Yottigation

[TimfromMersea]

When doing the YM Theory at night school, the lecturer explained that we needed to work out tidal heights and tide times to the nearest 10 cm and 1 minute.

Then he showed us a slide taken from a web site (the URL of which I knew once but have now forgotten!) and proudly told us that this web site demonstrated that at the local recording station (Felixstowe), on not one day in the last 2 weeks had either the tide height or tide time been in accordance with the prediction.

I put my hand up and said "What is the point of working it out to 10 cm and 1 minute then" and he said "because that's what the RYA say for the course".

Am I missing something obvious? Can anyone else understand this?!

 

[Croucher]

I was told by my lecturer that if you can do the calcs to this accuracy in the classroom then doing it for real in a bucking rolling boat will be a doddle. Seemed a reasonable answer.

 

[mel80]

It's the difference between error and bias. i.e. there will always be an error in the tidal forecast, due to weather etc, but the forecasts should not be biased. In other words, although it would be foolish to rely on being accurate to within 10cm or 1 min, you can still minimise the total error by calculting within those margins. Errors in the forecast can't be avoided; errors in calculations can.

 

[Koeketiene]

You will soon find that the RYA's YM Theory bears very little resemblance to real life.

Last time I worked out tidal heights was on my YM course. Since then, have been fine using th rule of 12ths. And add "a bit" to be on the safe side.

It's similar to working out CTS to within 2° - fine on a MOBO in flat seas, on a bouncy raggie in a bit of a blow I'm happy if you can keep within 10°.

 

[yoty]

You are being taught to pass a test - not sail a boat!

Try this for a rough guide: www.belfieldsoftware.co.uk

you get pictures too!

 

[Slow_boat]

Just like having a computer. Because you can.

I must admit that I'm happy to work it out to the nearest wave height plus a bit to within half an hour and let out about enough chain, give or take. I used to be able to do it in my head almost automatically but need to write things down these days.

I re did the yachtmaster theory a couple of years ago to try to catch up with the 20th century. The instructor was adamant that not checking chart datum would lead to gps position being out by 'as much as 300mtrs in mid channel'

Me- I'd be quite happy to be within quarter of a mile without a gps so 300mtrs will do me.

Instructor- This is a picture of a port entrance. If you were out 300mtrs you'ld be on those rocks.

Me- I think I would have noticed them or the lighthouse that is standing on them.

Instructor(with a gleam of triumph)-but what if it was foggy?

Me-Then you wouldn't catch me anywhere near that shore, gps or not.

Some fell on stony ground.

 

[Sailfree]

I asked a similar question and was told "the tide tables allow you to do it to the nearest minute, so do that and you have the most accurate estimate of what the tide is likely to be. From there you make an allowance for the effect of wind (surge) and atmospheric pressure. These are the variable that cause the tides to vary from the tables. Finally a sensible skipper then works out his draught, possible wave height (especially when crossing a river entrance bar) and applys the safety margin that the skipper is comfortable with".

Yes it is finally an estimation but one based on the best information rather than a rough guess.

Having once found the tide had turned 1hr earlier than predicted in Wotton Creek (I of W) and seen a number of boat that failed to make it either in or out of French Ports (Carteret, Paimpol & St Quay-Pontriaux) I vary my safety margin dependant on the consequences of getting it wrong!!

 

[Koeketiene]

[ QUOTE ]

Yes it is finally an estimation but one based on the best information rather than a rough guess.

[/ QUOTE ]

Rule of 12ths is an estimation too - but I can work that one out in my head.

How many people still know about the "rule of 12ths" anyway?

 

[whipper_snapper]

I suspect that one reason is that the examiner/instructor then knows exactly what the answer should be and so knows if you understand the method and have got the sums right.

I think that sensible estimation should be formally taught as part of the course. Personally, for a critical calculation, I round numbers and interpolate in my head with gay abandon,. But then I do it again 'from a different direction' if that makes sense. If I get about the same answer I know I am about right, or have screwed-up something more fundamental, like forgetting what month it is.

 

[mel80]

[ QUOTE ]

It's similar to working out CTS to within 2° - fine on a MOBO in flat seas, on a bouncy raggie in a bit of a blow I'm happy if you can keep within 10°.

[/ QUOTE ]

I've heard that argument before, but I'm not sure that I entirely see the logic. I would say the opposite, i.e. if you are able to steer a course accurately then the CTS calculation becomes less important as the total error will still be within acceptable bounds. Obviously that isn't the same as expecting the helmsman to follow the course accurately; it's just about cutting out errors where you can, and allowing for them where you can't.

 

[ChrisE]

Me, sir, I do. 1,2,3,3,2,1 twelths for each of the sixish hours of a tide works for height and speed - ish

 

[LymingtonPugwash]

agh! all this talk about tides is enough to make me choke on my ouzo!
Stick to the Med... we don't do tides here! /forums/images/graemlins/smile.gif More time for the important things associated with sailing! /forums/images/graemlins/smile.gif

 

[Twister_Ken]

>Me, sir, I do. 1,2,3,3,2,1 twelths for each of the sixish hours of a tide works for height and speed - ish<

Me too, but not in Poole Harbour or Soton Water!

 

[ChrisE]

To LymingtonP and TK, no marks for being smartarses /forums/images/graemlins/smile.gif

 

[LymingtonPugwash]

I'll drink to that Chris_E /forums/images/graemlins/smirk.gif

 

[peterb]

Tidal limits

[ QUOTE ]
I suspect that one reason is that the examiner/instructor then knows exactly what the answer should be and so knows if you understand the method and have got the sums right.

[/ QUOTE ]

It is exactly that. The instructor has a lot of papers to mark. If he sees that the answer is right to within 0.1 m and a couple of minutes, then he can just tick and pass on. If the answer is outside those limits, then he should look closely at the working to find out what has gone wrong.

There are lots of ways of estimating tidal heights; the RYA (more accurately, the Admiralty) method is just one, but one that works with the tide tables and tidal curves given in the almanacs. If you know the method, then getting the answer right is no problem. If you don't, then you probably need some help and the sooner the instructor knows, the better.

Having said that, in my experience tides and meteorology are the two worst-taught subjects in the RYA syllabus.

 

[MrCramp]

This is the site for looking at actual tide levels.
http://www.pol.ac.uk/ntslf/sadata_tgi_ntslf_v2.php?code=Felixstowe&span=1
I have seen the tide vary by a much as 1 metre from the prediction.

 

[peterb]

[ QUOTE ]
Rule of 12ths is an estimation too - but I can work that one out in my head.

How many people still know about the "rule of 12ths" anyway?

[/ QUOTE ]

"Rule of 12ths" was superb when depths were quoted in fathoms or feet, both divisible by 3. It's not so good in metres, unless you have that sort of brain.

Try "Rule of twelfths" at Dover, then look at the tidal curve. 5 hours to rise, 7 1/2 hours to fall. Alright, at Dover it's no big problem, deep water port and so on. But Rye, just round the corner, has the same tidal curve. You can just follow what's written in the pilot books, but you'll be a lot more restricted.

 

[Twister_Ken]

There speaketh a man obviously impressed with my choice in solomons.

 

[cookjwm]

You're stirring -find some work to do! /forums/images/graemlins/tongue.gif