RYA Tidal Calcs

[awol]

One of the things that annoyed me when I did the shore-based course was the calculation of tidal heights to umpteen decimal places (not actually true, but you know what I mean). http://www.ntslf.org/data/uk-network-real-time - just click on "RT" for a port near you - shows why carefully worked out clearance heights may not stop you clouting things.

 

[jac]

That link didn't work for me??

 

[jerrytug]

One of the things that annoyed me when I did the shore-based course was the calculation of tidal heights to umpteen decimal places (not actually true, but you know what I mean). http://www.ntslf.org/data/uk-network-real-time - just click on "RT" for a port near you - shows why carefully worked out clearance heights may not stop you clouting things.

I would be interested to hear all the other things that annoyed you as well, thanks.

 

[alant]

One of the things that annoyed me when I did the shore-based course was the calculation of tidal heights to umpteen decimal places (not actually true, but you know what I mean). http://www.ntslf.org/data/uk-network-real-time - just click on "RT" for a port near you - shows why carefully worked out clearance heights may not stop you clouting things.

As long as you have the method right, slight differences in arithmetic, would be taken account of.
IMO, anyone wanting accuracy of 0.5m in reality, hasn't been sailing.

 

[JumbleDuck]

One of the things that annoyed me when I did the shore-based course was the calculation of tidal heights to umpteen decimal places (not actually true, but you know what I mean).

Those calculations are unrealistic, but a good instructor will explain that they are unrealistic and why. However, s/he'll also explain that checking the answers to two decimal places is a simple way of making sure that the method as laid out is being applied properly, and that students aren't getting almost the right answer by chance. Same with passage planning; no, you won't really leave A at 06.28 and arrive with exactly 10cm under your keel as you enter B having made the trip at a steady 4.0kt, but it's a reasonable way of checking your working.

 

[awol]

I would be interested to hear all the other things that annoyed you as well, thanks.

You serious? Twas a long time ago and probably bore little similarity to today's offerings but you asked ..... I guess it all boiled down to the difference between sitting at a desk and bouncing around with water dripping off my head onto the chart table and fingers all wet, swollen and wrinkly. If I was able to sail a course to better than 5/10deg accuracy (other than motoring in a flat calm), or had more faith in the resolution of tidal atlas data then the pernickitiness of the navigation calcs and the time spent doing them may not have irked. The methodology was fine but a quick go/no go answer is what I tend to want and if I have to do sums to 0.1 accuracy to justify a "go" decision, I have to be pretty desperate and/or brave.

 

[LadyInBed]

As long as you have the method right, slight differences in arithmetic, would be taken account of.
IMO, anyone wanting accuracy of 0.5m in reality, hasn't been sailing.

The problem is that it has developed into a mathematical exercise more than understanding the principle. Tide calcs are treated as facts rather than predictions.

 

[jerrytug]

It's to demonstrate that you can work it out accurately, I'm sure the instructor said something like, 'In the real world, it won't be this precise, plus there are other factors such as the wind and the pressure'.
It's the same with chartwork in the Theory exams, you have to have a very sharp pencil and plot your course to steer to 1 degree accuracy, to show you can do it properly. What's wrong with that?

 

[capnsensible]

It's to demonstrate that you can work it out accurately, I'm sure the instructor said something like, 'In the real world, it won't be this precise, plus there are other factors such as the wind and the pressure'.
It's the same with chartwork in the Theory exams, you have to have a very sharp pencil and plot your course to steer to 1 degree accuracy, to show you can do it properly. What's wrong with that?

Indeed. General Notes To Instructors

'Please note that some of the navigation exercises are set to test theoretical skill and may not reflect fully the practical approach to a real situation.'

 

[SurreySailing]

I agree entirely that that theory courses push for a level of accuracy that you may not get aboard (I know I do!), however my argument for this is - at a massive and stable desk with generous time allowance and good lighting (in comparison to a chart table on a moving yacht as you approach a harbour in darkness) you should be able to get it 'spot on'!

If not your answer on board in a swell has little chance of being anywhere close ..... The key is to add a good safety allowance (0.5m minimum) to an accurate answer, not guesstimate the answer to start with.

