Rule of 12ths, how accurate is it?

[johnalison]

In the spirit of EU conformity I have abandoned twelfths and use the tide information in my plotter. However, any rough and ready shortcut in navigation is perfectly acceptable so long as you know that this is what you are doing and have some idea of the limits of your accuracy.

 

[NormanS]

Quote: "Electronics are more accurate...."

The other day there was a bit of a blow, and at Fort William, for example, the tide rose about 2m higher than predicted, and caused some flooding. In what way, in these circumstances, would electronics be more accurate?

 

[Jokani]

Quote: "Electronics are more accurate...."

In what way, in these circumstances, would electronics be more accurate?

My understanding is that neither would be accurate, they are predictions that do not take in to account circumstances that may arise in the future?

Unless I have misunderstood, circumstances aside, electonics would be more accurate?

 

[LadyInBed]

In one of his top tips in Yachting Monthly I recall Tom Cunliffe saying for a quicker working out, using rule of tenths is acceptable. I’ve done it that way for a while now and have yet to be caught out. Maths was never my strongpoint.

I've not come across the Rule of Tenths before. I found a write up at https://classicsailor.com/2018/04/rule-of-tenths/

 

[zoidberg]

'The Rule of Twelfths' is a useful starting point, for locations that are relatively standard in their topography e.g. relatively straight adjacent coastline, no tidal 'resonant chamber', insignificant hinterland. One needs to consider whether the other factors are likely to have an effect significant to what one intends. The difference of atmospheric pressure from Standard 1013mb, for example.... the effects of sustained onshore/offshore wind.... The effects of significant run-off rainfall on the nearby hills/moorlands ( not likely to concern the Sydney-based Neeves at present ).... the shape of the drainage basin....

As example of the latter two factors, the predicted 'curve' at the deep-waterTidal Reference point for Plymouth/Devonport is a quite regular sinusoid. Actual behaviour at jetties/moorings tucked up one of the arms of the Tamar estuary is markedly different, as those arms are broad, shallow mudflats with deeper gullies. If planning to come alongside, or dry out, at one of the tucked-away boatyards, it is prudent to ask the yard foreman first for advice. Sustained heavy rain up on the moors drains a lot of fresh water down into tributaries, modifying both High and Low Water times and heights.

While the prologues to Admiralty Tide Tables, etc. mentions such influences, they cannot be predicted and tabulated. That's when seamanship and experience is wanted, so best ask several locals - and average the result!

 

[LadyInBed]

It's not the time so much, 20 mins plus or minus would not phase me - 30 cm might be more critical.

The rule of twelfths isn't the significant factor if you are more concerned with the height of the tide.
Local to me we recently had Spring tide and a very low pressure area passing through, so the actual HW was considerably higher than that predicted, the time of HW wasn't much different to that predicted.

 

[JumbleDuck]

So "spot on" is the answer.

Yup. Sorry, I posted that in a hurry as I had an appointment and so didn't have time to give an explanation. It's a diagram I produced for a discussion here in (if the file dates are right) 2010. Basically it shows that for sinusoidal tides you're never more than 10% out and you can half that with some linear interpolation ("Rules of twelfths says 2m half an hour ago and 2.4m in half an hours, so it'll be about 2.2m now") around mid-tide. Since I mostly sail in areas with 4m range at most, that means at worst 40cm error, and I can live with that.

 

[Elessar]

Quote: "Electronics are more accurate...."

The other day there was a bit of a blow, and at Fort William, for example, the tide rose about 2m higher than predicted, and caused some flooding. In what way, in these circumstances, would electronics be more accurate?

In the way that the electronics display astronomical predictions more accurately.

 

[Elessar]

12 feet tides - unreal!!

50 ft where I'm from originally and learned to sail.......

 

[Little Dorrit]

I can't help thinking not so much 'how accurate the rule of 12ths is?' but what an anachronistic lot we really are.

 

[ghostlymoron]

Yes
Yes, but that's still just a prediction. There are lots of variables, like wind and atmospheric pressure. Most places where we sail the twelfths rule is perfect. It used to be even easier before metrification, when the tidal range was about 12 feet. ☺

That's what I meant by weather related variables.

 

[doug748]

"I can't help thinking not so much 'how accurate the rule of 12ths is?' but what an anachronistic lot we really are. "



The older I get the more I rely on the rule of 4:

Near HW
Near LW
1/2 tide ish going up
1/2 tide ish falling

.

 

[prv]

The other day there was a bit of a blow, and at Fort William, for example, the tide rose about 2m higher than predicted, and caused some flooding. In what way, in these circumstances, would electronics be more accurate?

