Tidal coefficient

[ylop]

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.

Accurate or precise?

Do you really mean 25%, ie a 7m tide could be just over 5m?

 

[capnsensible]

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.

I used to use the Reeve-Fawkes tide atlas for dozens of trips across the Channel and also in the Solent. I found it excellent. You used to be able to send the previous year's Cherbourg tide heights back to them and get a new one free of charge.

Yet another tool that many have never heard of in the electronic age. And like all the other handraulic methods, practice increases accuracy.

But once again, it's not compulsory, so does it matter?

 

[Chiara’s slave]

Accurate or precise?

Do you really mean 25%, ie a 7m tide could be just over 5m?

Think we are talking mostly about stream rates. 10% in height is roughly the possible variance, we certainly get that routinely in the Solent. But definitely 25% in stream rate.

 

[Chiara’s slave]

I used to use the Reeve-Fawkes tide atlas for dozens of trips across the Channel and also in the Solent. I found it excellent. You used to be able to send the previous year's Cherbourg tide heights back to them and get a new one free of charge.

Yet another tool that many have never heard of in the electronic age. And like all the other handraulic methods, practice increases accuracy.

But once again, it's not compulsory, so does it matter?

Of course, more info is a good thing. But we should be perfectly clear about the expected level of accuracy. And a number like 82% implies accuracy at that level. That is simply not so. 82% is the tidal model’s prediction, much like the % likelihood of rain in a weather model.

 

[dunedin]

Think we are talking mostly about stream rates. 10% in height is roughly the possible variance, we certainly get that routinely in the Solent. But definitely 25% in stream rate.

Yes stream rates can vary much more than heights. And the place where the stream flows faster and slower can change hugely - East of Corryvereckan can have a +6kt tide whilst a boat 100m away has -2kt, for no apparent reason. And 20 minutes later or a different day the boundary may be somewhere completely different.
But the coefficient at least tells me on a 40% day the confusion will be rather more moderate than on a 120% when the madness will be extreme and fast happening!
But it is always worth remembering that (Scottish) tides rarely read the tide tables.

 

[Fr J Hackett]

It's been a while! but I found that tidal coefficients were just a method of predicting tidal amplitude and was never sure other than in an empirical way that they related to tidal streams and their velocity ie with a big coefficient I could expect the tidal stream to be stronger than expected from data and as I invariably used Admiralty Tidal Stream data which is for mid Springs and Neaps applying it seemed of little value. I can see that it might interest coastal sailing with big tidal ranges ( Brittany) especially if you are marginal on a tidal gate but for me it was what it was and there was always the engine if needs must. If I really wanted to know I could just look at SOG and STW. I went with the flow.

 

[B27]

Accurate or precise?

Do you really mean 25%, ie a 7m tide could be just over 5m?

25% variance on tidal height would be relative to mean sea level perhaps?
So 75% of 3.5, + 3.5 = 6.125m.
The weather can certainly add 0.875m to a 7m tide.

I think currents are far more variable than heights, because currents have significant components of eddying and swirling around, rather than just flowing straight up and down the channel.
I think we also simplify the currents more, if you look at secondary ports, the time differences vary between springs and neaps.
That means a neap is not just a smaller version of a spring, it's a different shape.
That means there are places where the tide turns at different times relative to HW Devonport of whatever, between springs and neaps, so a simple factor is never the whole story.

People look for too much precision in all of the methods, even the simplest of the primary ports you find the reality deviating from the theory by a foot or more, so once you've passed the exam and proved you can do the method, there's little point in calculating anything to the nearest centimetre.

 

[Fr J Hackett]

Yes stream rates can vary much more than heights. And the place where the stream flows faster and slower can change hugely - East of Corryvereckan can have a +6kt tide whilst a boat 100m away has -2kt, for no apparent reason. And 20 minutes later or a different day the boundary may be somewhere completely different.
But the coefficient at least tells me on a 40% day the confusion will be rather more moderate than on a 120% when the madness will be extreme and fast happening!
But it is always worth remembering that (Scottish) tides rarely read the tide tables.

