I've just read Septembers PBO

[Peppermint]

I\'ve just read Septembers PBO

There's a letter in there about calculating the rule of 12th's. This is apparently difficult to do in your head so with much use of the calculator this guy has come up with a solution and a long letter. I'd reckon it was easy enough without all the fuss and if you need to simplify it can't you just call a 12th 8%.

I'm sure he's a lovely guy, and he explains that the RYA are at the root of his taking this on, but I think he should sail more and calculate less.

He does raise an interesting point though. If people are indeed working to the Nth degree is that why so many groundings are now being reported?

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[billmacfarlane]

Re: I\'ve just read Septembers PBO

My philosophy has always been if you've got to work it out to the nth degree then wait another half hour at least. I've always used the rule of twelfths and it's never let me down.......yet!!!

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[Guest]

Re: I\'ve just read Septembers PBO

Always a scrap of paper and a pencil for 12ths ..... never thought it needed much more .......


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

At last the practical way ...

If it aint enough now ----wait a bit !!

How many times have we all worked out a reasonably accurate tide and found a) high pressure keeps it down and we have to wait a bit, b) low pressure / surge gives more water and we could have gone in earlier ......

Mmmmmm so much for nth degree !!!!

Anyway I have a bilge keeler, keep pushing the mud till she slides over !!!! Joking !! sort of ......... I do have two tram lines leading from my marina berth in the mud !!!!!



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Sybarite]

Re: I\'ve just read Septembers PBO

The French have a term "le pied du marin". Whatever your 1/12ths calculation is, give yourself the extra "sailor's foot".

John

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[vyv_cox]

Tidal calcs

You highlight my biggest complaint about RYA training tidal calculations. I would never act upon the result achieved by the standard calculation "at what time will my boat drawing 1.4 metres be able to cross Keelbasher Rock?" I add at least 0.5 metre to such results and probably start later or earlier as required. So why do they teach it? Maybe this accounts for the increased number of groundings.

Except in very tricky situations, e.g. the Wats behind the Freisian Islands, I always use the 12ths rule and round down at every opportunity.

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[qsiv]

Re: Tidal calcs

I'd agree there - I like lots of water under the keel (and we draw 2.7m to start with) - in general anything less than a fathom of clear water on passage and I wont play. The only time I find I want any real precision is when anchoring - if it looks close according to twelths, then I'm afraid I go back either to port curves or perhaps these days to a reputable tidal calculator. From an insurance POV, I feel that I'm less likely to be considered negligent if using the Admiralty Totaltide, as this meets MCA and SOLAS requirements. Whats more it also does bridge clearance calcs, which always scare me more than grounding calcs - perhaps I do them less often, perhaps it is more easy to inflict damage. Last summer I redid the calculations for the Pertuis Breton bridge (La Rochelle to Ile de Re) umpteen times before convincing myself we should clear it by a metre or two - but I stillwent through at about .1 of a knot!

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[Guest]

and im on october PBO

it arrived this morning!!
stu

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[johndf]

Re: Tidal calcs

Anyone who suggest that you should use the twelfths rule without adding in a safety margin must be crazy. My RYA yachtmaster instructor was quite clear about the fact that the twelfths rule is a rough guide, subject to all sorts of errors and inaccuracies. I seem to remember that questions such as the one you quote were worded to say something like "At what time will my boat drawing 1.4 metres be able to cross Keelbasher Rock, with a clearance of 1 metre below the keel?"

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[Reap]

Re: Tidal calcs

Vyv I think you are incorrect to critisise the RYA on this subject.

As an instructor (practical) I always teach these quicker ways of tidal calcs.
Rule of twelfths is a good way. But also you can usually interpolate from the almanac by eye quite easily or by a quick calc on paper without resorting to graphs etc.
An Yachtmaster examiner would expect you to do it this way, and allow some extra for safety. But he would also expect you to be able to demonstrate you can do it accurately to the nth degree if necessary, otherwise how can you have a full of understanding of the calculation and therefore be able to safely use a short cut.

During my own training during Day Skipper theory, leaving a safety margin was always emphasised.

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[BrendanS]

Re: Tidal calcs

"At what time will my boat drawing 1.4 metres be able to cross Keelbasher Rock, with a clearance of 1 metre below the keel?"