Then with experience and confidence in your calculated answer (allowing for pressure) approaching with just 10cm clearance and a bottom of soft mud at slow speed on a rising tide is a great experience (having tested the accuracy of your depth sounder first).

My view is that when you properly get the theory you should make informed guesstimates, occasionally get it wrong and feel ok about it as you know what your doing.

 

[JumbleDuck]

My only quibble about the DS and CS theory courses as I did (and, in the case of DS, taught) them was that the RYA materials didn't mention the approximate / guesstimate nature of their calculations enough or in some cases at all; it was left to the instructors to explain that no, you can predict tidal heights to a centimeter or steer courses to a degree. I thought that was a shame, because there is some interesting stuff to cover there: if you are heading for X and know that you are most unlikely to end up sailing in through the harbour entrance without changing course, do you aim for X or to the left or to the right?

There's nothing wrong with checking that methods are used correctly, as long as you make sure that the methods themselves are not infallible and that the answers always need to be interpreted.

 

[capnsensible]

My only quibble about the DS and CS theory courses as I did (and, in the case of DS, taught) them was that the RYA materials didn't mention the approximate / guesstimate nature of their calculations enough or in some cases at all; it was left to the instructors to explain that no, you can predict tidal heights to a centimeter or steer courses to a degree. I thought that was a shame, because there is some interesting stuff to cover there: if you are heading for X and know that you are most unlikely to end up sailing in through the harbour entrance without changing course, do you aim for X or to the left or to the right?

There's nothing wrong with checking that methods are used correctly, as long as you make sure that the methods themselves are not infallible and that the answers always need to be interpreted.

It is important that Instructors read 'Guidance Notes For Instructors' for this very reason, a page in the answers booklet.

In addition, students should be made aware of the note on navigational accuracy inside the front cover of the student pack,

Its all there.....

 

[snowleopard]

I would be interested to hear all the other things that annoyed you as well, thanks.

Tidal diamonds. I had an argument on here that got quite heated. A poster was insisting that using diamonds to calculate tidal flow was the ONLY correct method. Despite me pointing out that if you were close to a headland the tide was likely to be doing something totally different to the diamond 5 miles offshore he stuck to his guns and got very heated. He maintained his stance when referring to being in a river where the tide, according to the nearest diamond, would be flowing across the river!

Of course an instructor worth his salt will teach the classroom method but temper it with explanations of how it might differ in reality. It's usually the half-informed pedants who insist there is only one way to do things. The only time the sort of precision served me well was when I did a tidal height calculation with adjustment for local pressure - it helped me win a £600 pair of binocs in a YM competition!

 

[Daedelus]

Tidal diamonds. I had an argument on here that got quite heated. A poster was insisting that using diamonds to calculate tidal flow was the ONLY correct method. Despite me pointing out that if you were close to a headland the tide was likely to be doing something totally different to the diamond 5 miles offshore he stuck to his guns and got very heated. He maintained his stance when referring to being in a river where the tide, according to the nearest diamond, would be flowing across the river!

You get them.

SWMBO was on a Yachtmaster shorebased when one of the students (the one who was always arguing with the instructor) announced he had done all the tidal calculations and it was impossible to get from Cherbourg to Jersey on one tide. SWMBO smiled sweetly and said it was, remarking she had done it. She was then accused of having an enormous and fast yacht and said it was in a Moody 31, whereupon the instructor joined in saying he'd done it in a Centaur. The class then explained to the know all that there was a back eddy which he had not noticed.

 

[oldbilbo]

Then with experience and confidence in your calculated answer (allowing for pressure) approaching with just 10cm clearance and a bottom of soft mud at slow speed on a rising tide is a great experience (having tested the accuracy of your depth sounder first).

All of "wot 'e said".....

Recalling a winter delivery from Guernsey to The Retreat Boatyard above Topsham Quay in seriously co-o-ld weather. I didn't relish spending the night leaning on a mudbank in the mid-Exe Estuary, so had to squeeze every cm. out of the calcs and the 'fudge factors'. I even allowed +2cm and 5-10mins for the brief stand in that short reach just after calculated HW when the ebb influence would not have started yet the river would still be flowing down. I reckoned on about +5cm at the tight stretch.