But you know that anyway by simply knowing where you are between neaps and springs which is what the TC is telling you in a mathematical format.

 

[capnsensible]

Of course, more info is a good thing. But we should be perfectly clear about the expected level of accuracy. And a number like 82% implies accuracy at that level. That is simply not so. 82% is the tidal model’s prediction, much like the % likelihood of rain in a weather model.

I think it's fair to adding practical experience of using them. Lots. In the same way that the long term bobbers about in boats have learnt to interpolate weather forecasts. As usual, practice, practice, practice. In my opinion.

 

[dunedin]

But you know that anyway by simply knowing where you are between neaps and springs which is what the TC is telling you in a mathematical format.

As noted earlier, there are Springs and there are SPRINGS ! As you probably know, you can immediately tell the difference between a 95% and 116% entering Gulf de Morbihan for example.
But agree each to their own methods. Some on here:
- stick rigidly to using UTC even when only sailing in BST
- stick rigidly to only using Dover rather than a local standard port, even when Dover is 500 miles away
- buy paper tide tables rather than use an app that gives official UKHO data instantly
All good, whatever people prefer.

 

[capnsensible]

As noted earlier, there are Springs and there are SPRINGS ! As you probably know, you can immediately tell the difference between a 95% and 116% entering Gulf de Morbihan for example.
But agree each to their own methods. Some on here:
- stick rigidly to using UTC even when only sailing in BST
- stick rigidly to only using Dover rather than a local standard port, even when Dover is 500 miles away
- buy paper tide tables rather than use an app that gives official UKHO data instantly
All good, whatever people prefer.

Other than using Dover as a standard port when the tide stream atlas is based on that, I agree with the rest!!

 

[capnsensible]

When deployed to places on our own yacht where there are no tide tables and having a dodgy echo sounder....rotating neon....I found a lead line very useful in sounding an anchorage. But I like a challenge....

 

[Chiara’s slave]

As noted earlier, there are Springs and there are SPRINGS ! As you probably know, you can immediately tell the difference between a 95% and 116% entering Gulf de Morbihan for example.
But agree each to their own methods. Some on here:
- stick rigidly to using UTC even when only sailing in BST
- stick rigidly to only using Dover rather than a local standard port, even when Dover is 500 miles away
- buy paper tide tables rather than use an app that gives official UKHO data instantly
All good, whatever people prefer.

We use whatever seems available and convenient, but mostly, a glance at what’s expected, and a long, hard look at the water, fixed objects, the chart, the depth sounder, and COG, SOG compared to STW and boat heading. Though in the Solent we do that without instruments, against other boats that are doing their own thinking. Which kind of explains my obsession with the subject. ‘Hiding’ behind known underwater obstructions, looking for signs like froth or a line of weed that there’s a tidal boundary. Various other cunning stunts too. The boats 20 metres apart in totally different tide is part of the fun of racing in the Solent. We find it applies everywhere else we sail too.

 

[Chiara’s slave]

When deployed to places on our own yacht where there are no tide tables and having a dodgy echo sounder....rotating neon....I found a lead line very useful in sounding an anchorage. But I like a challenge....

The norm on an XOD beating up the Lymington shore is a 5ft bamboo cane

 

[dunedin]

The norm on an XOD beating up the Lymington shore is a 5ft bamboo cane

I fitted a metal tip to the centreboard of our Enterprise when racing where I grew up - used to tack when it touched, and tack back in 3 boats lengths out. In the days when roll tacking was legal. In light winds sometimes beat 505s and the like round the windward mark, as they had to go further out into the tide. .
And no didn’t use tidal coefficients, or indeed tidal heights, then - plenty of moored boats to watch when they swing.
But very different sailing a deep keel cruiser when no other boats within 20 miles.

 

[Chiara’s slave]

I fitted a metal tip to the centreboard of our Enterprise when racing where I grew up - used to tack when it touched, and tack back in 3 boats lengths out. In the days when roll tacking was legal. In light winds sometimes beat 505s and the like round the windward mark, as they had to go further out into the tide. .
And no didn’t use tidal coefficients, or indeed tidal heights, then - plenty of moored boats to watch when they sing.
But very different sailing a deep keel cruiser when no other boats within 20 miles.

