Tidal coefficient

[ylop]

I tend to make it my business to know the spring range, or else not even a % coefficient is going to help you. I just don’t find it very hard to look at the days tide, and compare the range to mean springs. Is it nearly, is it about 3/4, etc. It’scas accurate as is needed.

But the point is that is multiple arithmetic operations - what's todays range; what's the spring range; how do they compare - its not rocket surgery but its just quite neat if someone else has done that for you. Which makes this ironic:

Spending too much time on the maths is a navigational fail, in my books.

 

[Chiara’s slave]

But the point is that is multiple arithmetic operations - what's todays range; what's the spring range; how do they compare - its not rocket surgery but its just quite neat if someone else has done that for you. Which makes this ironic:

The point is, you don’t need to. You don’t need a stream, or a height for that matter, to 1%, or even 10% accuracy, that’s too fine a margin. The tides themselves are, like the weather, only predictions. A very close association with tidal streams around here has taught me that observation is more important than a book or chart that tells me what it’s supposed to do. It’s chaos in action. Treating tidal coefficients as anything more than ‘it’s a big one’ or ‘it’s about halfway to neaps’ is a hiding to nothing. And you can get that info easily in any case. Do you not carry in your head a loose tide clock? And whether it’s springs or neaps? If you sail once a month or less, maybe not, I guess.

 

[franksingleton]

Monty Mariner. Where are you?

 

[Nimrod18]

UK springs corresponds to coefficient 95 (also called VE, Vives Eaux)
UK neaps corresponds to coefficient 45 (also called ME, Mortes Eaux)
Coefficients range is from 20 to 120 (it's not an absolute percentage value)
Similarly to UK, French tidal atlases are drawn for coefficients 45 and 95, except the 2+2 digits are inverted, if UK data reads 1234, French would be 3412

Roberto, great answer, thank you. Same to RunHardAground - I shall take a look next time I charter and dig a little deeper. Franksingleton, this thread has been very useful.

One follow up. I use the Imray Tides app, that has a “Np -> Sp” percentage at the top. Anyone informed enough to know if this in anyway relates to coefficients, or just tells you how far between the most adjacent neaps and springs you are? My instinct is the latter, but based on a very limited sample size of looking (I’m lazy)…

 

[ylop]

Do you not carry in your head a loose tide clock? And whether it’s springs or neaps? If you sail once a month or less, maybe not, I guess.

Not really - is it possible you are the odd one and prescribing how you do it as the only way thats useful?

I sail more often than once a month but not to a very regular pattern - it can be several days or even a few weeks in a row, two weekends in a row or 3 weeks in between. I suspect many people the RYA trains are far less frequent than that.

My sailing rarely needs an accurate depth, but does need to understand currents. A magic number without any arithmetic or estimates is quite neat - e.g. the spring tide at the start of July was a "small spring" whereas the one in about three weeks time is a "big spring". I remember fighting a bit of tide in July - perhaps if I was going the same way in Sept I would want to change my timing to suit. Of course I CAN work all that out by hand from a classic set of tide tables, but a number to give a clue at a glance seems like exactly the sort of thing to make life easier. Once on the water then of course what you see and feel is important - but my planning is often done several days ahead where I can't even see the sea.

I'm not telling you that you are doing it wrong or you should change your approach, i'm not sure why you feel it necessary to tell others that your method of gut feel is better - especially since your gut feels is based on calculating something very similar subconsciously simply with much less precision.

One follow up. I use the Imray Tides app, that has a “Np -> Sp” percentage at the top. Anyone informed enough to know if this in anyway relates to coefficients, or just tells you how far between the most adjacent meals and springs you are? My instinct is the latter, but based on a very limited sample size of looking (I’m lazy)…

What does it say for today? Coefficient is apparently 46/41 today - neaps are tomorrow so if its %age between adjacent spring/neap I'd expect a very different number.

 

[mrming]

I should think it’s unusual for most of us to be overly concerned about depth except when anchoring. But, even though I sail in another area of fast tidal streams, just a quick glance at how close to springs it is tells me all I need to know. I know that neap rates are about half of spring rates here. I’m not going to work out what 78% of 3.9 is and apply it to my passage plan. I just look at the buoys, pots, posts etc, and my watch, to know what’s happening and how soon it’s likely to change.

In our cruising ground of the East Coast, it’s all about depth. Drying creeks, bars, dodging the adverse tide or staying deep when it’s fair. Sneaking across a spitway at close to low water. The only critical instrument on the boat is the depth sounder.

 

[Frogmogman]

One follow up. I use the Imray Tides app, that has a “Np -> Sp” percentage at the top. Anyone informed enough to know if this in anyway relates to coefficients, or just tells you how far between the most adjacent meals and springs you are? My instinct is the latter, but based on a very limited sample size of looking (I’m lazy)…

Hmmm. On my Imray tides app it shows Coefficient at the top. I guess because I have a French phone.





In Brittany the coefficient is also something of which the non-sailing public seem aware - as a useful guide to when will be good for “pêche à pied”, for promoting awareness against being stranded by the tide etc.

 

[Chiara’s slave]

In our cruising ground of the East Coast, it’s all about depth. Drying creeks, bars, dodging the adverse tide or staying deep when it’s fair. Sneaking across a spitway at close to low water. The only critical instrument on the boat is the depth sounder.

I have cruised the east coast. It’s all about real world depth. If you have the depth that is predicted, it’s a pleasant surprise, generally The mud moves as well as the water.

