franksingleton
Well-known member
Somebody may well have asked - why does the RYA teach use of tidal coefficients? Or am I behind the pace? It just seems so easy and intuitive. Or does everyone use apps and not actually think about currents?
Tidal coefficient
- Thread starter franksingleton
- Start date
finestgreen
Active member
Easier and more intuitive than simple arithmetic on tidal ranges?
(Are tidal coefficients available outside France anyway?)
(Are tidal coefficients available outside France anyway?)
wonkywinch
Well-known member
I use them all the time to see how springy a spring might be. Occasionally you'll find tides half way between springs and neaps that are as strong as a "standard" spring.
My home made passage plan sheet has a space for % coefficient.
Since apps are only digital almanacs, I wouldn't trust them to factor in the coefficient.
I tend to use the Brest tables as it's pretty much the same expanse of water.
I don't use the "computation of rates" graph since I've been able to interpolate from a young age.
dunedin
Well-known member
I find the tidal coefficient is a very helpful way of knowing just how extreme a tidal flow will be - important if trying to sneak through somewhere like Kyle Rhea against the tide.
The only place I have found them listed in the UK is Tides for Fishing - Tide times and charts for Mallaig, Scotland and weather forecast for fishing in Mallaig in 2024.
The only place I have found them listed in the UK is Tides for Fishing - Tide times and charts for Mallaig, Scotland and weather forecast for fishing in Mallaig in 2024.
Sandy
Well-known member
I am not a fan of them. I want to know how high, high water is.
I get a daily email from KHM Plymouth with things such as '44% Mean Springs' totally meaningless. 4.7 metres give me a measurement to work with.
I get a daily email from KHM Plymouth with things such as '44% Mean Springs' totally meaningless. 4.7 metres give me a measurement to work with.
dunedin
Well-known member
Yes but perhaps you sail in shallow muddy waters.
Up here I am generally more interested in how FAST the water is going than how deep - is the tide going to be at peak 8 knots or rather less.
Heading off today against the tide (wish me luck !) - knowing the coefficient is 46/41 gives me an immediate and numerical reassurance that it is not going to be a fast stream today.
As I don’t mentally remember the full tidal range at Ullapool, the tide table with heights only doesn’t immediately tell me this.
ylop
Well-known member
Oh thanks - like you I am often much more interested in the current than the depth. A ready calculated figure could be useful.
Chiara’s slave
Well-known member
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.
Sandy
Well-known member
I am not sure I'd describe the Atlantic as 'shallow'.
On the French side of la manche they use coefficients all the time, and have some 'interesting' tidal flows, I still reach for the tidal atlas and can quickly work out even guesstimate the flow rate.
I suspect we all use different ways of getting to the same answer.
ylop
Well-known member
Day skipper theory would suggest otherwise! Lots of when can you enter / leave this harbour type calcs. Drying moorings. Short cuts across shallow areas etc.
If will be about 3 - because 80% (4/5) of 4 is 3.2. Not that I’m needing to be that precise either - I’m looking for a feel.
I’m not talking about when you are on the water - I’m thinking of “should we leave earlier” or “go the long way round” type questions.
I can of course compare to springs, but I was just thanking Dunedin for pointing out a convenient place that’s done the comparison for you. Knowing it’s a week before springs doesn’t give ME an intuitive feel for how “springy” it is especially as not all springs are the same. If you are sailing twice a week in the same patch I am sure that becomes intuitive - but if you are out less frequently or looking at new areas where you don’t intuitively know the typical spring range it just seems like a nice indicator.
wonkywinch
Well-known member
The coefficient is a useful guide to currents for me. The spring tides closest to the equinox are around 25% stronger than mean springs so that 4kt stream becomes 5kts. The coefficient tables give a much better feel than the days to/from springs/neaps on the almanac tide pages.
Height wise, the coefficient gives you a heads up that you may be at more risk of hitting the bottom in your favourite shallow anchorage/harbour entry.
Height wise, the coefficient gives you a heads up that you may be at more risk of hitting the bottom in your favourite shallow anchorage/harbour entry.
Chiara’s slave
Well-known member
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. Spending too much time on the maths is a navigational fail, in my books. Depth wise, there’s no other way of course, the tidal graph is needed to pass an exam. How often you look at that IRL, once a trip is probably it for me. Depends on how often you cross a bar, or attempt anything marginal. Perhaps having a lead mine on your boat makes it more vital. I wouldn’t know, I’ve never had one.
The Q
Well-known member
No idea about RYA teaching, but in my some what restricted tidal waters, tidal formula doesn't work very well.
So I use.
Norfolk-Broads.org Tide Table.
That plus local knowledge of the fact our tides flow out longer than incoming. Air pressure and wind direction also severely affect our tides.
