyone used to working with feet, fathoms, etc.

For anyone more at home working in decimal the rule of tenths might be easier to use.

So, instead of using twelfths uses percentages.
•10% for the 1st hour of range


•15% for the 2nd hour of range


•25% for the 3rd hour of range


•25% for the 4th hour of range


•15% for the 5th hour of range


•and 10% for the 6th hour of range


Read more: Rule of Twelfths for quick tidal estimates http://www.diy-wood-boat.com/Rule_of_twelfths.html#ixzz12j5QqBNi
Under Creative Commons License: Attribution
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I like that.

I am not sure about anyone else , but having been schooled in decimal (but before calculators became the norm) and usually being quite good with mental arithmetic, I have always wondered at people who say the rule of 12ths is easy to do in your head.

Trying to work out in my head 1/12th of a range of 4.9 (for example) is hard work for me.

Or maybe I am missing something and there is a simple trick akin to the one I use for VAT....10% + half of that (5%) + half of that (2.5%).

can anyone enlighten me?
 
reddragon2 said:
Trying to work out in my head 1/12th of a range of 4.9 (for example) is hard work for me.
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Personally, if I'm using the rule of twelfths, I am usually already approximating so, I'd probably use 4.8 m for the range - which is easily divisible by 12

The mental arithmetic might go:

HW 1808, call it 1800 (if I haven't already done so)

Range is about 4.8 m

It's now 1615, call it 1600 ... about 2 hrs before HW ... 3/12 left to rise.

3/12 is a quarter of 4.8 - that is 1.2 m left to rise

Interestingly, applying the rule of twelfths "formally" to the 4.9 m range gave 1.1 m (HW 1808, time "now" 1615)

I have assumed a flood tide. If I were working towards LW and clearance were of interest, I might even assume a range of 6 m to give me a safe (ish) margin. This gave me 1.5 m left to LW ... (rule of twelfths "formally" to the 4.9 m gives the same 1.1 m ...)

Of course applying this earlier in the tide could give a wider variation - but I'm not using the rule of twelfths when clearances and depths are critical - even in a place where I know the tidal curve is (more or less) suitable
 
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