Unable to allow for unknown very local silting, I reasoned that recent winter rains would have scoured rather than silted on the critical stretch - and was proved right. The boatyard people, highly experienced locals, were more than a little surprised to see us and didn't reckon any visitor would have got through on that neap. I didn't bet, but did get a bonus....

Yes, understanding the calcs does occasionally have its uses.


Edit: Apropos the OP's query, the RYA Training lot certainly expect the 'shorebased tutor' to explain the context, the reasons, and salt the lot with common sense. People take the course for a variety of reasons, and few intend to navigate to 1°/0.02nm accuracy. They are, however, usually keen to know it can be done when really needed and that gives something of a higher standard to aim for, if so wished.

And before there's a clamour that it can't..... just consider what occasions do call for that degree of accuracy of navigation - and how most of us achieve it.

 

[TPW]

Tidal Prediction Worksheet

In preparation last year for sailing in tidal waters the first time, I searched the web and made myself familiar with the ATT-method of tide calculation. As I do not like to do graphical or manual calculations (neither in general nor while sailing in unpleasant weather in particular ), I set up an EXCEL 2010 spreadsheet, called the Tidal Prediction Worksheet (TPW), to do all the wanted calculations, viz.
- tides at a secondary port,
- requested time and height at a standard or secondary port as well as
- under keel and masthead clearances.

The TPW incorporates the tide curves of several standard ports, so that tide curves need to be looked up in the Reeds or the ATT only for standard ports not supported by the TPW. The TPW can be used to verify tidal calculations in preparation of an examination and for navigation/planning purposes, all at own risk (!), of course.

I recently set up a web-page http://tidal-prediction-worksheet.jimdo.com where the TPW, a tide calculation example and an english manual is available for free download.

 

[Sandy]

One of the things that annoyed me when I did the shore-based course was the calculation of tidal heights to umpteen decimal places (not actually true, but you know what I mean). http://www.ntslf.org/data/uk-network-real-time - just click on "RT" for a port near you - shows why carefully worked out clearance heights may not stop you clouting things.

I think the clue is in the course name, Shorebased Theory course.

At sea I round everything up to the next 15 mins and round the keel up to the next full meter and never had a problem, but in the class room enjoy the mathematical modelling, perhaps I am a bit geeky, says cynical Scots engineer.

My brother was taught to fly by the RAF, on asking why he did the same exercise time and time again, I understand extreme boredom had set in, the instructor said, "it’s so you don't need to think about flying the aircraft, laddy".

By doing the theory in the comfort of a warm class room with endless supplies of hot coffee and easy access to the loos, for gentlemen of a certain age, when you are out there in a hooly and need to do a critical calculation to get over the bar you don't need to take 20 mins to work it out then worry if it is correct. You can work it out in your head without thinking too hard.

 

[Aardee]

My brother was taught to fly by the RAF, on asking why he did the same exercise time and time again, I understand extreme boredom had set in, the instructor said, "it’s so you don't need to think about flying the aircraft, laddy".

By doing the theory in the comfort of a warm class room with endless supplies of hot coffee and easy access to the loos, for gentlemen of a certain age, when you are out there in a hooly and need to do a critical calculation to get over the bar you don't need to take 20 mins to work it out then worry if it is correct. You can work it out in your head without thinking too hard.

As the old saying goes - don't practice until you get it right, practice until you stop getting it wrong

 

[Sandy]

Recalling a winter delivery from Guernsey to The Retreat Boatyard above Topsham Quay in seriously co-o-ld weather. I didn't relish spending the night leaning on a mudbank in the mid-Exe Estuary, so had to squeeze every cm. out of the calcs and the 'fudge factors'. I even allowed +2cm and 5-10mins for the brief stand in that short reach just after calculated HW when the ebb influence would not have started yet the river would still be flowing down. I reckoned on about +5cm at the tight stretch.

Unable to allow for unknown very local silting, I reasoned that recent winter rains would have scoured rather than silted on the critical stretch - and was proved right. The boatyard people, highly experienced locals, were more than a little surprised to see us and didn't reckon any visitor would have got through on that neap. I didn't bet, but did get a bonus....

Yes, understanding the calcs does occasionally have its uses.

Respect, as my daughter would say.

Out of interest what was the draught?

 

[Sandy]

As the old saying goes - don't practice until you get it right, practice until you stop getting it wrong

I like that