Because they reflect the fact that, as I can see by looking at the curve for Corpach next door, the tide there rises slightly faster than it falls. Whereas the rule of twelfths assumes they happen at the same rate. The difference in this case probably isn't enough to matter much, but nevertheless, one is a little more accurate than the other.

Your "bit of a blow" is a pointless obfuscation since neither method allows for it.

Pete

 

[AntarcticPilot]

"I can't help thinking not so much 'how accurate the rule of 12ths is?' but what an anachronistic lot we really are. "



The older I get the more I rely on the rule of 4:

Near HW
Near LW
1/2 tide ish going up
1/2 tide ish falling

.

My watch - a Casio G-Force - provides that, using the simple lunitidal interval to obtain the time of high water. It's a useful approximation!

As others have said, pretending that the actual height of the tide in inshore areas matches the predictions is a sure way of going aground. It'll be close a lot of the time, but it won't be a large minority of the time. Meteorological factors, river currents and basin effects away from tide-gauges mean that both the time and the height of the tide can be off substantially. For most purposes, the rule of 12ths is quite accurate enough; of course, local knowledge is needed to predict effects such as the dual tides around the Solent. My own rule is that if a passage depends on knowing the tide to 20cm, then I'm not going there - the actual tide could easily be 20cm less than the prediction!

 

[Refueler]

Your "bit of a blow" is a pointless obfuscation since neither method allows for it.

I would differ with you as basically tide tables paper or electronic assume standard atmospheric conditions and zero wind effects - making your original statement of Electronic more accurate wrong.

If you take Admiralty Tides and compare with the Wtides as example - you will find differences in height and time because of the different reference standards used.

 

[ditchcrawler]

If you find Twelfths difficulty then for the six hour cycle this approximaion works, accepting that the 6 hours rise or fall may be 6 hours and some minutes. First hour1 0%, second hour 15%, third hour 25%, fourth hour 25%, fifth hour 15% and sixth hour 10%.
Each calculation being a percentage of the range for the tide of the day..

 

[Uricanejack]

I have set of tide tables, they are issued by our Bureau of Meteorogy and give me high and low tide each day against time (plus moon phases) - that's it.

Which is why I asked.

Its not the time so much, 20 mins plus or minus would not phase me - 30 cm might be more critical.

We have, roughly, 2m tides max.

Here, Pittwater, its all very conventional - NE Tasmania - different ball game (6 knot tidal flows, lows coming in of Southern Ocean etc).

Jonathan

I’m not familiar with Australian tide tables. So I might be wrong.
Any official tide tables, I have used, have a method for interpolating intermediate heights and times of tide. The methods be it a graph or tables is in based on resolution of sine wave. So perhaps a bit closer than the rule of 12 th. But the predictions have thier limitations anyway.
Where I sail. Range may be 15ft An error of a foot or more is quite reasonable to expectation .
If your range is only 4 ft. An error of 1 ft is probably generous,

Unofficial tide tables I get from the tackle shop for fishing are ussualy limited to times of high and low water.

While I did suggest ussing apps, gps ect.
Just because the calculation is resolved to decimal points. How would I know the calculation is not just applying the rule of 12ths or a sine wave resolution to the same high and low water. Which would still be subject to the same limitations. As the graph or table in the tide tables. But less stepped. Ie just closer to JDs curve.

 

[CAPTAIN FANTASTIC]

I found that the rule of 12th works very well in the Bristol channel because the tidal sine curve period is symmetrical; unlike the Solent where the sine curve is unsymmetrical.

 

[Refueler]

I’m not familiar with Australian tide tables. So I might be wrong.
Any official tide tables, I have used, have a method for interpolating intermediate heights and times of tide. The methods be it a graph or tables is in based on resolution of sine wave. So perhaps a bit closer than the rule of 12 th. But the predictions have thier limitations anyway.
Where I sail. Range may be 15ft An error of a foot or more is quite reasonable to expectation .
If your range is only 4 ft. An error of 1 ft is probably generous,

Unofficial tide tables I get from the tackle shop for fishing are ussualy limited to times of high and low water.

While I did suggest ussing apps, gps ect.
Just because the calculation is resolved to decimal points. How would I know the calculation is not just applying the rule of 12ths or a sine wave resolution to the same high and low water. Which would still be subject to the same limitations. As the graph or table in the tide tables. But less stepped. Ie just closer to JDs curve.

I\ve highlighted a part in red ... c'mon - you know that that's not strictly correct. Official tide tables use a factor calculated for a particular location which alters the interpolation to try and mimic the curve at that location. HMSO Tide Tables have a whole section devoted to this at back of the book. As well as the curve at first page of location ... giving the predicted rate of tide at each time interval.

 

[LittleSister]

I can't help thinking not so much 'how accurate the rule of 12ths is?' but what an anachronistic lot we really are.

Yes, we need to get with these new modern tides: they're so different.