And my question back is always, are we talking about a flat calm, or 3 metre swell? Many of the courses never cover this bit.

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[john_morris_uk]

Re: RYA course content and undue conservatism

The RYA courses should/do cover this. Whenever I instruct theory or practical I always talk about the influence of weather and swell.

When I was in Audierne a week or two ago, as we arrived a boat was leaving. The owner (who happens to be a regular and interesting contributor to PBO) had a bit of a chat and said that he had to leave to pick up a bouy outside as otherwise it would mean getting up at 0500. It made me check and reckeck the tides as I was planning to leave at 0700.

Here are the facts. The pilot book says access at HW plus or minus two hours. However we had just come in and without concentrating on finding the very deepest water in the channel had never been in anything less than 4 metres. A few calculations later showed that at 0700 next morning, although it would be a couple of hours before low water there would only be 1.6 metres less than where we were on the tidle curve as we had entered. We draw 1.8 metres - therefore loads of water beneath the keel at 0700. (It was flat calm on a mud/sandy bottom)

We left at 0700 having had a pleasant run ashore and never had less than 1 metre beneath the keel.
We waved to the other yacht as we rounded the Raz.

Moral - sometimes accurate tidal calculations do matter - if only for peace of mind. (I did the calcs using 1/12's as well out of interest and they were reasonably close as they often are.)

If you are going to cut it fine, choose the circumstances!

Pilot books are always miserable and cynical and sometimes excessively conservative: you should think in a seamanlike way and act on the conditions that are affecting you...

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[vyv_cox]

Re: Tidal calcs

I don't think I was criticising the RYA, only the instructors of RYA courses. And books, of which I have several.

All of them make the point that the height of tide is not predictable to high degrees of accuracy, for a variety of reasons. Universally they go on to say: But we'll ignore that, calculate the tide height to two decimal places but don't forget to add a metre or so just in case. It seems to me that this gives the wrong message.

Personally, if I don't use the 12ths rule I always use the curves in Reeds, which seem to me to be far the easiest way to obtain an accurate value. Assuming I wanted one, that is.

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[Birdseye]

Re: Tidal calcs

I cant see an alternative to teaching the use of the 12ths rule, or tidal curves, and then explaining that neither approach is totally accurate or reliable for a variety of reasons - made even worse when using secondary ports. Trouble is, people do have difficulty understanding the concept of probability and of accuracy - the assumption is usually made that tides can be calculated to 3 decimal places, and you have to explain that is not correct.

A more serious problem these days is the number of young people who cannot do the rule of 12ths because they cannot divide by 12 (or multiply by .08). Some never see graphs at school and so dont understand what a tiidal curve is.

Mind you, all that is as nothing compared to trying to teach some people the "course to steer". Anybody ever use it in real life?

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[vyv_cox]

Re: Tidal calcs

I probably didn't explain myself as well as I might.

I can instruct this subject in two ways. One seems to me to be the standard one, definitely the one in my instruction books and always the one used in competitions in YM. This goes "Calculate for the keel depth, then add an estimated safety figure that seems to suit the conditions".

A safer one, it seems to me, is to do it the other way around. "What is a reasonable safety figure for today, bearing in mind waves, air pressure, etc. Add this to boat draught. Now calculate". The result will be the same but the message is very different.

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[ditchcrawler]

Re: Tidal calcs

Instead of twelths I use %s for the 6 tidal hours( which are more than 6 hours of course)
1sthour-10%
2ndhour-15%
3rdhour-25%
4thhour-25%
5thhour-15%
6thhour-10%

each of these being the % of the range against high or low water.
I find these easy to calculate allowing a margin for all the other varibles mentioned by others


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[Gunfleet]

Re: Tidal calcs

You could do it with coefficients if you want to. Or run the calculation in hex. In other words, what is the point of this?

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[MainlySteam]

Re: Tidal calcs

Assuming you are critcising the percentage series proposed - Working in twelfths is easy if working in fathoms, but inconvenient when working in meters (which, of course, most charts are in). It is easy to work in tenths (10 percents) in metres and the series proposed is both very easily remembered and works in tenths or half of a tenth (5 percents) making the calculations trivial mental ones.

It is easy enough on a calculator by assuming the rate of change in height is a pure sine wave, but now I can see that I can do it accurately enough in my head.

Will be adopting it myself.

John

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