 

[dunedin]

.......
I love arithmetic and maths and do complex head calcs, but I guess I sail to get away from such stuff

But that is why coefficients are so good - one number gives an immediate view of things.
Going into Gulf de Morbihan for the first time mid tide on a 116% coeff I knew things would be moving fast!

 

[RunAgroundHard]

In Reads spring and neap date is highlighted in amber and grey respectively. Also the usual full and new moon symbols, a day or so before spring and neap days.

There is always more than one way to skin a cat and neither coefficients or tide tables offer anything more efficient, when wondering about where you are in the cycle.

 

[Humblebee]

Can't say I've ever used coefficients although I must have been aware of them doing RYA qualifications years ago.
I think Chiara's Slave sums it up quite nicely. For me, I look at the times and heights of the day's tides, think about flood or ebb if going into a narrow channel eg Kyle Rhea, and use the rule of twelfth's to work out anchoring depth, length of rode etc.
As someone noted above, predictions are just that, predictions, and subject to other variables, especially wind and barometric pressure.

 

[AntarcticPilot]

Although height of tide USUALLY correlates with tidal stream, it doesn't always. Take a place like the Gulf of Corryvreckan. The immensely strong tidal stream there results from the difference in height of tide between the eastern and western ends, not from the tidal current as in an estuary or bay. So the strongest stream need not coincide with the highest tide.

 

[wonkywinch]

In Reads spring and neap date is highlighted in amber and grey respectively. Also the usual full and new moon symbols, a day or so before spring and neap days.

There is always more than one way to skin a cat and neither coefficients or tide tables offer anything more efficient, when wondering about where you are in the cycle.

Not true, as an example, the spring tide on 12th March was 50% stronger than the spring on 24th May.

 

[Sjp1960]

If I am interested in currents then I certainly use tidal coeff as it is a tiny bit easier than trying to compare tidal ranges or decide where the day sits between the last neap/ spring and following one.

I don’t however try and use it to work to 2 decimal places, I just decide based on the % whether it’s springish averagish, neapish and take the higher, average or lower of the two current values. If the coeff is more than 95% then I note to myself that the currents will be even bigger than the value in the tidal atlas.

I record it in the log book, so if I have done a passage before I can quickly compare the currents today with the previous trip, when thinking about how long it might take.

 

[RunAgroundHard]

Not true, as an example, the spring tide on 12th March was 50% stronger than the spring on 24th May.

My post is factual. Reeds Small Boat Almanac displays the data as I describe.

 

[capnsensible]

Just another tool that some like to use. Some don't. Does it matter?

Personally I like to know how much tide there is over datum in northern waters. And an indication of the likely rate and direction of the tide stream. Its great to have stuff on an easy app or plotter. But not everyone who sails owns their own boat so are restricted in the kit available. Knowing more than one way to skin the tidal cat can be very useful. In my opinion....

 

[bluerm166]

For anyone who is interested in using the rate of tidal streams here is a page from the comprehensive 'The Yachtsmans Manual of Tides' Fastnet and Brest to the Wash and Den Helder by Michael Reeve-Fowkes which you may find a copy of on a specialist second hand site such as Abe Books.
In this case the base tidal streams are shown on the charts as a single mean value for every hour and the table bottom left allows conversion using the days coefficient (bottom row) as well as the days height at Cherbourg .

There are separate volumes covering local areas but I see from the Western Channel (Channel Islands,Western Approaches,Fastnet,St Georges Channel South,Bristol Channel) edition which I have inherited that these don't show the coefficient .Since I am in the East and unlikely now to go West I could forward this Western version FOC to anyone with a keen interest - it is no longer mint .

 

[Sandy]

For anyone who is interested in using the rate of tidal streams here is a page from the comprehensive 'The Yachtsmans Manual of Tides' Fastnet and Brest to the Wash and Den Helder by Michael Reeve-Fowkes which you may find a copy of on a specialist second hand site such as Able Books.
In this case the base tidal streams are shown on the charts as a single mean value for every hour and the table bottom left allows conversion using the days coefficient (bottom row) as well as the days height at Cherbourg .

There are separate volumes covering local areas but I see from the Western Channel (Channel Islands,Western Approaches) edition which I have inherited that these don't show the coefficient .Since I am in the East and unlikely now to go West I could forward this Western version FOC to anyone with a keen interest - it is no longer mint .

You will need

https://www.bloomsbury.com/media/zwubu1bf/cherbourg-tide-tables-2024.pdf

To use this.

 

[Chiara’s slave]

You will need

https://www.bloomsbury.com/media/zwubu1bf/cherbourg-tide-tables-2024.pdf

To use this.

And a healthy dose of scepticism. There is no way of being as accurate as is implicit in that table. I daresay the authors know this perfectly well, but for the ‘new to this’ or ‘blind faith’ communities, it needs a 25% disclaimer.

 

[Black Sheep]

Just another tool that some like to use. Some don't. Does it matter?

Yes, it does matter, because not all of us are familiar with that tool.

I'm finding the conversation very interesting; I was vaguely aware that the French use coefficients, so if nothing else a knowledge of them would be useful when interpreting French tidal information. And it sounds like a tool that might suit the way I think. Something for me to investigate further.

So I thank Frank for bringing the subject up, and various contributors for helping clarify how coefficients work.

A great example of an information-sharing forum!