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?
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.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?
Yes but perhaps you sail in shallow muddy waters.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.
Oh thanks - like you I am often much more interested in the current than the depth. A ready calculated figure could be useful.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.
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.Oh thanks - like you I am often much more interested in the current than the depth. A ready calculated figure could be useful.
I am not sure I'd describe the Atlantic as 'shallow'.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.
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.
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 going to work out what 78% of 3.9 is
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.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.
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.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.
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.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.
That is my understanding of it...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.
UK springs corresponds to coefficient 95 (also called VE, Vives Eaux)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.
... 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.