But you do have instruments to tell you what is really happening. If I had limitless time, I’d like to produce a log of real against predicted tidal streams.

Note to self… should probably post less on forums.

 

[obmij]

There can be a bigger difference between a small spring (June for example) and an equinoctical spring as there is between a small spring and the corresponding neap.

So the 'at a glance' information in the almanac - this day is springs and this day is neaps - could actually be a little misleading. An infrequent boater could easily be caught out if their previous experience of a spring tide was one of a relatively low coefficient. I can think of several tidal races where this really would matter.

If course, if you are familiar with an area and know the ranges then you will be aware if what you are heading out I to is a monster of a tide or not. Likewise many will know just based on the time of year - but without this knowledge I think the coefficient way of presenting would be helpful to almost everyone.

I keep an eye on coefficient all the time. It's just another layer of information to absorb almost without being concious of it. It's also interesting when an absolute beast turns up.

 

[bluerm166]

A plan is only a 'say if' review of what you might encounter and may for one reason or another ' damn,I won't have the car on that day .....' fail at an early stage.But its still worth making the review.

The point about accuracy is taken but these coefficients are derived from tidal height records and tidal flow surveys .
So if the coefficients are inaccurate then so must be the tide heights,yet we still think it worthwhile showing LAT state on charts.
Sailing in known waters allows rules of thumb to be sufficient.But if you can cross the channel from the east coast to the continent ,say to belgium,I take my hat off to you if you can ,armed only with the tidal heights at each end and of course a chart ,calculate your likely track. I suggest that tides are considerably more predictable than the weather.

 

[Chiara’s slave]

A plan is only a 'say if' review of what you might encounter and may for one reason or another ' damn,I won't have the car on that day .....' fail at an early stage.But its still worth making the review.

The point about accuracy is taken but these coefficients are derived from tidal height records and tidal flow surveys .
So if the coefficients are inaccurate then so must be the tide heights,yet we still think it worthwhile showing LAT state on charts.
Sailing in known waters allows rules of thumb to be sufficient.But if you can cross the channel from the east coast to the continent ,say to belgium,I take my hat off to you if you can ,armed only with the tidal heights at each end and of course a chart ,calculate your likely track. I suggest that tides are considerably more predictable than the weather.

On a large scale, like crossing the channel, they may well be. But the ‘gusts and lulls’ in the tide maybe a lot less predictable. So, if you’re a boat that hoists the sails, sheets them in, and sets off on a pre ordained course, you’ll find the tide stream accuracy mediumly good. If you wonder why you’re a mile off track, that’ll be why. Even 3 or 4 miles. If that’s of little consequence as it is when crossing the channel there (usually) no problem. Don’t try that at St Malo, where cross track error is a very serious matter indeed.

 

[capnsensible]

The norm on an XOD beating up the Lymington shore is a 5ft bamboo cane

You can extend the range of your soundings by tieing a lead line to the end of your stick and crouching in the bow. Around 2m of line is plenty.

I consider myself very lucky to have been taught coastal navigation by 3 different people who were Members of the Royal Institute of Navigation, amongst others. The are lots of ways, as I mentioned, to do this. Happily for me it was in the kinda decca/ GPS changeover.

When I sail others boats in a gadget free environment, I remain happy!

Another thing that I found fascinating was reading about ww2 minesweeper navigators operating in the Dover Staits. Transits, bearings, tide height and tide stream calculations to a very high accuracy using the tables of the era. Whist getting shot at by enemy aircraft!!

However, the great thing about plotters, etc is the way they've opened up navigation and made it a lot easier so encouraging more peeps to have a go. On Thursday, we are going out on my oppos Fairline 47 as I want to show him some 25 to 30 knot quick nav techniques whilst we go for a whizz around the Island of Lobos. Electronics will be involved.....and transits and clearance bearings!