High air pressure lowers the north sea, our water levels reduce, tide flows out more. Easterly and southerly wind has a similar effect.
Northerly and westerly winds increase our water levels, add low air pressure at the same time and we get floods especially with gales.
So I use.
Norfolk-Broads.org Tide Table.
That plus local knowledge of the fact our tides flow out longer than incoming. Air pressure and wind direction also severely affect our tides.
High air pressure lowers the north sea, our water levels reduce, tide flows out more. Easterly and southerly wind has a similar effect.
Northerly and westerly winds increase our water levels, add low air pressure at the same time and we get floods especially with gales.
Nimrod18
Member
I’ve not seen this methodology before though I can see its value for flow rates, particularly when planning / replanning: therefore trying to translate into my predominantly UK based sailing. (For what it is worth, when it comes to tidal heights, my instinct is that raw height values would suit me better).
Question for those familiar with the coefficient system: is there a way to translate the coefficient value into the admiralty tidal stream atlas (or similar) for UK waters? I am working on the principle that I need a baseline coefficient flow rate value to work from (I assume 100%).
Example, a random flow arrow on the Admiralty flow rate chart for Portland indicates 26,52 (not to suck eggs, 2.6kts at neaps, 5.2kts at springs). Is 5.2kts a 100% spring, or a 120% spring (120 is the maximum according to the French website)?
And if 100% spring, then the neap figure should never be below 50% (2.6kts = 0.5 x 5.2kts), yet the coefficient values for 2024 dip down to 30 (and, according to the French website) could be as low as 20%; in which case the neap value on the flow atlas should be 0.2 x 5.2kts = 1.0kts.
I hope my question makes sense. To reiterate: in Uk waters where do you get a baseline flow rate on which to apply the coefficient.
Thanks in advance.
Question for those familiar with the coefficient system: is there a way to translate the coefficient value into the admiralty tidal stream atlas (or similar) for UK waters? I am working on the principle that I need a baseline coefficient flow rate value to work from (I assume 100%).
Example, a random flow arrow on the Admiralty flow rate chart for Portland indicates 26,52 (not to suck eggs, 2.6kts at neaps, 5.2kts at springs). Is 5.2kts a 100% spring, or a 120% spring (120 is the maximum according to the French website)?
And if 100% spring, then the neap figure should never be below 50% (2.6kts = 0.5 x 5.2kts), yet the coefficient values for 2024 dip down to 30 (and, according to the French website) could be as low as 20%; in which case the neap value on the flow atlas should be 0.2 x 5.2kts = 1.0kts.
I hope my question makes sense. To reiterate: in Uk waters where do you get a baseline flow rate on which to apply the coefficient.
Thanks in advance.
Chiara’s slave
Well-known member
The figures on charts, or tidal stream atlases, are for mean springs and mean neaps.
Frogmogman
Well-known member
Before I had a boat in Brittany tidal coefficient really wasn’t something that I paid much attention to. That changed quite quickly, as everybody here refers to it a lot.
It’s now one of the first things I look at before I start any planning, as it gives an immediate feel to the springiness of the tide, I also find it a useful tool for ready reckoning on tidal flows.
Sandy
Well-known member
That is my understanding of it...
KHM Plymouth daily email gave 125% on a 5.8 metre high water the other day.
If you have a look in the front of a Tidal Atlas there is a table for the computation of rates and they extend well above mean springs.
Roberto
Well-known member
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
oldmanofthehills
Well-known member
Sailing as we do in the tide funnel of western english channel or eastern Britol Channel the coefficient would not be be that useful. Tide maps show many currents going round in circles with great variance on points a few miles apart. The Navigator is always unsucessfully trying to get full agreement between Navionics plotter, Paper Chart Tidal Maps and other Current info sources
And locally currents can vary with back eddies which the locals cunningly use by seemingly heading close to large spiky rocks.
Its enough I know its more at springs and less at neaps and lags the tide by 2 hours in west english channel but matches tide in bristol channel
I love arithmetic and maths and do complex head calcs, but I guess I sail to get away from such stuff
And locally currents can vary with back eddies which the locals cunningly use by seemingly heading close to large spiky rocks.
Its enough I know its more at springs and less at neaps and lags the tide by 2 hours in west english channel but matches tide in bristol channel
I love arithmetic and maths and do complex head calcs, but I guess I sail to get away from such stuff
RunAgroundHard
Well-known member
Tidal Coefficients can be found in PBO / Reads Small Craft Almanac, Chapter 5 - Tides, Page 135, the very first page after the chapter index.
No idea why the RYA does not teach iit, but likely because it is just a Quick Look and the RYA are more interested in teaching how to calculate specific heights and velocities.